White dwarf-neutron star binaries generate detectable gravitational radiation. We construct Newtonian equilibrium models of corotational white dwarf-neutron star (WDNS) binaries in circular orbit and find that these models terminate at the Roche limit. At this point the binary will undergo either stable mass transfer (SMT) and evolve on a secular time scale, or unstable mass transfer (UMT), which results in the tidal disruption of the WD. The path a given binary will follow depends primarily on its mass ratio. We analyze the fate of known WDNS binaries and use population synthesis results to estimate the number of LISA-resolved galactic binaries that will undergo either SMT or UMT. We model the quasistationary SMT epoch by solving a set of simple ordinary differential equations and compute the corresponding gravitational waveforms. Finally, we discuss in general terms the possible fate of binaries that undergo UMT and construct approximate Newtonian equilibrium configurations of merged WDNS remnants. We use these configurations to assess plausible outcomes of our future, fully relativistic simulations of these systems. If sufficient WD debris lands on the NS, the remnant may collapse, whereby the gravitational waves from the inspiral, merger, and collapse phases will sweep from LISA through LIGO frequency bands. If the debris forms a disk about the NS, it may fragment and form planets.

Current models of pulsar gamma-ray emission use the magnetic field of a rotating dipole in vacuum as a first approximation to the shape of plasma-filled pulsar magnetosphere. In this paper we revisit the question of gamma-ray light-curve formation in pulsars in order to ascertain the robustness of the "two-pole caustic" (TPC) and "outer gap" (OG) models based on the vacuum magnetic field. We point out an inconsistency in the literature on the use of the relativistic aberration formula, where in several works the vacuum field was treated as known in the instantaneously corotating frame, rather than in the laboratory frame. With the corrected formula, the TPC model using the vacuum field may no longer produce sharp peaks in the light curve. The OG model is less affected by this change, but the range of magnetic inclination angles and viewing geometries resulting in double-peaked light curves is reduced. In a realistic magnetosphere, the modification of field structure near the light cylinder due to plasma effects may change the shape of the polar cap and the location of the emission zones. We study the sensitivity of the light curves to different shapes of the polar cap for static and retarded vacuum dipole fields. In particular, we consider polar caps traced by the last open field lines and compare them to circular polar caps. We find that the TPC model is very sensitive to the shape of the polar cap, and a circular polar cap can lead to four peaks of emission. The OG model is less affected by polar cap shapes, but is subject to big uncertainties of applying the vacuum field near the light cylinder. We conclude that deviations from vacuum field can lead to large uncertainties in the pulse shapes, and a more realistic force-free field should be applied to the study of pulsar high energy emission.

(Abridged) Gamma-ray emission from pulsars has long been modeled using a vacuum dipole field. This approximation ignores changes in the field structure caused by the magnetospheric plasma and strong plasma currents. We present the first results of gamma-ray pulsar light curve modeling using the more realistic field taken from 3D force-free magnetospheric simulations. Having the geometry of the field, we apply several prescriptions for the location of the emission zone, comparing the light curves to observations. We find that the conventional two-pole caustic model fails to produce double-peak pulse profiles, mainly because the size of the polar cap in force-free magnetosphere is larger than the vacuum field polar cap. The conventional outer-gap model is capable of producing only one peak under general conditions, because a large fraction of open field lines does not cross the null charge surface. We propose a novel "annular gap" model, where the high-energy emission originates from a thin layer on the open field lines just inside of the current sheet that bounds the open flux tube. The emission from this layer generates two strong caustics on the sky map due to the effect we term "Sky Map Stagnation" (SMS). It is related to the fact that force-free field asymptotically approaches the field of a rotating split monopole, and the photons emitted on such field lines in the outer magnetosphere arrive to the observer in phase. The double-peak light curve is a natural consequence of SMS. We show that most features of the currently available gamma-ray pulsar light curves can be reasonably well reproduced and explained with the force-free field model. Association of the emission region with the current sheet will guide more detailed future studies of the magnetospheric acceleration physics.

Timing noise in the data on accretion-powered millisecond pulsars (AMP) appears as irregular pulse phase jumps on timescales from hours to weeks. A large systematic phase drift is also observed in the first discovered AMP SAX J1808.4-3658. To study the origin of these timing features, we use here the data of the well studied 2002 outburst of SAX J1808.4-3658. We develop first a model for pulse profile formation accounting for the screening of the antipodal emitting spot by the accretion disk. We demonstrate that the variations of the visibility of the antipodal spot associated with the receding accretion disk cause a systematic shift in Fourier phases, observed together with the changes in the pulse form. We show that a strong secondary maximum can be observed only in a narrow intervals of inner disk radii, which explains the very short appearance of the double-peaked profiles in SAX J1808.4-3658. By directly fitting the pulse profile shapes with our model, we find that the main parameters of the emitting spot such as its mean latitude and longitude as well as the emissivity pattern change irregularly causing small shifts in pulse phase, and the strong profile variations are caused by the increasing inner disk radius. We finally notice that significant variations in the pulse profiles in the 2002 and 2008 outbursts of SAX J1808.4-3658 happen at fluxes differing by a factor of 2, which can be explained if the inner disk radius is not a simple function of the accretion rate, but depends on the previous history.

Using XMM-Newton and Chandra, we achieved phase-connected timing of the 105 ms X-ray pulsar PSR J1852+0040 that provides the first measurement of the spin-down rate of a member of the class of Central Compact Objects (CCOs) in supernova remnants. We measure P-dot = 8.68(9)E-18, and find no evidence for timing noise or variations in X-ray flux over 4.8 yr. In the dipole spin-down formalism, this implies a surface magnetic field strength B_s = 3.1E10 G, the smallest ever measured for a young neutron star, and consistent with being a fossil field. In combination with upper limits on B_s from other CCO pulsars, this is strong evidence in favor of the "anti-magnetar" explanation for their low luminosity and lack of magnetospheric activity or synchrotron nebulae. While this dipole field is small, it is able to prevent accretion of enough fall-back material to account for the observed X-ray luminosity of L_x = 5.3E33(d/7.1 kpc}^2 erg/s, which instead must be residual cooling. The spin-down luminosity of PSR J1852+0040, E-dot = 3.0E32 erg/s, is an order-of-magnitude smaller than L_x. Fitting of the X-ray spectrum to two blackbodies finds small emitting radii, R_1 = 1.9 km and R_2 = 0.45 km, for components of kT_1 = 0.30 keV and kT_2 = 0.52 keV, respectively. Such small, hot regions are ubiquitous among CCOs, and are not yet understood in the context of the anti-magnetar picture because anisotropic surface temperature is usually attributed to the effects of strong magnetic fields.

Gamma-ray emission from pulsars is thought to arise from accelerating regions in pulsar's outer magnetosphere. The shape of the light curves is thus sensitive to the details of the magnetic geometry of the magnetosphere. In this work, we show the first calculations of light curves from the more realistic force-free field under the framework of conventional emission models. We compare the properties of gamma-ray emission between the commonly used vacuum dipole magnetic field and the new force-free field. We discuss the role of the polar cap shape and aberration effect on the appearance of the light curves as well as the formation of caustics on the sky map. With the force-free field, the double-peak pulse profile is best reproduced if the emission zone lies in a thin layer just outside the current sheet, and the peaks are mainly contributed from regions near the light cylinder. The conventional two-pole caustic model can produce up to four peaks in general, while the conventional outer-gap model can normally produce only one peak. These results will be useful for interpreting Fermi telescope observations.

A hypothetical time-variation of the gravitational constant $G$ would make neutron stars expand or contract, so the matter in their interiors would depart from beta equilibrium. This induces non-equilibrium weak reactions, which release energy that is invested partly in neutrino emission and partly in internal heating. Eventually, the star arrives at a stationary state in which the temperature remains nearly constant, as the forcing through the change of $G$ is balanced by the ongoing reactions. Using the surface temperature of the nearest millisecond pulsar (PSR J0437$-$4715) inferred from ultraviolet observations and results from theoretical modelling of the thermal evolution, we estimate two upper limits for this variation: (1) $|\dot G/G| < 2 \times 10^{-10}\mathrm{yr}^{-1},$ if the fast, "direct Urca" reactions are allowed, and (2) $|\dot G/G|<4\times 10^{-12}\mathrm{yr}^{-1},$ considering only the slower, "modified Urca" reactions. The latter is among the most restrictive upper limits obtained by other methods.

