  The quasi-persistent neutron star low-mass X-ray binary MXB 1659-29 went into
quiescence in 2001, and we have followed its quiescent X-ray evolution since.
Observations over the first 4 years showed a rapid drop in flux and temperature
of the neutron star atmosphere, interpreted as cooling of the neutron star
crust which had been heated during the 2.5 year outburst. However, observations
taken approximately 1400 and 2400 days into quiescence were consistent with
each other, suggesting the crust had reached thermal equilibrium with the core.
Here we present a new Chandra observation of MXB 1659-29 taken 11 years into
quiescence and 4 years since the last Chandra observation. This new observation
shows an unexpected factor of ~3 drop in count rate and change in spectral
shape since the last observation, which cannot be explained simply by continued
cooling. Two possible scenarios are that either the neutron star temperature
has remained unchanged and there has been an increase in the column density,
or, alternatively the neutron star temperature has dropped precipitously and
the spectrum is now dominated by a power-law component. The first scenario may
be possible given that MXB 1659-29 is a near edge-on system, and an increase in
column density could be due to build-up of material in, and a thickening of, a
truncated accretion disk during quiescence. But, a large change in disk height
may not be plausible if standard accretion disk theory holds during quiescence.
Alternatively, the disk may be precessing, leading to a higher column density
during this latest observation.
