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The quiescent spectrum of neutron star low-mass X-ray binaries typicallyconsists of two components - a thermal component associated with emission fromthe neutron star surface, and a non-thermal power-law component whose origin isnot well understood. Spectral fitting of neutron star atmosphere models to thethermal component is one of the leading methods for measuring the neutron starradius. However, it has been known for years that the X-ray spectra ofquiescent neutron stars vary between observations. While most quiescentvariability is explained through a variable power-law component, the brightestand best-studied object, Cen X-4, requires a change in the thermal componentand such thermal variability could be a problem for measuring neutron starradii. In this paper, we significantly increase the number of sources whosequiescent spectra have been studied for variability. We examine 9 potentialquiescent neutron stars with luminosities <1E34 erg/s over the course ofmultiple Chandra observations of the globular clusters NGC 6440 and Terzan 5and find no strong evidence for variability in the effective temperature in 7of the 9 sources. Two sources show a potential change in temperature, thoughthis depends on the exact model fitted. CX1 in NGC 6440 is equally well fit bya variable thermal component or a variable power law. Therefore, the resultsare inconclusive and we cannot exclude or require thermal variability in thatsource. CX5 in NGC 6440 shows a potential change in temperature, though thisdepends on whether a power-law is included in the spectral fit or not. Thissuggests that thermal variability may not be widespread among quiescent neutronstars with luminosities < 1E34 erg/s, and hence thermal radiation remains apromising means to constraining neutron star radii.

Investigating Variability of Quiescent Neutron Stars in the Globular Clusters NGC 6440 and Terzan 5