Magnetorotational instability (MRI) has been suggested to lead a rapid growth of the magnetic field in core collapse supernovae and produce departures from spherical syymmetry that can be important in determining the explosion mechanism. We address the problem of stability in differentially rotating magnetized proto-neutron stars at the beginning of their evolution. Criteria for MRI in proto-neutron stars are derived without simplying assumptions about a weak magnetic field and are substantially different from the standard condition. If the magnetic field is strong, MRI can occur only in the neighbourhood of the region where the spherical radial component of the magnetic field vanishes. The growth rate of MRI is relatively low except for perturbations with very small scales which usually are not detected in numerical simulations. We find that MRI in proto-neutron stars grows more slowly than than the double diffusive instability analogous the Goldreich-Schubert-Fricke instability in ordinary stars.

Pulsars and central engines of long gamma ray burst -- collapsars -- may produce highly magnetized (Poynting flux dominated) outflows expanding in a dense surrounding (interstellar medium or stellar material). For certain injection conditions, the magnetic flux of the wind cannot be accommodated within the cavity. In this case, ideal (non-dissipative) MHD models, similar to the Kennel and Coroniti (1984) model of the Crab nebular, break down (the so-called sigma problem). This is typically taken to imply that the wind should become particle-dominated on scales much smaller than the size of the cavity. The wind is then slowed down by a fluid-type (low magnetization) reverse shock. Recent Fermi results, indicating that synchrotron spectrum of the Crab nebula extends well beyond the upper limit of the most efficient radiation reaction-limited acceleration, contradict the presence of a low sigma reverse shock.
We propose an alternative possibility, that the excessive magnetic flux is destroyed in a reconnection-like process in two regions: near the rotational axis and near the equator. We construct an example of such highly magnetized wind having two distinct reconnection regions and suggest that these reconnection cites are observed as tori and jets in pulsar wind nebulae. The model reproduces, qualitatively, the observed morphology of the Crab nebula. In parts of the nebular the dissipation occurs in a relativistically moving wind, alleviating the requirements on the acceleration rate.

GS 1826-238 is a well-studied X-ray bursting neutron star in a low mass binary system. Thermal Comptonisation by a hot electron cloud is a widely accepted mechanism accounting for its high energy emission, while the nature of most of its soft X-ray output is not completely understood. A further low energy component is typically needed to model the observed spectra: pure blackbody and Comptonisation-modified blackbody radiation by a lower temperature (a few keV) electron plasma were suggested to explain the low energy data. We studied the steady emission of GS 1826-238 by means of broad band (X to soft Gamma-rays) measurements obtained by the INTEGRAL observatory in 2003 and 2006. The newly developed, up-to-date Comptonisation model CompTB is applied for the first time to study effectively the low-hard state variability of a low-luminosity neutron star in a low-mass X-ray binary system. We confirm that the 3-200 keV emission of \GS is characterised by Comptonisation of soft seed photons by a hot electron plasma. A single spectral component is sufficient to model the observed spectra. At lower energies, no direct blackbody emission is observed and there is no need to postulate a low temperature Compton region. Compared to the 2003 measurements, the plasma temperature decreased from 20 to 14 keV in 2006, together with the seed photons temperature. The source intensity was also found to be 30% lower in 2006, whilst the average recurrence frequency of the X-ray bursts significantly increased. Possible explanations for this apparent deviation from the typical limit-cycle behaviour of this burster are discussed.

We report the discovery of the second accreting millisecond X-ray pulsar (AMXP) in the globular cluster NGC 6440. Pulsations with a frequency of 205.89 Hz were detected with the Rossi X-Ray Timing Explorer on August 30th and October 1st, 2009, during the decay of ~4 days outburst of a newly X-ray transient source in NGC 6440. By studying the Doppler shift of the pulsation frequency we find that the system is an ultra-compact binary with an orbital period of 57.3 minutes and a projected semi-major axis of 6.22 light-milliseconds. Based on the mass function, we estimate a lower limit to the mass of the companion to be 0.0067 M_sun (assuming a 1.4 M_sun neutron star). This new pulsar shows the shortest outburst recurrence time among AMXPs (~1 month). If this behaviour does not cease, this AMXP has the potential to be one of the best sources in which to study how the binary system and the neutron star spin evolve. Furthermore, the characteristics of this new source indicates that there might exist a population of AMXPs undergoing weak outbursts which are undetected by current all-sky X-ray monitors. NGC 6440 is the only globular cluster to host two known AMXPs, while no AMXPs have been detected in any other globular cluster.

We have identified a new transient luminous low-mass X-ray binary, NGC 6440 X-2, with Chandra/ACIS, RXTE/PCA, and Swift/XRT observations of the globular cluster NGC 6440. The discovery outburst (July 28-31, 2009) peaked at L_X~1.5*10^36 ergs/s, and lasted for <4 days above L_X=10^35 ergs/s. Three other outbursts (May 29-June 4, Aug. 29-Sept. 1, and Oct. 1-3, 2009) have been observed with RXTE/PCA (identifying millisecond pulsations, Altamirano et al. 2009a) and Swift/XRT (confirming a positional association with NGC 6440 X-2). Optical and infrared imaging did not detect a clear counterpart, with best limits of V>21, B>22 in quiescence from archival HST imaging, g'>22 during the third outburst from Gemini-South GMOS imaging, and J>~18.5$ and K>~17 during the second outburst from CTIO 4-m ISPI imaging.
Archival Chandra X-ray images of the core do not detect the quiescent counterpart, and place a bolometric luminosity limit of L_{NS}< 5.6*10^31 ergs/s (one of the lowest measured) for a hydrogen atmosphere neutron star. A followup Chandra observation finds marginal evidence of enhanced quiescent emission at L_X (0.5-10 keV)~6*10^31 ergs/s 10 days into quiescence.
NGC 6440 X-2 currently shows the shortest recurrence time (32 days) of any known X-ray transient, although regular outbursts were not visible in the bulge scans before early 2009. Fast, low-luminosity transients like NGC 6440 X-2 may be easily missed by current X-ray monitoring.

Using pulsewidth data for 872 isolated radio pulsars we test the hypothesis that pulsars evolve through a progressive narrowing of the emission cone combined with progressive alignment of the spin and magnetic axes. The new data provide strong evidence for the alignment over a time-scale of about 1 Myr with a log standard deviation of around 0.8 across the observed population. This time-scale is shorter than the time-scale of about 10 Myr found by previous authors, but the log standard deviation is larger. The results are inconsistent with models based on magnetic field decay alone or monotonic counter-alignment to orthogonal rotation. The best fits are obtained for a braking index parameter n_gamma approximately equal to 2.3, consistent the mean of the six measured values, but based on a much larger sample of young pulsars. The least-squares fitted models are used to predict the mean inclination angle between the spin and magnetic axes as a function of log characteristic age. Comparing these predictions to existing estimates it is found that the model in which pulsars are born with a random angle of inclination gives the best fit to the data. Plots of the mean beaming fraction as a function of characteristic age are presented using the best-fitting model parameters.

We investigate the emission mechanism and evolution of pulsars that are associated with supernova remnants.
We used imaging techniques in both the optical and near infrared, using images with very good seeing (<0.6) to study the immediate surroundings of the Crab pulsar. In the case of the infrared, we took two data sets with a time window of 75 days, to check for variability in the inner part of the Crab nebula. We also measure the spectral indices of all these wisps, the nearby knot, and the interwisp medium, using our optical and infrared data. We then compared the observational results with the existing theoretical models.
We report variability in the three nearby wisps located to the northwest of the pulsar and also in a nearby anvil wisp in terms of their structure, position, and emissivity within the time window of 75 days. All the wisps and the inner knot display red spectra with similar spectral indices. Similarly, the interwisp medium regions also show red spectra similar to those of the wisps. Also, based on archival HST data and our IR data, we find that the inner knot remains stationary for a time period of 13.5 years. The projected average velocity relative to the pulsar for this period is < 8 km/s.
By comparing the spectral indices of the structures in the inner Crab with the current theoretical models, we find that the Del Zanna et al. (2006) model for the synchrotron emission fits our observations, although the spectral index is at the flatter end of their modelled spectra.

This paper presents a numerical study over a wide parameter space of the likelihood of the dynamical bar-mode instability in differentially rotating magnetized neutron stars. The innovative aspect of this study is the incorporation of magnetic fields in such a context, which have thus far been neglected in the purely hydrodynamical simulations available in the literature. The investigation uses the Cosmos++ code which allows us to perform three dimensional simulations on a cylindrical grid at high resolution. A sample of Newtonian magneto-hydrodynamical simulations starting from a set of models previously analyzed by other authors without magnetic fields has been performed, providing estimates of the effects of magnetic fields on the dynamical bar-mode deformation of rotating neutron stars. Overall, our results suggest that the effect of magnetic fields are not likely to be very significant in realistic configurations. Only in the most extreme cases are the magnetic fields able to suppress growth of the bar mode.

