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 t ransients " are observed both in outburst (when the high accretion rate mak es 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-r ay emission from three neutron star transients (Aql X-l, Cen X-4, and 4U 16 08-522) were well represented by thermal emission from the neutron star's h ydrogen atmosphere and that the emitting area was consistent with the whole surface. Previous blackbody spectral fits (which are not accurate represen tations of the thermal spectrum) severely underestimated the true emitting area. In this paper, we fit hydrogen atmosphere models to the X-ray data fo r 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 greater than or simila r to 3 M-circle dot. While the neutron stars are similar in their intrinsic X-ray spectra (that is, similar effective temperatures and emission area r adii similar to 10 km), the spectra of two black hole candidates are signif icantly different, and the spectra of the remaining four are consistent wit h a very large parameter space that includes the neutron stars. The spectra l differences between the neutron stars and black hole candidates favors th e interpretation that the quiescent neutron star emission is predominantly thermal emission from the neutron star surface. Higher quality data from Ch andra, XMM, and ASTRO-E will yield a much better contrast. There are many t ransients which do not have clear neutron star characteristics (such as typ e 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 th e 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 d istinguishing between neutron stars and black holes. The faint X-ray source s in globular clusters-thought to be either cataclysmic variables or quiesc ent neutron stars-are a class of objects which can be investigated in this manner.