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The extremely faint quiescent NS SXT 1H 1905+00: constrainst on the NS EoS

Peter G Jonker

SRON Utrecht / CfA Cambridge

Abstract:

Observations of black hole and neutron star Soft X-ray Transients (SXTs) with Chandra and XMM-Newton turned out to have a profound impact on two important areas of high energy astrophysics. First of all, comparing the quiescent luminosity of neutron star SXTs with that of black hole SXTs it was found that black hole (BH) SXTs are systematically fainter in quiescence than neutron stars (e.g. Narayan, Garcia, & McClintock 1997, Garcia et al. 2001). This has been interpreted as evidence for advection of energy across a BH event horizon. Despite many objections to this interpretation, alternative explanations for the difference in quiescent luminosity, and neutron stars which turned out to be fainter than initially found to be rule, none of the neutron star SXTs is as faint as the BH SXT A 0620-00 in quiescence. Secondly, in observations of neutron star SXTs in quiescence which allow for a spectral study, the spectrum was found to be well-fit by a neutron star atmosphere model (NSA) sometimes supplemented with a power-law component. Well established theories about the time averaged mass accretion rates in neutron star SXTs, the pycnonuclear reactions taking place in the neutron star crust combined with neutron star cooling theory predictions, yield a neutron star core temperature. This hot neutron star core, moderated by the neutron star atmosphere, is thought to be observed during the quiescent phase of neutron star SXTs. In theory, a NSA-fit provides means to measure the mass and radius of the neutron star and hence constrain the equation of state (EoS) of matter at supranuclear densities. The description of the relations between pressure and density of matter (the EoS) under the extreme conditions encountered in neutron stars is one of the ultimate goals of the study of neutron stars. We recently observed the neutron star SXT 1H1905+000 in quiescence with ACIS-S. However, the source was not detected even though the distance and interstellar extinction are well known. This means that the source (thermal) luminosity in the 0.5-10 keV band is lower than 10³¹ erg s^-1. From this and from the fact that it is known from binary evolution theory that the time averaged mass accretion rate cannot be much less than 10^-12 Msun per year, we conclude that the neutron star must be so massive that only EoSs with a nucleonic core can exist.

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