The surface of hot neutron stars is covered by a thin atmosphere. If there is accretion after neutron star formation, the atmosphere could be composed of light elements (H or He); if no accretion takes place or if thermonuclear reactions occur after accretion, heavy elements (for example, Fe) are expected. Despite detailed searches, observations have been unable to confirm the atmospheric composition of isolated neutron stars. Here we report an analysis of archival observations of the compact X-ray source in the centre of the Cassiopeia A supernova remnant. We show that a carbon atmosphere neutron star (with low magnetic field) produces a good fit to the spectrum. Our emission model, in contrast with others, implies an emission size consistent with theoretical predictions for the radius of neutron stars. This result suggests that there is nuclear burning in the surface layers and also identifies the compact source as a very young (~330-year-old) neutron star.

We show that energy deposited into an expanding supernova remnant by a highly magnetic (B ~ 5 x 10^14 G) neutron star spinning at an initial period of P ~ 2-20 ms can substantially brighten the light curve. For magnetars with parameters in this range, the rotational energy is released on a timescale of days to weeks, which is comparable to the effective diffusion time through the supernova remnant. The late time energy injection can then be radiated without suffering overwhelming adiabatic expansion losses. The magnetar input also produces a central bubble which sweeps ejecta into an internal dense shell, resulting in a prolonged period of nearly constant photospheric velocity in the observed spectra. We derive analytic expressions for the light curve rise time and peak luminosity as a function of B, P and the properties of the supernova ejecta that allow for direct inferences about the underlying magnetar in bright supernovae. We perform numerical radiation hydrodynamical calculations of a few specific instances and compare the resulting light curves to observed events. Magnetar activity is likely to impact more than a few percent of all core collapse supernovae, and may naturally explain some of the brightest events ever seen (e.g., SN 2005ap and SN 2008es) at L > 10^44 ergs/s.

Following an initial explosion that might be launched either by magnetic interactions or neutrinos, a rotating magnetar radiating according to the classic dipole formula could power a very luminous supernova. While some 56Ni might be produced in the initial explosion, the peak of the light curve in a Type I supernova would not be directly related to its mass. In fact, the peak luminosity would be most sensitive to the dipole field strength of the magnetar. The tail of the light curve could resemble radioactive decay for some time but, assuming complete trapping of the pulsar emission, would eventually be brighter. Depending on the initial explosion energy, both high and moderate velocities could accompany a very luminous light curve.

An interesting new high-energy pulsar sub-population is emerging following early discoveries of gamma-ray millisecond pulsars (MSPs) by the Fermi Large Area Telescope (LAT). We present results from 3D emission modeling, including the Special Relativistic effects of aberration and time-of-flight delays and also rotational sweepback of B-field lines, in the geometric context of polar cap (PC), outer gap (OG), and two-pole caustic (TPC) pulsar models. In contrast to the general belief that these very old, rapidly-rotating neutron stars (NSs) should have largely pair-starved magnetospheres due to the absence of significant pair production, we find that most of the light curves are best fit by TPC and OG models, which indicates the presence of narrow accelerating gaps limited by robust pair production -- even in these pulsars with very low spin-down luminosities. The gamma-ray pulse shapes and relative phase lags with respect to the radio pulses point to high-altitude emission being dominant for all geometries. We also find exclusive differentiation of the current gamma-ray MSP population into two MSP sub-classes: light curve shapes and lags across wavebands impose either pair-starved PC (PSPC) or TPC / OG-type geometries. In the first case, the radio pulse has a small lag with respect to the single gamma-ray pulse, while the (first) gamma-ray peak usually trails the radio by a large phase offset in the latter case. Finally, we find that the flux correction factor as a function of magnetic inclination and observer angles is typically of order unity for all models. Our calculation of light curves and flux correction factor for the case of MSPs is therefore complementary to the "ATLAS paper" of Watters et al. for younger pulsars.

The pulse shapes detected during multiple outbursts of SAX J1808 are analyzed in order to constrain the neutron star's mass and radius. We use a hot-spot model with a small disk-scattering component to jointly fit data from two different epochs, under the restriction that the star's mass and radius and the binary's inclination do not change from epoch to epoch. All other parameters describing the spot location, emissivity, and relative fractions of blackbody to Comptonized radiation are allowed to vary with time. The joint fit of data from the 1998 "slow decay" and the 2002 "end of outburst maximum" epochs using the constraint i<90 degrees leads to the 3 sigma confidence constraint on the neutron star mass 0.8 M_sun < M < 1.7 M_sun and equatorial radius 5 km < R < 13 km. Inclinations as low as 41 degrees are allowed. The best-fit models with M > 1.0 M_sun from joint fits of the 1998 data with data from other epochs of the 2002 and 2005 outbursts also fall within the same 3 sigma confidence region. This 3 sigma confidence region allows a wide variety of hadronic equations of state, in contrast with an earlier analysis (Leahy et al 2008) of only the 1998 outburst data that only allowed for extremely small stars.

The first study of migration-induced resonances in a pair of Earth-like planets has been performed by Papaloizou and Szuszkiewicz (2005). They concluded that in the case of disparate masses embedded in a disc with the surface density expected for a minimum mass solar nebula at 5.2 au, the most likely resonances are ratios of large integers, such as 8:7. For equal masses, planets tend to enter into the 2:1 or 3:2 resonance. In Papaloizou and Szuszkiewicz (2005) the two low-mass planets have masses equal to 4 Earth masses, chosen to mimic the very well known example of two pulsar planets which are close to the 3:2 resonance. That study has stimulated quite a few interesting questions. One of them is considered here, namely how the behaviour of the plan- ets close to the mean-motion resonance depends on the actual values of the masses of the planets. We have chosen a 3:2 commensurability and investigated the outcome of an orbital migration in the vicinity of this resonance in the case of a pair of equal mass super-Earths, whose mass is either 5 or 8 Earth masses.

Pulsars are amongst the most stable rotators known in the Universe. Over many years some millisecond pulsars rival the stability of atomic clocks. Comparing observations of many such stable pulsars may allow the first direct detection of gravitational waves, improve the Solar System planetary ephemeris and provide a means to study irregularities in terrestrial time scales. Here we review the goals and status of current and future pulsar timing array projects.

The neutrino-antineutrino annihilation into electron-positron pairs near the surface of compact general relativistic stars could play an important role in supernova explosions, neutron star collapse, or for close neutron star binaries near their last stable orbit. General relativistic effects increase the energy deposition rates due to the annihilation process. We investigate the deposition of energy and momentum due to the annihilations of neutrinos and antineutrinos in the equatorial plane of the rapidly rotating neutron and quark stars, respectively. We analyze the influence of general relativistic effects, and we obtain the general relativistic corrections to the energy and momentum deposition rates for arbitrary stationary and axisymmetric space-times. We obtain the energy and momentum deposition rates for several classes of rapidly rotating neutron stars, described by different equations of state of the neutron matter, and for quark stars, described by the MIT bag model equation of state and in the CFL (Color-Flavor-Locked) phase, respectively. Compared to the Newtonian calculations, rotation and general relativistic effects increase the total annihilation rate measured by an observer at infinity. The differences in the equations of state for neutron and quark matter also have important effects on the spatial distribution of the energy deposition rate by neutrino-antineutrino annihilation.

(Abridged.) The mean-square current quadrupole moment associated with vorticity fluctuations in high-Reynolds-number turbulence in a differentially rotating neutron star is calculated analytically, as are the amplitude and decoherence time of the resulting, stochastic gravitational wave signal. The calculation resolves the subtle question of whether the signal is dominated by the smallest or largest turbulent eddies: for the Kolmogorov-like power spectrum observed in superfluid spherical Couette simulations, the wave strain is controlled by the largest eddies, and the decoherence time approximately equals the maximum eddy turnover time. For a neutron star with spin frequency $\nu_s$ and Rossby number $Ro$, at a distance $d$ from Earth, the root-mean-square wave strain reaches $h_{RMS} \approx 3\times 10^{-24} Ro^3 (\nu_s / 30 Hz)^3 (d/1 kpc)^{-1}$. A cross-correlation search can detect such a source in principle, because the signal decoheres over the time-scale $\tau_c \approx 10^{-3} Ro^{-1} (\nu_s / 30 Hz)^{-1} s$, which is adequately sampled by existing long-baseline interferometers. Hence hydrodynamic turbulence imposes a fundamental noise floor on gravitational wave observations of neutron stars, although its polluting effect may be muted by partial decoherence in the hectohertz band, where current continuous-wave searches are concentrated, for the highest frequency (and hence most powerful) sources.

Nancay radiotelescope is involved in high precision timing since 20 years. Since 2004, a coherent dedispersion instrumentation enables numerous routine observations on more than 200 pulsars using half of the time if this 100-meters class radiotelescope. Two main programs are currently conducted. A large set of young and old pulsars is timed for a multi-wavelength approach, complementary to the very successful high energy observations of pulsars done by FERMI. A set of highly stable millisecond pulsars is monitored as our contribution to the European Pulsar Timing Array in order to probe the cosmological Gravitational Wave Background.

