Why soft X-ray transients can remain in the low/hard state during outburst

Abstract

In the canonical understanding of transient X-ray sources the accretion during quiescence occurs via a geometrically thin disk in the outer part and via an advection-dominated hot coronal flow/ADAF in the inner part. The inner part important for the radiation yields a hard spectrum. In most sources the luminosity increase during outburst causes a change over to a soft spectrum which can be understood as that of a multi-color black body disk reaching inward to the marginally stable orbit. A few transient sources do not display this transition to the soft state. We show that this can be understood as due to relatively low peak luminosities in outburst in systems with short orbital periods and therefore less mass accumulated in the then smaller accretion disk. This is in agreement with the observations which show that most likely short orbital period systems remain in the hard spectral state.