  The neutron star low-mass X-ray binary and intermittent millisecond X-ray
pulsar HETE J1900.1-2455 returned to quiescence in late 2015, after a prolonged
accretion outburst of ~10 yr. Using a Chandra observation taken ~180 d into
quiescence we detect the source at a luminosity of ~4.5E31 (D/4.7 kpc)^2 erg/s
(0.5-10 keV). The X-ray spectrum can be described by a neutron star atmosphere
model with a temperature of ~54 eV for an observer at infinity. We perform
thermal evolution calculations based on the 2016 quiescent data and a <98 eV
temperature upper limit inferred from a Swift observation taken during an
unusually brief (<2 weeks) quiescent episode in 2007. We find no evidence in
the present data that the thermal properties of the crust, such as the heating
rate and thermal conductivity, are different than those of non-pulsating
neutron stars. Finding this neutron star so cold after its long outburst
imposes interesting constraints on the heat capacity of the stellar core; these
become even stronger if further cooling were to occur.