We have studied the various properties of magnetically deformed atoms, replaced by deformed Wigner-Seitz cells, at the outer crust region of strongly magnetized neutron stars (magnetars) using a relativistic version of Thomas-Fermi model in cylindrical coordinates.

The accretion disk around a compact object is a nonlinear general relativistic system involving magnetohydrodynamics. Naturally the question arises whether such a system is chaotic (deterministic) or stochastic (random) which might be related to the associated transport properties whose origin is still not confirmed. Earlier, the black hole system GRS 1915+105 was shown to be low dimensional chaos in certain temporal classes. However, so far such nonlinear phenomena have not been studied fairly well for neutron stars which are unique for their magnetosphere and kHz quasi-periodic oscillation (QPO). On the other hand, it was argued that the QPO is a result of nonlinear magnetohydrodynamic effects in accretion disks. If a neutron star exhibits chaotic signature, then what is the chaotic/correlation dimension? We analyze RXTE/PCA data of neutron stars Sco X-1 and Cyg X-2, along with the black hole Cyg X-1 and the unknown source Cyg X-3, and show that while Sco X-1 and Cyg X-2 are low dimensional chaotic systems, Cyg X-1 and Cyg X-3 are stochastic sources. Based on our analysis, we argue that Cyg X-3 may be a black hole.

We present radial velocity observations of four extremely low-mass (0.2 Msol) white dwarfs. All four stars show peak-to-peak radial velocity variations of 540 - 710 km/s with 1.0 - 5.9 hr periods. The optical photometry rules out main-sequence companions. In addition, no milli-second pulsar companions are detected in radio observations. Thus the invisible companions are most likely white dwarfs. Due to the loss of angular momentum through gravitational radiation, three of the systems will merge within 500 Myr. The remaining system will merge within a Hubble time. The mass functions for three of the systems imply companions more massive than 0.44 Msol; thus those are carbon/oxygen core white dwarfs. However, the chance of a supernova Ia event is only 1% to 5%. These systems will most likely form single R Coronae Borealis stars, providing evidence for a white dwarf + white dwarf merger mechanism for these unusual objects. One of the systems, SDSS J105353.89+520031.0 has a 70% chance of having a low-mass white dwarf companion. This system will probably form a single helium-enriched subdwarf O star. All four white dwarf systems have unusal mass ratios of < 0.2-0.8$ that may also lead to the formation of AM CVn systems. The unknown inclination angles prohibit a definitive conclusion about the future of these systems.

We present the results of a search for transient radio bursts of between 0.125 and 32 millisecond duration in two archival pulsar surveys of intermediate galactic latitudes with the Parkes multibeam receiver. Fourteen new neutron stars have been discovered, seven of which belong to the recently identified "rotating radio transients" (RRATs) class. Here we describe our search methodology, and discuss the new detections in terms of how the RRAT population relates to the general population of pulsars. The new detections indicate (1) that the galactic z-distribution of RRATs in the surveys closely resembles the distribution of pulsars, with objects up to 0.86 kpc from the galactic plane; (2) where measurable, the RRAT pulse widths are similar to that of individual pulses from pulsars of similar period, implying a similar beaming fraction; and (3) our new detections span a variety of nulling fractions, and thus we postulate that the RRATs may simply be nulling pulsars that are only "on" for less than a pulse period. Finally, the newly discovered object PSR J0941-39 may represent a link between pulsars and RRATs. This bizarre object was discovered as an RRAT, but in follow-up observations often appeared as a bright (~10 mJy) pulsar with a low nulling fraction. It is obvious therefore that a neutron star can oscillate between being an RRAT and a pulsar. Crucially, the sites of the RRAT pulses are coincident with the pulsar's emission, implying that the two emission mechanisms are linked, and that RRATs are not just pulsars observed from different orientations.

The inner 10 pc of our galaxy contains many counterpart candidates of the very high energy (VHE; > 100 GeV) gamma-ray point source HESS J1745-290. Within the point spread function of the H.E.S.S. measurement, at least three objects are capable of accelerating particles to very high energies and beyond, and of providing the observed gamma-ray flux. Previous attempts to address this source confusion were hampered by the fact that the projected distances between those objects were of the order of the error circle radius of the emission centroid (34", dominated by the pointing uncertainty of the H.E.S.S. instrument). Here we present H.E.S.S. data of the Galactic Centre region, recorded with an improved control of the instrument pointing compared to H.E.S.S. standard pointing procedures. Stars observed during gamma-ray observations by optical guiding cameras mounted on each H.E.S.S. telescope are used for off-line pointing calibration, thereby decreasing the systematic pointing uncertainties from 20" to 6" per axis. The position of HESS J1745-290 is obtained by fitting a multi-Gaussian profile to the background-subtracted gamma-ray count map. A spatial comparison of the best-fit position of HESS J1745-290 with the position and morphology of candidate counterparts is performed. The position is, within a total error circle radius of 13", coincident with the position of the supermassive black hole Sgr A* and the recently discovered pulsar wind nebula candidate G359.95-0.04. It is significantly displaced from the centroid of the supernova remnant Sgr A East, excluding this object with high probability as the dominant source of the VHE gamma-ray emission.

A phase transition from paramagnetism to ferromagnetism in neutron star interior is explored. Since there is $^3$P$_2$ neutron superfluid in neutron star interior, it can be treated as a system of magnetic dipoles. Under the presence of background magnetic field, the magnetic dipoles tend to align in the same direction. Below a critical temperature, there is a phase transition from paramagnetism to ferromagnetism. And this gives a convenient explanation of the strong magnetic field of magnetars. In our point of view, there is an upper limit for the magnetic field strength of magnetars. The maximum field strength of magnetars is about $(3.0-4.0)\times 10^{15}$ G. This can be tested directly by further investigations.
Magnetars are instable due to the ultra high Fermi energy of electrons. The Landau column becomes a very long cylinder along the magnetic field, but it is very narrow and the Fermi energy of electron gas is given as $E_F(e)\approx40(B/B_{cr})^{1/4}$ when $B\gg B_{cr}$. $E_F(e)\approx90MeV$ When $B\sim10^{15}$ G. Hence, the electron capture process $e^-+p\to n+\nu_e$ will be happen rapidly. Thus the $^3$P$_2$ Cooper pairs will be destroyed quickly by the outgoing neutrons with high energy. It will cause the isotropic superfluid disappear and then the magnetic field induced by the $^3$P$_2$ Cooper pairs will be also disappear. These energy will immediately be transmitted into thermal energy and then transformed into the radiation energy with X-ray - soft $\gamma$-ray. We may get a conclusion that the activity of magnetars originates from instability caused by the high Fermi energy of electrons in extra strong magnetic field.

Markov Chain Monte Carlo (MCMC) methods have revolutionised Bayesian data analysis over the years by making the direct computation of posterior probability densities feasible on modern workstations. However, the calculation of the prior predictive, the Bayesian evidence, has proved to be notoriously difficult with standard techniques. In this work a method is presented that lets one calculate the Bayesian evidence using nothing but the results from standard MCMC algorithms, like Metropolis-Hastings. This new method is compared to other methods like MultiNest, and greatly outperforms the latter in several cases. One of the toy problems considered in this work is the analysis of mock pulsar timing data, as encountered in pulsar timing array projects. This method is expected to be useful as well in other problems in astrophysics, cosmology and particle physics.

We report on gamma-ray observations of the Crab Pulsar and Nebula using 8 months of survey data with the Fermi Large Area Telescope (LAT). The high quality light curve obtained using the ephemeris provided by the Nancay and Jodrell Bank radio telescopes shows two main peaks stable in phase with energy. The first gamma-ray peak leads the radio main pulse by (281 \pm 12 \pm 21) mus, giving new constraints on the production site of non-thermal emission in pulsar magnetospheres. The improved sensitivity and the unprecedented statistics afforded by the LAT enable precise measurement of the Crab Pulsar spectral parameters: cut-off energy at E_c = (5.8 \pm 0.5 \pm 1.2) GeV, spectral index of Gamma = (1.97 \pm 0.02 \pm 0.06) and integral photon flux above 100 MeV of (2.09 \pm 0.03 \pm 0.18) x 10^{-6} cm^{-2} s^{-1}. The first errors represent the statistical error on the fit parameters, while the second ones are the systematic uncertainties. Pulsed gamma-ray photons are observed up to ~ 20 GeV which precludes emission near the stellar surface, below altitudes of around 4 to 5 stellar radii in phase intervals encompassing the two main peaks. The spectrum of the nebula in the energy range 100 MeV - 300 GeV is well described by the sum of two power-laws of indices Gamma_{sync} = (3.99 \pm 0.12 \pm 0.08) and Gamma_{IC} = (1.64 \pm 0.05 \pm 0.07), corresponding to the falling edge of the synchrotron and the rising edge of the inverse Compton components, respectively. This latter, which links up naturally with the spectral data points of Cherenkov experiments, is well reproduced via inverse Compton scattering from standard Magnetohydrodynamics (MHD) nebula models, and does not require any additional radiation mechanism.

