The soft X-ray source RX J1856.5-3754 is the brightest and nearest of the
so-called isolated neutron stars![[*]](/icons/foot_motif.gif){kind=icon}
(for a review, see Treves et al. 2000). These objects
have X-ray spectra which appear to be entirely thermal, indicating
that the emission arises from the photosphere and that there is little
if any contamination from ill-understood emission processes such as
those occurring in magnetospheres in radio pulsars and accretion flows
in X-ray binaries. Therefore, these objects offer perhaps the best
hope of modeling neutron-star spectra, and inferring the effective
temperature, surface gravity, and gravitational redshift. In
principle, this could lead to unique constraints on the equation of
state of matter in the neutron star interiors (e.g., Lattimer & Prakash 2001).
Given these possible gains, ever since its discovery in
1996 by Walter et al., RX J1856.5-3754 has been the subject of
much observational attention. Walter & Matthews (1997) used the Hubble Space
Telescope (HST) to discover a very faint,
,
optical counterpart; its flux is roughly consistent with seeing the
Rayleigh-Jeans tail of the
spectrum. Further HST observations were used by Walter (2001) to measure the parallax,
while Pons et al. (2001) used HST, ROSAT, ASCA, and EUVE to measure the broad-band spectral energy distribution.
Pons et al. (2001) also presented detailed model atmospheres for a variety of compositions, with which they were able to model the broad-band spectrum satisfactorily. This leads to strong constraints on the temperature. When combined with the parallax, however, the inferred radii are too small for realistic neutron star models. Pons et al. suggest the surface may not have a uniform temperature distribution. If so, the broad-band spectrum can be used to set only weak constraints on the equation of state.
If one could observe spectral features in the spectrum, one might be able to measure the surface gravity and gravitational redshift without much ambiguity (Paerels 1997). In this respect, the first spectrum at good resolution, taken with XMM of RX J0720.4-3125, was disappointing, as no features were seen (Paerels et al. 2001). Recent results on RX J1856.5-3754 appear similarly disappointing, with neither X-ray spectra taken with Chandra (Burwitz et al. 2001), nor ultra-violet spectra taken with HST (Pons et al. 2001) showing strong features.
The use of RX J1856.5-3754 to address the fundamental issues in physics and astrophysics mentioned above would benefit from - or even require - understanding the nature of the source. Walter et al. (1996) suggested it could be a young, cooling neutron star, or a neutron star kept hot by accretion from the interstellar medium. An alternative would be that it is a few million-year old magnetar, as was suggested for RX J0720.4-3125 on the basis of its 8.4-s periodicity (Kulkarni & van Kerkwijk 1998). An indication that RX J1856.5-3754 might be young is its proper motion, which Walter (2001) found to point away from the nearby Sco-Cen association. This has led him to the plausible suggestion that RX J1856.5-3754 was born in this association about a million years ago. For a young neutron star, however, it is hard to understand the lack of X-ray pulsations. Could it be that this neutron star has no significant magnetic field? Almost certainly, the interpretation of the high resolution X-ray spectra of this source will depend on knowing the composition of the atmosphere, the strength of the magnetic field, and the level of non-thermal emission.
In an effort to understand the nature of this important but enigmatic source, we have undertaken a series of observations. In this paper, we report on the first optical spectrum of RX J1856.5-3754 and on accurate optical-UV photometry. In our spectra, we find evidence for a nebula around RX J1856.5-3754. Those observations and the interpretation of the nebula will be the subject of the next paper (van Kerkwijk & Kulkarni 2001).
The organisation of this paper is as follows. We describe our observations in Sects. 2 and 3, and the reduction in Sect. 4. We pay particular attention to accurate calibration, since some of our results turn out to be discrepant with previously published ground-based results. We also re-analyse the HST imaging, taking particular care to correct for systematic effects affecting faint stars. We discuss the results in Sects. 5 and 6.
Copyright ESO 2001