CONSTRAINTS ON THE ILLUMINATION MODEL FOR SOFT-X-RAY TRANSIENTS

We study the evolution of secondaries under the effect of illumination by X-ray fluxes comparable to their intrinsic flux. We show that the time scale for a radius increase by a fraction of a scale height is compatible with the illumination model for soft X-ray transients only if the companion star is a subgiant or a main sequence star with a period longer than about 9 hr. Similarly, we show that these conditions are also required in order to avoid a second outburst immediately following the first one. In short period systems, the illumination model may apply if there are intrinsic fluctuations of the secondary radius in the vicinity of the L1 region, due for example to magnetic activity; this would have observable consequences on the regularity of outbursts.