Association of recurrent soft x-ray transients with x-ray burst sources

Hakucho observations have revealed that the soft transients, 2S1608-522, 2S1715-321, CenX-4 and Aq1X-1, all generate X-ray burst. This result makes it convincing that these sources involve neutron stars. Furthermore, the optical counterpart of Aq1X-1, when X-ray quiescent, is identified to be a ~KO dwarf, thus providing evidence that Aq1X-1 is a binary of a late dwarf and a neutron star.

Soft transients in the flare state are similar in X-ray and optical properties to so-called bulge sources. This and the observed high association of soft transients and bulge sources with bursters seem to indicate similarities of both to Aq1X-1. Flare-up of the soft transients is considered to be caused by episodic mass accretion.