RX J185635-3754: The Spectrum

We have detected the object in

The EUVE SW spectrum was obtained in June and July 1997. It represents about 480,000 seconds of good data. This spectrum falls about a factor of 2 below the deconvolved ROSAT PSPC flux at these wavelengths. The ROSAT PSPC effective area is not well known shortward of about 0.2 keV. Previous observations of soft sources (e.g., Napiwotski et al. A&A 278, 478) suggest that the PSPC effective area is underestimated by nearly a factor of two in this region, which would bring the two observations into agreement. This, of course, means that the temperature and column inferred from the PSPC spectrum alone are incorrect.

The spectral energy distribution is approximately thermal from 10 to 6600 Angstroms (5 orders of magnitude below the peak in F<em>lambda</em>). The plot at left shows the deconvolved ROSAT PSPC spectrum (the thin green line is the 57 eV black body folded through the PSPC response); the EUVE spectrum, as well as the fluxes from the HST/WFPC2 and the V detection and the upper limit at R, from the ground.

No pulsations have yet been detected in X-rays (precise limit not available).

How do we know that this must be a neutron star? The X-ray-to-visual flux ratio fX/fV is about 70,000, which excludes most other known classes of objects. Furthermore, the small X-ray column density strongly suggests that the star is foreground to the 130-parsec-distant R CrA molecular cloud.

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