Hysteresis in the transition between hard and soft spectral states of low-mass X-ray binaries

Details Hysteresis in the transition between hard and soft spectral states of low-mass X-ray binaries Hysteresis in the transition between hard and soft spectral states of low-mass X-ray binaries

Aug 2005

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005aspc..330..349m&link_type=abstract

The Astrophysics of Cataclysmic Variables and Related Objects, Proceedings of ASP Conference Vol. 330. Edited by J.-M. Hameury a

Astronomy and Astrophysics

Astrophysics

Scientific paper

An interesting feature found in X-ray binary observations is the change between a soft and a hard spectrum. Such transitions were observed for both neutron star and black hole systems. The soft and hard spectral state are understood as originating from accretion via a disk which reaches inward to the compact object, in the first case, or an advection-dominated hot coronal flow/ADAF in the inner part and accretion via a disk only in the outer regions, in the second case.
In X-ray transients the change over from the hard to the soft spectrum occurs during the luminosity rise in outburst when the ADAF region disappears due to the increased accretion rate. The change back is connected with the decreasing accretion rate in outburst decline. In connection with these spectral transitions a special feature is observed, a hysteresis in the luminosity at the transition: the hard-soft transition is found at a luminosity higher by a factor of 3 to 5 than that at the soft-hard transition.
The change over from disk accretion to a hot coronal flow can be investigated by studying the interaction of a hot corona with the cool disk underneath. The location of the transition follows from the balance between mass flow rate in the thin disk and the rate at which gas evaporates into the corona. We here explain the hysteresis as caused by a different amount of Compton cooling of the corona by radiation from the inner region, very little for the hard photons of the hot inner flow at the change from hard to soft state, and more for the soft photons from the inner disk, in case of the change from soft to hard state.

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