We report the discovery of an unusual source of extended X-ray emission CXOU J184846.3-013040 (`The Stem') located on the outskirts of the globular cluster GLIMPSE-C01. No point-like source falls within the extended emission which has an X-ray luminosity L_X =10^{32} ergs/s and a physical size of 0.1 pc at the inferred distance to the cluster. These X-ray properties are consistent with the pulsar wind nebula (PWN) of an unseen pulsar located within the 95-percent confidence error contour of unidentified Fermi gamma-ray source 0FGL J1848.6-0138. However, we cannot exclude an alternative interpretation that postulates X-ray emission associated with a bow shock produced from the interaction of the globular cluster and interstellar gas in the Galactic plane. Analysis of the X-ray data reveals that `The Stem' is most significant in the 2-5 keV band, which suggests that the emission may be dominated by non-thermal bremsstrahlung from suprathermal electrons at the bow shock. If the bow shock interpretation is correct, these observations would provide compelling evidence that GLIMPSE-C01 is shedding its intracluster gas during a galactic passage. Such a direct detection of gas stripping would help clarify a crucial step in the evolutionary history of globular clusters. Intriguingly, the data may also accommodate a new type of X-ray source.

Eight Accreting Millisecond X-ray Pulsars (AMXPs) are known to date. Optical and NIR observations carried out during quiescence give a unique opportunity to constrain the nature of the donor star and to investigate the origin of the observed quiescent luminosity at long wavelengths. Using data obtained with the ESO-Very Large Telescope, we performed a deep optical and NIR photometric study of the fields of XTE J1814-338 and of the ultracompact systems XTE J0929-314 and XTE J1807-294 during quiescence in order to look for the presence of a variable counterpart. If suitable candidates were found, we also carried out optical spectroscopy. We present here the first multi-band (VR) detection of the optical counterpart of XTE J1814-338 in quiescence together with its optical spectrum. The optical light curve shows variability in both bands consistent with a sinusoidal modulation at the known 4.3 hr orbital period and presents a puzzling decrease of the V-band flux around superior conjunction that may be interpreted as a partial eclipse. The marginal detection of the very faint counterpart of XTE J0929-314 and deep upper limits for the optical/NIR counterpart of XTE J1807-294 are also reported. We also briefly discuss the results reported in the literature for the optical/NIR counterpart of XTE J1751-305. Our findings are consistent with AMXPs being systems containing an old, weakly magnetized neutron star, reactivated as a millisecond radio pulsar during quiescence which irradiates the low-mass companion star. The absence of type I X-ray bursts and of hydrogen and helium lines in outburst spectra of ultracompact (P_orb < 1 hr) AMXPs suggests that the companion stars are likely evolved dwarf stars.

We have investigated the pulse shape evolution of the Crab pulsar emission in the hard X-ray domain of the electromagnetic spectrum. In particular, we have studied the alignment of the Crab pulsar phase profiles measured in the hard X-rays and in other wavebands. To obtain the hard X-ray pulse profiles, we have used six year (2003-2009, with a total exposure of about 4 Ms) of publicly available data of the SPI telescope on-board of the INTEGRAL observatory, folded with the pulsar time solution derived from the Jodrell Bank Crab Pulsar Monthly Ephemeris. We found that the main pulse in the hard X-ray 20-100 keV energy band is leading the radio one by $8.18\pm0.46$ milliperiods in phase, or $275\pm15 \mu s$ in time. Quoted errors represent only statistical uncertainties.Our systematic error is estimated to be $\sim 40 \mu s$ and is mainly caused by the radio measurement uncertainties. In hard X-rays, the average distance between the main pulse and interpulse on the phase plane is $0.3989\pm0.0009$. To compare our findings in hard X-rays with the soft 2-20 keV X-ray band, we have used data of quasi-simultaneous Crab observations with the PCA monitor on-board the Rossi X-Ray Timing Explorer (RXTE) mission. The time lag and the pulses separation values measured in the 3-20 keV band are $0.00933\pm0.00016$ (corresponding to $310\pm6 \mu s$) and $0.40016\pm0.00028$ parts of the cycle, respectively. While the pulse separation values measured in soft X-rays and hard X-rays agree, the time lags are statistically different. Additional analysis show that the delay between the radio and X-ray signals varies with energy in the 2 - 300 keV energy range. We explain such a behaviour as due to the superposition of two independent components responsible for the Crab pulsed emission in this energy band.

We study non-axisymmetric oscillations of rapidly and differentially rotating relativistic stars in the Cowling approximation. Our equilibrium models are sequences of relativistic polytropes, where the differential rotation is described by the relativistic $j$-constant law. We show that a small degree of differential rotation raises the critical rotation value for which the quadrupolar f-mode becomes prone to the CFS instability, while the critical value of $T/|W|$ at the mass-shedding limit is raised even more. For softer equations of state these effects are even more pronounced. When increasing differential rotation further to a high degree, the neutral point of the CFS instability first reaches a local maximum and is lowered afterwards. For stars with a rather high compactness we find that for a high degree of differential rotation the absolute value of the critical $T/|W|$ is below the corresponding value for rigid rotation. We conclude that the parameter space where the CFS instability is able to drive the neutron star unstable is increased for a small degree of differential rotation and for a large degree at least in stars with a higher compactness.

We report the detection of pulsed gamma-rays for PSRs J0631+1036, J0659+1414, J0742-2822, J1420-6048, J1509-5850 and J1718-3825 using the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope (formerly known as GLAST). Although these six pulsars are diverse in terms of their spin parameters, they share an important feature: their gamma-ray light curves are (at least given the current count statistics) single peaked. For two pulsars there are hints for a double-peaked structure in the light curves. The shapes of the observed light curves of this group of pulsars are discussed in the light of models for which the emission originates from high up in the magnetosphere. The observed phases of the gamma-ray light curves are, in general, consistent with those predicted by high-altitude models, although we speculate that the gamma-ray emission of PSR J0659+1414, possibly featuring the softest spectrum of all Fermi pulsars coupled with a very low efficiency, arises from relatively low down in the magnetosphere. High-quality radio polarization data are available showing that all but one have a high degree of linear polarization. This allows us to place some constraints on the viewing geometry and aids the comparison of the gamma-ray light curves with high-energy beam models.

On 2009 January 22 numerous strong bursts were detected from the anomalous X-ray pulsar 1E 1547.0-5408. Swift/XRT and XMM-Newton/EPIC observations carried out in the following two weeks led to the discovery of three X-ray rings centered on this source. The ring radii increased with time following the expansion law expected for a short impulse of X-rays scattered by three dust clouds. Assuming different models for the dust composition and grain size distribution, we fit the intensity decay of each ring as a function of time at different energies, obtaining tight constrains on the distance of the X-ray source. Although the distance strongly depends on the adopted dust model, we find that some models are incompatible with our X-ray data, restricting to 4-8 kpc the range of possible distances for 1E 1547.0-5408. The best-fitting dust model provides a source distance of 3.91 +/- 0.07 kpc, which is compatible with the proposed association with the supernova remnant G 327.24-0.13, and implies distances of 2.2 kpc, 2.6 kpc and 3.4 kpc for the dust clouds, in good agreement with the dust distribution inferred by CO line observations towards 1E 1547.0-5408. However, dust distances in agreement with CO data are also obtained for a set of similarly well-fitting models that imply a source distance of about 5 kpc. A distance of about 4-5 kpc is also favored by the fact that these dust models are already known to provide good fits to the dust-scattering halos of bright X-ray binaries.

We present a deep Chandra X-ray Observatory study of the peculiar binary radio millisecond pulsar PSR J1740--5340 and candidate millisecond pulsars (MSPs) in the globular cluster NGC 6397. The X-rays from PSR J1740--5340 appear to be non-thermal and exhibit variability at the binary period. These properties suggest the presence of a relativistic intrabinary shock formed due to interaction of a relativistic rotation-powered pulsar wind and outflow from the unusual "red-straggler/sub-subgiant" companion. We find the X-ray source U18 to show similar X-ray and optical properties to those of PSR J1740--5340, making it a strong MSP candidate. It exhibits variability on timescales from hours to years, also consistent with an intrabinary shock origin of its X-ray emission. The unprecedented depth of the X-ray data allows us to conduct a complete census of MSPs in NGC 6397. Based on the properties of the present sample of X-ray--detected MSPs in the Galaxy we find that NGC 6397 probably hosts no more than 6 MSPs.

