RX J185635-3754 was discovered as an anomalously-bright, extremely soft X-ray source in a ROSAT PSPC image of the R CrA molecular cloud. The ROSAT PSPC spectrum is best fit as a 57eV black body, with nH=1.4x1020 cm-2. The low column suggests that the source is foreground to the 120 pc-distant molecular cloud. At this distance the luminosity of the source is about 5x1031 erg s-1, and the emitting area is about 480 km2. We identified the optical counterpart in a pair of HST/WFPC2 images as 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. The spectral energy distribution is consistent with a Si-ash or Fe atmosphere.
I will review the data, and then present some new evidence, including the soft X-ray spectrum from EUVE, new ground-based photometric detections, the ASCA spectrum, and the possible detection of a proper motion, that RX J185635-3754 is a million year old, cooling neutron star. Future observations of the parallax and proper motion from HST, and the X-ray spectrum from AXAF, will yield a more definitive picture, but for now it appears that RX J185635-3754 affords our best opportunity to observe and characterize the photosphere of a neutron star.
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