Why do X-ray binaries such as GRS-1915 have active and quiet states?
Such objects are surrounded by an accretion disk, that is a disk of gas, the gas being accreted onto the central object. However, at this stage of its life, the accretion disk around the black hole of a binary system like GRS-1915+105 is strongly depleted. Therefore, the accretion rate is not sufficient to support a coutinuous burst. Matter fills the outer disk until a critical density is reached, that triggered an outburst. Mass from the companion star is lost by a Roche lobe overflow mechanism.

References:


X-ray binaries in general: Remillard &amp; McClintock 2006
For the outburst X-ray transients in particular: Dubus et al. 2001