Publication Date:
2015-08-06
Description:
In quiescent low-mass X-ray binaries (qLMXBs) containing neutron stars, the origin of the thermal X-ray component may be either release of heat from the core of the neutron star, or continuing low-level accretion. In general, heat from the core should be stable on time-scales 〈10 4 yr, while continuing accretion may produce variations on a range of time-scales. While some quiescent neutron stars (e.g. Cen X-4, Aql X-1) have shown variations in their thermal components on a range of time-scales, several others, particularly those in globular clusters with no detectable non-thermal hard X-rays (fit with a power law), have shown no measurable variations. Here, we constrain the spectral variations of 12 low-mass X-ray binaries in three globular clusters over ~10 years. We find no evidence of variations in 10 cases, with limits on temperature variations below 11 per cent for the seven qLMXBs without power-law components, and limits on variations below 20 per cent for three other qLMXBs that do show non-thermal emission. However, in two qLMXBs showing power-law components in their spectra (NGC 6440 CX 1 and Terzan 5 CX 12) we find marginal evidence for a 10 per cent decline in temperature, suggesting the presence of continuing low-level accretion. This work adds to the evidence that the thermal X-ray component in quiescent neutron stars without power-law components can be explained by heat deposited in the core during outbursts. Finally, we also investigate the correlation between hydrogen column density ( N H ) and optical extinction ( A V ) using our sample and current models of interstellar X-ray absorption, finding N H (cm –2 ) = (2.81 ± 0.13) x 10 21 A V .
Print ISSN:
0035-8711
Electronic ISSN:
1365-2966
Topics:
Physics
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