RX J185635-3754 - an Isolated Neutron Star

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We discovered the X-ray source RX J185635-3754 serendipidously in a ROSAT PSPC image of the Corona Australis dark cloud taken in 1992. The source is bright, at 3.6 counts per second, and has the spectrum of a 57 eV (660,000K) black body. The neutron star is the bright source to the lower right.

The ROSAT HRI image yielded an improved position. The neutron star is the bright object in the center. The image is approximately 50 arcmin across.

Ground-based optical observations revealed no optical counterpart to about V=23. HST images of the region did. These are the central 14X14 arcseconds of the PC camera, in the F300W (wide U) and F606W (wide V) filters. Cosmic rays have been cleaned in the immediate vicinity of the neutron star. The arrow points North; the neutron star is marked. The candidate has V=25.6, and an equivalent U-V color of -1.2. The optical source is located at 18h 56m 35.41s -37deg 54' 35.8" (epoch J2000, equinox 1996.7, referred to the Digital Sky Survey reference frame).

These data have been published in Discovery of a Nearby Isolated Neutron Star. F.M. Walter, S.J. Wolk, & R. Neuhauser. Nature, 379, 233 (1996), and in The Optical Counterpart of the Isolated Neutron Star RX~J185635-3754. F.M. Walter & L.D. Matthews. Nature, 389, 358 (Sept. 25, 1997). There is a typo in the Nature paper: the declination is off by 27 arcsec. The correct position is given above.

The spectrum is remarkably close to thermal. This lets us start to place constraints on the radius, and on the equation of state. The star is foreground to the CrA molecular cloud, at about 130 parsecs. We can fit the X-ray data and the 3000A flux with a Greenstein and Hartke model, with the temperature varying sinusoidally over the surface. This forces R_infinity to be less than 14 km (at 130 pc). If the true distance is less than about 100 parsecs, and the mass is 1.4 solar masses, we can exclude most conventional equations of state (see Lattimer & Prakash 1997). Future observations will tell us the distance. Further details are in the Nature paper.

We got some publicity on this. Hubble Sees a Neutron Star Alone in Space. is the press release from STScI. The HST color composite publicity shot is made by taking the ratio of the F300W to the F606W image. The neutron star (marked by the arrow) is white. Blue points are cosmic rays.

Further HST observations have revealed the distance to be 61 parsecs (200 light years). The neutron star is moving across the sky at a rate of 1/3 of a second of arc each year, from west to east. The star appears to have left the Upper Scorpius Association about 0.9 million years ago, about the same time that a supernove ejected the runaway O star zeta Oph. Perhaps RX J1856 and zeta Oph were members of a binary system, and RX J1856 is the remnant of the star that exploded. This is reported in a paper to appear in the Astrophysical Journal on 1 March 2001 (astro-ph/0009031). The press release is at the STScI web site.

M. van Kerkwijk & S. Kulkarni have observed a bowshock aroung RX J1856. See the ESO press release.

At the 2000 HEAD meeting, J. Pons reported on fits to the spectral energy distribution. The data seem to exclude a Hydrogen or Helium surface, but are consistent with either a black body or a heavy element (Fe or Si-ash) atmosphere. The resultant radius of about 6 km appears small.

Planned or unanalyzed observations include:

Collaborators on this program include P. An, S. Kulkarni, J. Lattimer, J. Lim, L.D. Matthews, J. Pons, M. Prakash, R. Neuhaeuser, and S. Wolk.

Recent Updates

We presented a poster on this star at the 1997 HEAD meeting (4-7 November). More details are provided therein.

We presented an invited talk on this star at the June 1998 AAS meeting in the "Pulsars in the UV and Visible" special session.

I presented an invited talk on Isolated Neutron Stars at the UCSB ITP on 5 October 2000. The viewgraphs are available at the ITP conference site. Audio is available too. An update of this is available here.

I presented an invited talk on the astrophysical evidence for (or against) the existence of quark stars at the Strange Quark Matter 2003 conference on 16 March 2003.

This page written and maintained by F.M. Walter. Last recorded update: 18 March 2003.