Thu 1/13/00 
Fred Walter 
SUNY

        The Isolated Neutron Star RX J185635-3754:
    Everything you ever wanted to know about neutron stars?

RX J185635-3754 is a bright, extremely soft (57 eV), X-ray source
foreground to the R CrA molecular cloud. At the distance of the cloud,
the blackbody luminosity is 5X10^31 erg/s, and the emitting area is
480 km^2. The optical counterpart is a faint (V=25.6) blue (U-V= -1.4)
object. The U and V fluxes are close to the extrapolation of the X-ray
black body to long wavelengths. The small size and high temperature
of the object suggest that it is an isolated neutron star. There is no
evidence for a hard non-thermal tail, radio emission, or variability. The
emitting area suggests emission from most, if not all, of the surface,
and not just from a polar cap.

Since then, we have obtained further X-ray observations, using the EUVE,
ASCA, and SAX. The spectral energy distribution is consistent with a
Si-ash or Fe atmosphere. Second-epoch HST observations reveal a large
proper motion (0.3"/yr). The evidence suggests that RX J185635-3754 is
a 2 million year old, cooling neutron star without a strong magnetic
field, and that it originated in the Upper Cen-Lupus OB association. I
will conclude with a discussion of what future observations with AXAF,
XMM, and the HST promise to reveal.