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-raytransient. We observed the globular cluster with Chandra on two occasions whenthe bright transient was in its quiescent state in July 2000 and June 2003(both observations were made nearly 2 years after the end of their precedingoutbursts). The quiescent spectrum during the first observation is wellrepresented by a two component model (a neutron-star atmosphere model plus apower-law component which dominates at energies above 2 keV). During the secondobservation (which was roughly of equal duration to the first observation) wefound that the power-law component could no longer be detected. Our spectralfits indicate that the effective temperature of the neutron-star surface wasconsistent between the two observations. We conclude that the effect of thechange in power-law component caused the 0.5-10 keV flux to be a factor of ~2lower during the second observation compared to the first observation. Wediscuss plausible explanations for the variations, including variable residualaccretion onto the neutron star magnetosphere or some variation in theinteraction of the pulsar wind with the matter still outflowing from thecompanion star.Comment: 18 pages, 3 color figs, 1 b&w figures, 3 tables; discussion expanded; accepted for publication in Ap