Program
|
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.
[PDF]Go back to the program page
|