We present broadband (gamma-ray, X-ray, near-infrared, optical, and radio) observations of the gamma-ray burst (GRB) 090709A and its afterglow in an effort to ascertain the origin of this high-energy transient. Previous analyses suggested that GRB 090709A exhibited quasi-periodic oscillations with a period of 8.06 s, a trait unknown in long-duration GRBs but typical of flares from soft gamma-ray repeaters. When properly accounting for the underlying shape of the power-density spectrum of GRB 090709A, we find no conclusive (> 3 sigma) evidence for the reported periodicity. In conjunction with the location of the transient (far from the Galactic plane and absent any nearby host galaxy in the local universe) and the evidence for extinction in excess of the Galactic value, we consider a magnetar origin relatively unlikely. A long-duration GRB, however, can account for the majority of the observed properties of this source. GRB 090709A is distinguished from other long-duration GRBs primarily by the large amount of obscuration from its host galaxy (A_K,obs >~ 2 mag).

Giant pulses observed in a number of pulsars show a record brightness temperature which corresponds to the high energy density of 10^15 erg/cm^3. Comparable densities of energy in the radio-frequency region are attainable in a cavity-resonator being the pulsar internal vacuum gap. Energy emission through the breaks accidentally appearing in the magnetosphere of open field lines corresponds to the giant pulses. The emitted energy is defined by the break area, which causes a power dependence of break occurrence probability. The observed localization of giant pulses as to the average pulse is explained by radiation through a waveguide near the magnetic axis or through a slot on the border of the open field lines. Separate discharges may be superimposed on the radiation through the breaks forming the fine structure of giant pulses with duration up to some nanoseconds. Coulomb repulsion of particles in the puncture spark in the gap leads to spark rotation around its axis in the crossed fields, which provokes the appearance of observed circular polarization of giant pulses. The correlation between GP phase and the phase of the hard pulsar radiation (X-ray and gamma) also is naturally explained.
Thus, a wide range of events observed at giant pulses can be explained from the viewpoint, that the internal vacuum gap is a cavity-resonator stimulated by discharges and radiating through the breaks in the magnetosphere.

[abridged] We present a unified picture for the evolution of star clusters on the two-body relaxation timescale. We use direct N-body simulations of star clusters in a galactic tidal field starting from different multi-mass King models, up to 10% of primordial binaries and up to Ntot=65536 particles. An additional run also includes a central Intermediate Mass Black Hole. We find that for the broad range of initial conditions we have studied the stellar mass function of these systems presents a universal evolution which depends only on the fractional mass loss. The structure of the system, as measured by the core to half mass radius ratio, also evolves toward a universal state, which is set by the efficiency of heating on the visible population of stars induced by dynamical interactions in the core of the system. Interactions with dark remnants are dominant over the heating induced by a moderate population of primordial binaries (3-5%), especially under the assumption that most of the neutron stars and black holes are retained in the system. All our models without primordial binaries undergo a deep gravothermal collapse in the radial mass profile. However their projected light distribution can be well fitted by medium concentration King models (with parameter W0 ~ 8), even though there tends to be an excess over the best fit for the innermost points of the surface brightness. This excess is consistent with a shallow cusp in the surface brightness (mu(R) ~ R^{-v} with v ~ 0.4-0.7), like it has been observed for many globular clusters from high-resolution HST imaging. Classification of core-collapsed globular clusters based on their surface brightness profile is likely to fail in systems that have already bounced back to lower concentrations.

The early part of the gravitational wave signal of binary neutron star inspirals can potentially yield robust information on the nuclear equation of state. The influence of a star's internal structure on the waveform is characterized by a single parameter: the tidal deformability lambda, which measures the star's quadrupole deformation in response to the companion's perturbing tidal field. We calculate lambda for a wide range of equations of state and find that the value of lambda spans an order of magnitude for the range of equation of state models considered.
An analysis of the feasibility of discriminating between neutron star equations of state with gravitational wave observations of the early part of the inspiral reveals that the measurement error in lambda increases steeply with the total mass of the binary. Comparing the errors with the expected range of lambda, we find that Advanced LIGO observations of binaries at a distance of 100 Mpc will probe only unusually stiff equations of state, while the proposed Einstein Telescope is likely to see a clean tidal signature.

We present evidence for an identical behavior of the precession of the accretion disk and that of the neutron star (NS) in Her X-1, based on investigating the well established 35 day modulation in Her X-1 in two different ways: 1) following the turn-ons, thought to be due to the precession of the accretion disk, and 2) following the re-appearance of the shape of the pulse profiles, which we assume to be due to precession of the NS. The turn-on evolution and the evolution of the phase-zero values of the precessing NS (as determined from the pulse profiles) track each other very closely. Since the turn-on evolution is strongly correlated with the pulse period evolution, this means that there is also a strong correlation between the spin and the precession of the NS. There is a very strong physical coupling between the NS and the accretion disk, we suggest through physical feedback in the binary system. The apparent long-term stability of the 35 d clock may be due to the interior of the NS, the coupling of which to the observable surface effects is of general importance for the physics of super-dense, highly magnetized material.

We present the long-term monitoring of the High Mass X-ray Binary GX 301-2 performed with the SuperAGILE instrument on-board the AGILE mission. The source was monitored in the 20-60 keV energy band during the first year of the mission from 2007 July 17 to 2008 August 31, covering about one whole orbital period and three more pre-periastron passages for a total net observation time of about 3.7 Ms. The SuperAGILE dataset represents one of the most continuous and complete monitoring at hard X-ray energies of the 41.5 day long binary period available to date. The source behaviour was characterized at all orbital phases in terms of hard X-ray flux, spectral hardness, spin period history, pulsed fraction and pulse shape profile. We also complemented the SuperAGILE observations with the soft X-ray data of the RossiXTE/ASM. Our analysis shows a clear orbital modulation of the spectral hardness, with peaks in correspondence of the pre-periastron flare and near phase 0.25. The hardness peaks we found could be related with the wind-plus-stream accretion model proposed in order to explain the orbital light curve modulation of GX 301-2. Timing analysis of the pulsar spin period shows that the secular trend of the about 680 s pulse period is consistent with the previous observations, although there is evidence of a slight decrease in the spin-down rate. The analysis of the hard X-ray pulsed emission also showed a variable pulse shape profile as a function of the orbital phase, with substructures detected near the passage at the periastron, and a clear modulation of the pulsed fraction, which appears in turn strongly anti-correlated with the source intensity.

We analyse the influence of rotation on shapes of pulse profiles of fast-rotating (millisecond) pulsars. Corotation has two opposing effects: 1) the caustic enhancement of the trailing side (TS) by aberration and retardation (AR), which squeezes the emission into a narrower phase interval; 2) the weakening of the TS caused by the asymmetry of curvature radiation about the dipole axis. Analysis of the radii of curvature of electron trajectories in the inertial observer's frame (IOF) enables these two effects to be considered together. We demonstrate that for dipolar magnetic field lines on the TS there exists a `caustic phase' beyond which no emission can be observed. This phase corresponds to the zero (or minimum) curvature of the IOF trajectories and maximum bunching of the emission. The maximum gradient of polarisation angle (PA) in the S-shaped PA curve is also associated with the curvature minimum and occurs at exactly the same phase. The asymmetry of trajectory curvature with respect to the dipole axis affects the curvature emissivity and the efficiency of pair production, suggesting a minimum at the caustic phase. Emission over a fixed range of altitudes, as expected in millisecond pulsars, leads to broad leading profiles and sharp peaks with a cutoff phase on the TS. We apply our results to the main pulse of the 5 ms pulsar J1012+5307.

The present operation of the ground-based network of gravitational-wave laser interferometers in "enhanced" configuration brings the search for gravitational waves into a regime where detection is highly plausible. The development of techniques that allow us to discriminate a signal of astrophysical origin from instrumental artefacts in the interferometer data and to extract the full range of information are some of the primary goals of the current work. Here we report the details of a Bayesian approach to the problem of inference for gravitational wave observations using a network of instruments, for the computation of the Bayes factor between two hypotheses and the evaluation of the marginalised posterior density functions of the unknown model parameters. The numerical algorithm to tackle the notoriously difficult problem of the evaluation of large multi-dimensional integrals is based on a technique known as Nested Sampling, which provides an attractive alternative to more traditional Markov-chain Monte Carlo (MCMC) methods. We discuss the details of the implementation of this algorithm and its performance against a Gaussian model of the background noise, considering the specific case of the signal produced by the in-spiral of binary systems of black holes and/or neutron stars, although the method is completely general and can be applied to other classes of sources. We also demonstrate the utility of this approach by introducing a new coherence test to distinguish between the presence of a coherent signal of astrophysical origin in the data of multiple instruments and the presence of incoherent accidental artefacts, and the effects on the estimation of the source parameters as a function of the number of instruments in the network.

