We identify two candidate magnetars in archival X-ray observations of HESS detected shell-type SNRs. X-ray point sources in CTB 37B coincident with HESS J1713-381 and in G353.6-0.7 coincident with HESS J1731-347 both have AXP-like spectra, much softer than those of ordinary, rotation powered pulsars, and no optical/IR counterparts. The spectrum of CXOU J171405.7-381031 in CTB 37B has a hard excess above 6 keV, which may be similar to such components seen in some AXPs. A new Chandra observation of this object reveals a highly significant pulsed signal at P = 3.82 s with pulsed fraction f_p = 0.31. Analysis of an XMM-Newton observation of the second candidate, XMMU J173203.3-344518 in G353.6-0.7, yields only marginal evidence for a 1 s period. If it is not a magnetar, then it could be a weakly magnetized central compact object (CCO). Considering that these HESS sources previously attributed to the SNR shells are possibly centrally peaked, we hypothesize that their pulsars may contribute to diffuse TeV emission. These identifications potentially double the number of magnetar/SNR associations in the Galaxy, and can be used to investigate the energetics and asymmetries of the supernovae that give rise to magnetars.
HD 15137 is an intriguing runaway O-type binary system that offers a rare opportunity to explore the mechanism by which it was ejected from the open cluster of its birth. Here we present recent blue optical spectra of HD 15137 and derive a new orbital solution for the spectroscopic binary and physical parameters of the O star primary. We also present the first XMM-Newton observations of the system. Fits of the EPIC spectra indicate soft, thermal X-ray emission consistent with an isolated O star. Upper limits on the undetected hard X-ray emission place limits on the emission from a proposed compact companion in the system, and we rule out a quiescent neutron star in the propellor regime or a weakly accreting neutron star. An unevolved secondary companion is also not detected in our optical spectra of the binary, and it is difficult to conclude that a gravitational interaction could have ejected this runaway binary with a low mass optical star. HD 15137 may contain an elusive neutron star in the ejector regime or a quiescent black hole with conditions unfavorable for accretion at the time of our observations.
The population of stellar black holes (SBHs) in the Galaxy and galaxies generally is poorly known in both number and distribution. SBHs are the fossil record of the massive stars in galaxy evolution and may have produced some (if not all) of the intermediate mass (\gsim100\Msun) black holes (IMBHs) and, in turn, the central supermassive black holes (SMBHs) in galactic nuclei. For the first time, a Galaxy-wide census of accreting black holes, and their more readily recognizable tracer population, accreting neutron stars (NSs), could be measured with a wide-field hard X-ray imaging survey and soft X-ray and optical/IR prompt followup -- as proposed for the EXIST mission.
We discuss possible radio bursts which can be generated during binary neutron stars mergers associated with short gamma-ray bursts. Low-frequency radio band appear to be advantageous due to the time delay of a radio signal propagating in the intergalactic medium, which makes it possible to use short gamma-ray burst alerts to search for specific regions on the sky by low-frequency radio instruments, especially LOFAR. The LOFAR sensitivity will allow significant detection of such bursts up to redshifts z~0.3 with a rate of ~25 events per year.
We consider the motion of charged particles in the vacuum magnetospheres of rotating neutron stars with a strong surface magnetic field, B>10^(12) G. The electrons and positrons falling into the magnetosphere or produced in it are shown to be captured by the force-free surface EB=0. Using the Dirac-Lorentz equation, we investigate the dynamics of particle capture and subsequent motion near the force-free surface. The particle energy far from the force-free surface has been found to be determined by the balance between the power of the forces of an accelerating electric field and the intensity of curvature radiation. When captured, the particles perform adiabatic oscillations along the magnetic field lines and simultaneously move along the force-free surface. We have found the oscillation parameters and trajectories of the captured particles. We have calculated the characteristic capture times and energy losses of the particles through the emission of both bremsstrahlung and curvature photons by them. The capture of particles is shown to lead to a monotonic increase in the thickness of the layer of charged plasma accumulating near the force-free surface. The time it takes for a vacuum magnetosphere to be filled with plasma has been estimated.
The gamma-ray survey of the sky by the Fermi Gamma-ray Space Telescope offers both opportunities and challenges for multiwavelength and multi-messenger studies. Gamma-ray bursts, pulsars, binary sources, flaring Active Galactic Nuclei, and Galactic transient sources are all phenomena that can best be studied with a wide variety of instruments simultaneously or contemporaneously. Identification of newly-discovered gamma-ray sources is largely a multiwavelength effort. From the gamma-ray side, a principal challenge is the latency from the time of an astrophysical event to the recognition of this event in the data. Obtaining quick and complete multiwavelength coverage of gamma-ray sources can be difficult both in terms of logistics and in terms of generating scientific interest. The Fermi LAT team continues to welcome cooperative efforts aimed at maximizing the scientific return from the mission through multiwavelength studies.
Geminga is the second brightest persistent source in the GeV gamma-ray sky. Discovered in 1975 by SAS-2 mission, it was identified as a pulsar only in the 90s, when ROSAT detected the 237 ms X-ray periodicity, that was later also found by EGRET in gamma rays. Even though Geminga has been one of the most intensively studied isolated neutron star during the last 30 years, its interest remains intact especially at gamma-ray energies, where instruments like the Large Area Telescope (LAT) aboard the Fermi mission will provide an unprecedented view of this pulsars. We will report on the preliminary results obtained on the analysis of the first year of observations. We have been able to do precise timing of Geminga using solely gamma rays, producing a timing solution and allowing a deep study of the evolution of the light curve with energy. We have also measured and studied the high-energy cutoff in the phase-averaged spectrum and produced a detailed study of the spectral evolution with phase.
Following an extremely interesting idea \cite{R1}, published long ago, the work function at the outer crust region of a strongly magnetized neutron star is obtained using relativistic version of Thomas-Fermi type model. In the present scenario, the work function becomes anisotropic; the longitudinal part is an increasing function of magnetic field strength, whereas the transverse part diverges. An approximate estimate of the electron density in the magnetosphere due to field emission and photo emission current, from the polar cap region are obtained.
Here we briefly report on first results of self-consistent simulation of non-stationary electron-positron cascades in the polar cap of pulsar using specially developed hybrid PIC/Monte-Carlo numerical code. We consider the case of Ruderman-Sutherland cascade -- when particles cannot be extracted from the surface of the neutrons star.
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