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In quiescent low-mass X-ray binaries (qLMXBs) containing neutron stars, theorigin of the thermal X-ray component may be either release of heat from thecore of the neutron star, or continuing low-level accretion. In general, heatfrom the core should be stable on timescales $<10^4$ years, while continuingaccretion may produce variations on a range of timescales. While some quiescentneutron stars (e.g. Cen X-4, Aql X-1) have shown variations in their thermalcomponents on a range of timescales, several others, particularly those inglobular clusters with no detectable nonthermal hard X-rays (fit with apowerlaw), have shown no measurable variations. Here, we constrain the spectralvariations of 12 low mass X-ray binaries in 3 globular clusters over $\sim10$years. We find no evidence of variations in 10 cases, with limits ontemperature variations below 11% for the 7 qLMXBs without powerlaw components,and limits on variations below 20% for 3 other qLMXBs that do show non-thermalemission. However, in 2 qLMXBs showing powerlaw components in their spectra(NGC 6440 CX 1 & Terzan 5 CX 12) we find marginal evidence for a 10% decline intemperature, suggesting the presence of continuing low-level accretion. Thiswork adds to the evidence that the thermal X-ray component in quiescent neutronstars without powerlaw components can be explained by heat deposited in thecore during outbursts. Finally, we also investigate the correlation betweenhydrogen column density (N$_H$) and optical extinction (A$_V$) using our sampleand current models of interstellar X-ray absorption, finding $N_H ({\rmcm}^{-2}) = (2.81\pm0.13)\times10^{21} A_V$.

Limits on thermal variations in a dozen quiescent neutron stars over a decade