We investigate non-axisymmetric low frequency modes of a rotating and magnetized neutron star, assuming that the star is threaded by a dipole magnetic field whose strength at the stellar surface, $B_0$, is less than $\sim 10^{12}$G, and whose magnetic axis is aligned with the rotation axis. For modal analysis, we use a neutron star model composed of a fluid ocean, a solid crust, and a fluid core, where we treat the core as being non-magnetic assuming that the magnetic pressure is much smaller than the gas pressure in the core. Here, we are interested in low frequency modes of a rotating and magnetized neutron star whose oscillation frequencies are similar to those of toroidal crust modes of low spherical harmonic degree and low radial order. For a magnetic field of $B_0\sim 10^7$G, we find Alfv\'en waves in the ocean have similar frequencies to the toroidal crust modes, and we find no $r$-modes confined in the ocean for this strength of the field. We calculate the toroidal crustal modes, the interfacial modes peaking at the crust/core interface, and the core inertial modes and $r$-modes, and all these modes are found to be insensitive to the magnetic field of strength $B_0\ltsim10^{12}$G. We find the displacement vector of the core $l^\prime=|m|$ $r$-modes have large amplitudes around the rotation axis at the stellar surface even in the presence of a surface magnetic field $B_0\sim10^{10}$G, where $l^\prime$ and $m$ are the spherical harmonic degree and the azimuthal wave number of the $r$-modes, respectively. We suggest that millisecond X-ray variations of accretion powered X-ray millisecond pulsars can be used as a probe into the core $r$-modes destabilized by gravitational wave radiation.

Very high-energy (VHE; E>100 GeV) gamma-rays have been detected from a wide range of astronomical objects, such as SNRs, pulsars and pulsar wind nebulae, active galactic nuclei, gamma-ray binaries, molecular clouds, and possibly star-forming regions as well. At lower energies, sources detected using the Large Area Telescope (LAT) aboard Fermi provide a rich set of data which can be used to study the behaviour of cosmic accelerators in the GeV to TeV energy bands. In particular, the improved angular resolution in both bands compared to previous instruments significantly reduces source confusion and facilitates the identification of associated counterparts at lower energies. In this paper, a comprehensive search for VHE gamma-ray sources which are spatially coincident with Galactic Fermi/LAT bright sources is performed, and the available GeV to TeV spectra of coincident sources are compared. It is found that bright LAT GeV sources are correlated to TeV sources, in contrast to previous studies using EGRET data. Moreover, a single spectral component seems unable to describe the MeV to TeV spectra of some coincident GeV/TeV sources. It is suggested that gamma-ray pulsars are accompanied by VHE gamma-ray emitting nebulae, a notion that can be tested by VHE observations of these pulsars.

Young isolated radio-quiet neutron stars are still hot enough to be detectable at X-ray and optical wavelengths due to their thermal emission and can hence probe cooling curves. An identification of their birth sites can constrain their age. For that reason we try to identify the parent associations for four of the so-called Magnificent Seven neutron stars for which proper motion and distance estimates are available. We are tracing back in time each neutron star and possible birth association centre to find close encounters. The associated time of the encounter expresses the kinematic age of the neutron star which can be compared to its characteristic spin-down age. Owing to observational uncertainties in the input data, we use Monte-Carlo simulations and evaluate the outcome of our calculations statistically. RX J1856.5-3754 most probably originated from the Upper Scorpius association about 0.3 Myr ago. RX 0720.4-3125 was either born in the young local association TWA about 0.4 Myr ago or in Tr 10 0.5 Myr in the past. Also RX J1605.3+3249 and RBS 1223 seem to come from a nearby young association such as the Sco-Cen complex or the extended Corona-Australis association. For RBS 1223 also a birth in Sct OB2 is possible. We also give constraints on the observables as well as on the radial velocity of the neutron star. Given the birth association, its age and the flight time of the neutron star, we estimate the mass of the progenitor star. Some of the potential supernovae were located very nearby (<100pc) and thus should have contributed to the 10Be and 60Fe material found in the Earth's crust. In addition we reinvestigate the previously suggested neutron star/ runaway pair PSR B1929+10/ zeta Ophiuchi and conclude that it is very likely that both objects were ejected during the same supernova event.

We present recent results of 3D magnetohydrodynamic simulations of neutron stars with small misalignment angles, as regards the features in light curves produced by regular movements of the hot spots during accretion onto the star. In particular, we show that the variation of position of the hot spot created by the infalling matter, as observed in 3D simulations, can produce high frequency Quasi Periodic Oscillations with frequencies associated with the inner zone of the disk. Simulations show that the usual assumption of a fixed hot spot near the polar region is valid only for misalignment angles relatively large. Otherwise, two phenomena challenge the assumption: one is the presence of Rayleigh-Taylor instabilities at the disk-magnetospheric boundary (e.g. Kulkarni & Romanova 2008), which produce tongues of accreting matter that can reach the star almost anywhere between the equator and the polar region; the other one is the motion of the hot spot around the magnetic pole during stable accretion (e.g. Romanova et al. 2004). In this paper we start by showing that both phenomena are capable of producing short-term oscillations in the light curves. We then use Monte Carlo techniques to produce model light curves based on the features of the movements observed, and we show that the main features of kHz QPOs can be reproduced. Finally, we show the behavior of the frequencies of the moving spots as the mass accretion rate changes, and the discovery of a mechanism for the production of double QPO peaks.

[abridged] Three years of optical monitoring of the low-mass X-ray binary (LMXB) 4U 1957+11 is presented. The source was observed in V, R and i-bands using the Faulkes Telescopes North and South. The light curve is dominated by long-term variations which are correlated (at the > 3 sigma level) with the RXTE ASM soft X-ray flux. The variations span one magnitude in all three filters. We find no evidence for periodicities in our light curves, contrary to a previous short-timescale optical study in which the flux varied on a 9.3-hour sinusoidal period by a smaller amplitude. The optical spectral energy distribution is blue and typical of LMXBs in outburst, as is the power law index of the correlation beta = 0.5, where F_{nu,OPT} propto F_X^beta. The discrete cross-correlation function reveals a peak at an X-ray lag of 2 - 14 days, which could be the viscous timescale. However, adopting the least squares method the strongest correlation is at a lag of 0 +/- 4 days, consistent with X-ray reprocessing on the surface of the disc. We therefore constrain the optical lag behind X-ray to be between -14 and +4 days. In addition, we use the optical - X-ray luminosity diagram for LMXBs as a diagnostic tool to constrain the nature of the compact object in 4U 1957+11, since black hole (BH) and neutron star (NS) sources reside in different regions of this diagram. If the system contains a BH (as is the currently favoured hypothesis), its distance must exceed ~ 20 kpc for the optical and X-ray luminosities to be consistent with other soft state BH systems. For distances < 20 kpc, the data lie in a region of the diagram populated only by NS sources. 4U 1957+11 is unique: it is either the only BH LMXB to exist in an apparent persistent soft state, or it is a NS LMXB which behaves like a black hole.

We address the existence of globally neutral neutron star configurations in contrast with the traditional ones constructed by imposing local neutrality. The equilibrium equations describing this system are the Einstein-Maxwell equations which must be solved self-consistently with the general relativistic Thomas-Fermi equation and $\beta$-equilibrium condition. To illustrate the application of this novel approach we adopt the Baym, Bethe, and Pethick (1971) strong interaction model of the baryonic matter in the core and of the white-dwarf-like material of the crust. We illustrate the crucial role played by the boundary conditions satisfied by the leptonic component of the matter at the interface between the core and the crust. For every central density an entire new family of equilibrium configurations exists for selected values of the Fermi energy of the electrons at the surface of the core. Each such configuration fulfills global charge neutrality and is characterized by a non-trivial electrodynamical structure. The electric field extends over a thin shell of thickness $\sim \hbar/(m_e c)$ between the core and the crust and becomes largely overcritical in the limit of decreasing values of the crust mass.

SGR 1550-5418 (previously known as AXP 1E 1547.0-5408) went into three active bursting episodes in 2008 October and in 2009 January and March, emitting hundreds of typical Soft Gamma Repeater (SGR) bursts in soft gamma rays. The second episode was especially intense, and our untriggered burst search on Fermi/GBM data (8-1000 keV) revealed ~450 bursts emitted over 24 hours during the peak of this activity. Using the GBM data, we identified a ~150-s-long enhanced persistent emission during 2009 January 22 that exhibited intriguing timing and spectral properties: (i) clear pulsations up to ~110 keV at the spin period of the neutron star (P ~2.07 s, the fastest of all magnetars), (ii) an additional (to a power-law) blackbody component required for the enhanced emission spectra with kT ~17 keV, (iii) pulsed fraction that is strongly energy dependent and highest in the 50-74 keV energy band. A total isotropic-equivalent energy emitted during this enhanced emission is estimated to be 4.3 x 10^{40} ergs. We conclude that the enhanced emission detected in the persistent flux of SGR 1550-5418 may be a transitional event between an intermediate burst and a giant SGR flare. The estimated area of the blackbody emitting region ~0.044 km^2 (roughly a few x 10^{-5} of the neutron star area) is the smallest "hot spot" ever measured for a magnetar and most likely corresponds to the size of magnetically-confined plasma near the neutron star surface.

