A method for distinguishing between transiently accreting neutron stars and black holes, in quiescence

Neutron stars and black holes often reside in binaries where the accretion rate onto the compact object varies by orders of magnitude. These "X-ray transients" are observed both in outburst (when the high accretion rate makes them X-ray bright) and quiescence (when the accretion rate is very low, or potentially zero). In a previous paper, we showed that the quiescent X-ray emission from three neutron star transients (Aq1 X-1, Cen X-4, and 4U 1608-522) were well represented by thermal emission from the neutron star's hydrogen atmosphere and that the emitting area was consistent with the whole surface. Previous blackbody spectral fits (which are not accurate representations of the thermal spectrum) severely underestimated the true emitting area. In this paper, we fit hydrogen atmosphere models to the X-ray data for four neutron stars (the three from the previous paper, plus 4U 2129 + 47) and six black hole candidates (A0620-00, GS 2000 + 25, GS 1124-68, GS 2023 + 33, GRO J1655-40, and GRO J0422 + 32) with masses ≳ 3 M_⊙. While the neutron stars are similar in their intrinsic X-ray spectra (that is, similar effective temperatures and emission area radii ∼ 10 km), the spectra of two black hole candidates are significantly different, and the spectra of the remaining four are consistent with a very large parameter space that includes the neutron stars. The spectral differences between the neutron stars and black hole candidates favors the interpretation that the quiescent neutron star emission is predominantly thermal emission from the neutron star surface. Higher quality data from Chandra, XMM, and ASTRO-E will yield a much better contrast. There are many transients which do not have clear neutron star characteristics (such as type I X-ray bursts or coherent pulsations) and where the mass of the compact object is not constrained. In these cases, it is ambiguous as to whether the compact object is a neutron star or black hole. Our work suggests that an X-ray spectral comparison in quiescence provides an additional means for distinguishing between neutron stars and black holes. The faint X-ray sources in globular clusters - thought to be either cataclysmic variables or quiescent neutron stars - are a class of objects which can be investigated in this manner.