  Deep Chandra LETG+HRC-S observations of the isolated neutron star candidate
RX J1856.5-3754 have been analysed to search for metallic and resonance
cyclotron spectral features and for pulsation behaviour. As found from earlier
observations, the X-ray spectrum is well-represented by a ~ 60 eV (7e5 K)
blackbody. No unequivocal evidence of spectral line or edge features has been
found, arguing against metal-dominated models. The data contain no evidence for
pulsation and we place a 99% confidence upper limit of 2.7% on the
unaccelerated pulse fraction over a wide frequency range from 1e-4 to 100 Hz.
We argue that the derived interstellar medium neutral hydrogen column density
of 8e19 <= N_H <= 1.1e20 per sq. cm favours the larger distance from two recent
HST parallax analyses, placing RX J1856.5-3754 at ~ 140 pc instead of ~ 60 pc,
and in the outskirts of the R CrA dark molecular cloud. That such a
comparatively rare region of high ISM density is precisely where an isolated
neutron star re-heated by accretion of interstellar matter would be expected is
either entirely coincidental, or current theoretical arguments excluding this
scenario for RX J1856.5-3754 are premature. Taken at face value, the combined
observational evidence -- a lack of spectral and temporal features and an
implied radius at infinity of 3.8-8.2 km that is too small for current neutron
star models -- points to a more compact object, such as allowed for quark
matter equations of state.

Astrophysics