We report the detection of very-high-energy (VHE) gamma-ray emission from supernova remnant (SNR) G106.3+2.7. Observations performed in 2008 with the VERITAS atmospheric Cherenkov gamma-ray telescope resolve extended emission overlapping the elongated radio SNR. The 7.3 sigma (pre-trials) detection has a full angular extent of roughly 0.6deg by 0.4deg. Most notably, the centroid of the VHE emission is centered near the peak of the coincident 12CO (J = 1-0) emission, 0.4deg away from the pulsar PSR J2229+6114, situated at the northern end of the SNR. Evidently the current-epoch particles from the pulsar wind nebula are not participating in the gamma-ray production. The VHE energy spectrum measured with VERITAS is well characterized by a power law dN/dE = N_0(E/3 TeV)^{-G} with a differential index of G = 2.29 +/- 0.33stat +/- 0.30sys and a flux of N_0 = (1.15 +/- 0.27stat +/- 0.35sys)x 10^{-13} cm^{-2} s^{-1} TeV^{-1}. The integral flux above 1 TeV corresponds to ~5 percent of the steady Crab Nebula emission above the same energy. We describe the observations and analysis of the object and briefly discuss the implications of the detection in a multiwavelength context.

We intended to measure the radial velocity curve of the supergiant companion to the eclipsing high mass X-ray binary pulsar EXO1722-363 and hence determine the stellar masses of the components.
We used a set of archival K$_{\rm s}$-band infrared spectra of the counterpart to EXO1722-363 obtained using ISAAC on the VLT, and cross-correlated them in order to measure the radial velocity of the star.
The resulting radial velocity curve has a semi-amplitude of $24.5 \pm 5.0$ km s$^{-1}$. When combined with other measured parameters of the system, this yields masses in the range 1.5 $\pm$ 0.4 - 1.6 $\pm$ 0.4 M$_{\odot}$ for the neutron star and 13.6 $\pm$ 1.6 - 15.2 $\pm$ 1.9 M$_{\odot}$ for the B0--1 Ia supergiant companion. These lower and upper limits were obtained under the assumption that the system is viewed edge-on (i = 90$^\circ$) for the lower limit and the supergiant fills its Roche lobe ($\beta = 1$) for the upper limit respectively. The system inclination is constrained to $i>75^{\circ}$ and the Roche lobe-filling factor of the supergiant is $\beta>0.9$. Additionally we were able to further constrain our distance determination to be 7.1 $\le$ d $\le$ 7.9 kpc for EXO1722-363. The X-ray luminosity for this distance range is 4.7 $\times$ 10$^{35}$ $\le$ L$_{\rm X}$ $\le$ 9.2 $\times$ 10$^{36}$ erg s$^{-1}$.
EXO1722-363 therefore becomes the seventh of the ten known eclipsing X-ray binary pulsars for which a dynamical neutron star mass solution has been determined. Additionally EXO1722-363 is the first such system to have a neutron star mass measurement made utilising near-infrared spectroscopy.

Fermi Large Area Telescope (LAT) has recently detected 8 gamma-ray millisecond pulsars (MSPs), providing an unprecedented opportunity to probe the magnetospheres of these low-spin-down pulsars. We performed 3D emission modeling, including various Special Relativistic effects, in the context of pair-starved polar cap (PSPC), slot gap (SG), and outer gap (OG) pulsar models. Most of the light curves are best fit by SG and OG models, surprisingly indicating the presence of narrow accelerating gaps limited by robust pair production. All model fits imply high-altitude emission, and we observe exclusive differentiation of the current gamma-ray MSP population into two sub-classes: light curve shapes and lags across wavebands impose either PSPC or SG / OG-type geometries.

We intended to measure the radial velocity curve of the supergiant companion to the eclipsing high mass X-ray binary pulsar EXO1722-363 and hence determine the stellar masses of the components.
We used a set of archival K$_{\rm s}$-band infrared spectra of the counterpart to EXO1722-363 obtained using ISAAC on the VLT, and cross-correlated them in order to measure the radial velocity of the star.
The resulting radial velocity curve has a semi-amplitude of $24.5 \pm 5.0$ km s$^{-1}$. When combined with other measured parameters of the system, this yields masses in the range 1.5 $\pm$ 0.4 - 1.6 $\pm$ 0.4 M$_{\odot}$ for the neutron star and 13.6 $\pm$ 1.6 - 15.2 $\pm$ 1.9 M$_{\odot}$ for the B0--1 Ia supergiant companion. These lower and upper limits were obtained under the assumption that the system is viewed edge-on (i = 90$^\circ$) for the lower limit and the supergiant fills its Roche lobe ($\beta = 1$) for the upper limit respectively. The system inclination is constrained to $i>75^{\circ}$ and the Roche lobe-filling factor of the supergiant is $\beta>0.9$. Additionally we were able to further constrain our distance determination to be 7.1 $\le$ d $\le$ 7.9 kpc for EXO1722-363. The X-ray luminosity for this distance range is 4.7 $\times$ 10$^{35}$ $\le$ L$_{\rm X}$ $\le$ 9.2 $\times$ 10$^{36}$ erg s$^{-1}$.
EXO1722-363 therefore becomes the seventh of the ten known eclipsing X-ray binary pulsars for which a dynamical neutron star mass solution has been determined. Additionally EXO1722-363 is the first such system to have a neutron star mass measurement made utilising near-infrared spectroscopy.

Fermi Large Area Telescope (LAT) has recently detected 8 gamma-ray millisecond pulsars (MSPs), providing an unprecedented opportunity to probe the magnetospheres of these low-spin-down pulsars. We performed 3D emission modeling, including various Special Relativistic effects, in the context of pair-starved polar cap (PSPC), slot gap (SG), and outer gap (OG) pulsar models. Most of the light curves are best fit by SG and OG models, surprisingly indicating the presence of narrow accelerating gaps limited by robust pair production. All model fits imply high-altitude emission, and we observe exclusive differentiation of the current gamma-ray MSP population into two sub-classes: light curve shapes and lags across wavebands impose either PSPC or SG / OG-type geometries.

We constrain the cosmological density of cosmic string loops using two observational signatures -- gravitational microlensing and the Kaiser-Stebbins effect. Photometry from RXTE and CoRoT space missions and pulsar timing from Parkes Pulsar Timing Array, Arecibo and Green Bank radio telescopes allow us to probe cosmic strings in a wide range of tensions $G\mu/c^2=10^{-16}\div10^{-10}$. We find that pulsar timing data provide the most stringent constraints on the abundance of light strings at the level $\Omega_s \sim 10^{-3}$. Future observational facilities such as the Square Kilometer Array will allow one to improve these constraints by orders of magnitude.

New astrophysical instruments such as skA (square kilometer Array) and IXO (formerly Constellation X) promise the discovery of tens of thousands of new isolated rotating neutron stars (pulsars), neutron stars in low-mass X-ray binaries (LMXBs), anomalous X-ray pulsars (AXPs), and soft gamma repeaters (SGRs). Many of these neutron stars will experience dramatic density changes over their active lifetimes, driven by either stellar spin-up or spin-down, which may trigger phase transitions in their dense baryonic cores. More than that, accretion of matter onto neutron stars in LMXBs is believed to cause pycno-nuclear fusion reactions in the inner crusts of neutron stars. The associated reaction rates may be drastically altered if strange quark matter would be absolutely stable. This paper outlines the investigative steps that need to be performed in order to explore the thermal response of neutron stars to rotationally-driven phase transitions in their cores as well as to nuclear burning scenarios in their crusts. Such research complements the exploration of the phase diagram of dense baryonic matter through particle collider experiments, as performed at RHIC in the USA and as planned at the future Facility for Antiproton and Ion Research (FAIR) in Darmstadt, Germany.

The International Pulsar Timing Array project combines observations of pulsars from both Northern and Southern hemisphere observatories with the main aim of detecting ultra-low frequency (~10^-9 to 10^-8 Hz) gravitational waves. Here we introduce the project, review the methods used to search for gravitational waves emitted from coalescing supermassive binary black-hole systems in the centres of merging galaxies and discuss the status of the project.