The supersoft X-ray sources (SSS) are
a class of luminous point sources
10
36-1037 ergs s-1, emitting mostly
in the energy range of 20-60 eV.
The most promising model for binary SSS invokes a white dwarf
undergoing steady nuclear burning
as a result of very rapid accretion
(
/yr) from a Roche lobe-filling
companion star (e.g. van den Heuvel et al. 1992).
Whereas the bulk of the energy should be released near the
white dwarf into the X-ray region,
the optical and infrared spectra should be dominated by the other
components of the system, namely the accretion disk and eventually the
secondary star.
Most persistent SSS have been found in the Magellanic Clouds,
and only few of them in the galaxy, since presumably
their soft X-rays are easily absorbed by the interstellar
hydrogen near the galactic plane.
MR Vel (RX J0925.7-4758) is one of the few known galactic SSS
and it has been relatively well studied in X-rays
and optical wavelengths.
The orbital period is unusually
long for an SSS (
d,
Schmidtke & Cowley 2001). During the orbital cycle,
the star shows cyclic
variations in the V magnitude between 17.1 and 17.3,
whereas the B-V and V-R colours
show no change (Schmidtke et al. 2000).
The great strength of the
interstellar absorption lines seen in
the optical region are consistent with the
position of the source behind the Vela Sheet molecular cloud.
The inclination has been constrained to i= 55
10
(Matsumoto & Mennickent 2000).
The optical emission lines show a large velocity amplitude,
implying masses of 1-2
for the donor star,
which must be a giant in order to fill its Roche lobe, and
0.5-1.7
for the compact star (Bearda et al. 2002).
The X-ray spectrum is rather complex,
and cannot be explained by the usual models of photo-ionized plasma
or stellar atmospheres combined with thermal plasma
in collisional ionization equilibrium (Bearda et al. 2002; Motch et al. 2002).
In spite of the recent progress made to understand the unusual properties of MR Vel, the infrared region has not yet been explored. This is also true for most SSS, which have been historically studied in X-rays and the optical region. Infrared observations of SSS could, in principle, be combined with X-rays and optical data, in order to analyse the overall spectral energy distribution, aiming to constrain models for the emitting region and get insights on stellar parameters, energetics and distances. In this paper we provide the first description of the infrared spectrum of MR Vel, and combine this information with optical data to model the optical-IR spectral energy distribution.
Copyright ESO 2003