next previous
Up: Infrared spectroscopy of the


1 Introduction

The supersoft X-ray sources (SSS) are a class of luminous point sources $L_{\rm X} \sim$ 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 ( $\dot{M} \sim 10^{-7}~M_{\odot}$/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 ( $4.0287822 \pm 0.0000026$ 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 $\pm$ 10$^{\circ }$(Matsumoto & Mennickent 2000). The optical emission lines show a large velocity amplitude, implying masses of 1-2 $M_{\odot}$ for the donor star, which must be a giant in order to fill its Roche lobe, and 0.5-1.7 $M_{\odot}$ 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.


next previous
Up: Infrared spectroscopy of the

Copyright ESO 2003