  We report on observations of the unusual neutron-star binary system FIRST
J102347.6+003841 carried out using the XMM-Newton satellite. This system
consists of a radio millisecond pulsar in an 0.198-day orbit with a ~0.2
solar-mass Roche-lobe-filling companion, and appears to have had an accretion
disk in 2001. We observe a hard power-law spectrum (\Gamma = 1.26(4)) with a
possible thermal component, and orbital variability in X-ray flux and possibly
hardness of the X-rays. We also detect probable pulsations at the pulsar period
(single-trial significance ~4.5 sigma from an 11(2)% modulation), which would
make this the first system in which both orbital and rotational X-ray
pulsations are detected. We interpret the emission as a combination of X-rays
from the pulsar itself and from a shock where material overflowing the
companion meets the pulsar wind. The similarity of this X-ray emission to that
seen from other millisecond pulsar binary systems, in particular 47 Tuc W (PSR
J0024-7204W) and PSR J1740-5340, suggests that they may also undergo disk
episodes similar to that seen in J1023 in 2001.
