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