X-ray variability during the quiescent state of the neutron star X-ray transient in the globular cluster NGC 6440

The globular cluster NGC 6440 is known to harbor a bright neutron star X-ray transient. We observed the globular cluster with Chandra on two occasions when the bright transient was in its quiescent state, in 2000 July and 2003 June ( both observations were made nearly 2 yr after the end of their preceding outbursts). The quiescent spectrum during the first observation is well represented by a two-component model ( a neutron star atmosphere model plus a power-law component that dominates at energies above 2 keV). During the second observation ( which was roughly of equal duration to the first observation) we found that the power-law component could no longer be detected. Our spectral fits indicate that the effective temperature of the neutron star surface was consistent between the two observations. We conclude that the effect of the change in the power-law component caused the 0.5 - 10 keV flux to be a factor of similar to 2 lower during the second observation compared to the first observation. We discuss plausible explanations for the variations, including variable residual accretion onto the neutron star magnetosphere or some variation in the interaction of the pulsar wind with the matter still outflowing from the companion star.