Spectra of thermally unstable slim discs

Thermal instability driven by radiation pressure might be relevant for intr insically bright accreting sources. The most promising candidate where this instability seems to be at work is one of the two known galactic superlumi nal sources, GRS 1915+105 (Belloni et al. 1997). In spite of being of relev ance. this scenario has not yet been confirmed by proper time-dependent mod elling. Non-linear time-dependent calculations performed by Szuszkiewicz an d Miller (1998) show that thermally unstable discs undergo limit-cycle beha viour with successive evacuation and refilling of the central parts of the disc. This evolution is very similar to the one proposed by Belloni et al. (1997) in their phenomenological model. Further investigations are needed t o confirm the thermal instability being operational in this source. First o f all the spectra emitted from the disc during its evolution should be calc ulated and compared with observations. Here such spectra are computed assum ing local blackbody emission from the best studied transonic disc model.