Why Can Soft X-Ray Transients Remain in the Low/Hard State during Outburst

In the canonical understanding of transient X-ray sources the accretionduring quiescence occurs via a geometrically thin disk in the outer part andvia an advection-dominated hot coronal flow/ADAF in the inner part. The innerpart important for the radiation yields a hard spectrum. In most sources theluminosity increase during outburst causes a change over to a soft spectrumwhich can be understood as that of a multi-color black body disk reachinginward to the marginally stable orbit. A few transient sources do not displaythis transition to the soft state. We show that this can be understood as dueto relatively low peak luminosities in outburst in systems with short orbitalperiods and therefore less mass accumulated in the then smaller accretion disk.This is in agreement with the observations which show that most likely shortorbital period systems remain in the hard spectral state.Comment: 6 pages, A&A in pres