Bibcode
Wijnands, R.; Degenaar, N.; Armas Padilla, M.; Altamirano, D.; Cavecchi, Y.; Linares, M.; Bahramian, A.; Heinke, C. O.
Referencia bibliográfica
Monthly Notices of the Royal Astronomical Society, Volume 454, Issue 2, p.1371-1386
Fecha de publicación:
12
2015
Número de citas
78
Número de citas referidas
76
Descripción
We search the literature for reports on the spectral properties of
neutron star low-mass X-ray binaries when they have accretion
luminosities between 1034 and 1036 erg
s-1, corresponding to roughly 0.01-1 per cent of the
Eddington accretion rate for a neutron star. We found that in this
luminosity range the photon index (obtained from fitting a simple
absorbed power law in the 0.5-10 keV range) increases with decreasing
0.5-10 keV X-ray luminosity (i.e. the spectrum softens). Such behaviour
has been reported before for individual sources, but here we demonstrate
that very likely most (if not all) neutron star systems behave in a
similar manner and possibly even follow a universal relation. When
comparing the neutron star systems with black hole systems, it is clear
that most black hole binaries have significantly harder spectra at
luminosities of 1034-1035 erg s-1.
Despite a limited number of data points, there are indications that
these spectral differences also extend to the
1035-1036 erg s-1 range, but above a
luminosity of 1035 erg s-1 the separation between
neutron star and black hole systems is not as clear as below. In
addition, the black hole spectra only become softer below luminosities
of 1034 erg s-1 compared to 1036 erg
s-1 for the neutron star systems. This observed difference
between the neutron star binaries and black hole ones suggests that the
spectral properties (between 0.5 and 10 keV) at
1034-1035 erg s-1 can be used to
tentatively determine the nature of the accretor in unclassified X-ray
binaries. More observations in this luminosity range are needed to
determine how robust this diagnostic tool is and whether or not there
are (many) systems that do not follow the general trend. We discuss our
results in the context of properties of the accretion flow at low
luminosities and we suggest that the observed spectral differences
likely arise from the neutron star surface becoming dominantly visible
in the X-ray spectra. We also suggest that both the thermal component
and the non-thermal component might be caused by low-level accretion on
to the neutron star surface for luminosities below a few times
1034 erg s-1.
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