Disc outbursts in various types of binary systems

I discuss some aspects of the Disc Instability Model applied to outbursts in various types of binary systems. I lament the general lack of interest in the subject.Comment: The Golden Age of Cataclysmic Variables and Related Objects, F. Giovannelli & L. Sabau-Graziati (eds.), Mem. SAIt. (2012) 83 N.2 (in press

X-rays from Quiescent Low-Mass X-ray Binary Transients

I argue that it is very unlikely that X-rays from quiescent black-hole low-mass X-ray binary transients are emitted by coronae of companion stars. I show that in a simple model in which these X-rays are emitted by an ADAF filling the inner part of an unsteady, dwarf-nova type disc, the X-ray luminosity is correlated with the orbital period. I predict what values of X-ray luminosities from black-hole transient systems should be observed by Chandra and XMM-Newton.Comment: 9 pages, 2 figures, Astronomy and Astrophysics, in press. Slightly revised versio

Black hole accretion discs

This is an introduction to models of accretion discs around black holes. After a presentation of the non-relativistic equations describing the structure and evolution of geometrically thin accretion discs we discuss their steady-state solutions and compare them to observation. Next we describe in detail the thermal-viscous disc instability model and its application to dwarf novae for which it was designed and its X-ray irradiated-disc version which explains the soft X--ray transients, i.e. outbursting black-hole low-mass X-ray binaries. We then turn to the role of advection in accretion flows onto black holes illustrating its action and importance with a toy model describing both ADAFs and slim discs. We conclude with a presentation of the general-relativistic formalism describing accretion discs in the Kerr space-time.Comment: 41 pages, 11 figures, Chapter 1 in: "Astrophysics of Black Holes - From fundamental aspects to latest developments", Ed. Cosimo Bambi, Springer: Astrophysics and Space Science Library 440 (2016). Minor errors correcte

Why Do Black--Hole X-ray Binaries Tend to Be Transient?

Black hole X-ray binaries are transient probably because their discs are subject to the same thermal-viscous instability which is present in dwarf nova binary systems. I discuss applications of the dwarf-nova instability model to transient, low-mass, X-ray binary systems. When disc truncation and X-ray irradiation are taken into account this model is capable of reproducing the basic properties of X-ray binary outbursts (Dubus et al. 2000).Comment: 6 pages, 2 figures; Proceedings of the ESO Workshop on "Black Holes in Binaries and Galactic Nuclei: Diagnostics, Demography and Formation", in honour of Prof. R. Giacconi, in pres

Anomalous Z Cam stars: a response to mass-transfer outbursts

Recent observations of two unusual Z Cam systems, V513 Cas and IW And have shown light curves that seem to contradict the disc-instability model for dwarf novae: outbursts are appearing during standstills of the system when according to the model, the disc is supposed to be in a hot quasi-equilibrium state. We investigate what additional physical processes need to be included in the model to reconcile it with observations of such anomalous Z Cam systems. We used our code for modeling thermal-viscous outbursts of the accretion discs and determined what types of mass-transfer variations reproduce the observed light curves. Outbursts of mass transfer (with a duration of a few days, with a short rise time and an exponential decay) from the stellar companion will account for the observed properties of V513 Cas and IW And, provided they are followed by a short but significant mass-transfer dip. The total mass involved in outbursts is of the order of 10$^{23}$g. We studied the possible origins of these mass transfer outbursts and showed that they most probably result from a giant flare near the secondary star surface, possibly due to the absence of star spots in the $L_1$ region.Comment: 5 pages, 2 figures. Misprints corrected, in particular "inside-out" should have read "outside-in

The disappearance and reformation of the accretion disc during a low state of FO Aquarii

FO Aquarii, an asynchronous magnetic cataclysmic variable (intermediate polar) went into a low-state in 2016, from which it slowly and steadily recovered without showing dwarf nova outbursts. This requires explanation since in a low-state, the mass-transfer rate is in principle too low for the disc to be fully ionized and the disc should be subject to the standard thermal and viscous instability observed in dwarf novae. We investigate the conditions under which an accretion disc in an intermediate polar could exhibit a luminosity drop of 2 magnitudes in the optical band without showing outbursts. We use our numerical code for the time evolution of accretion discs, including other light sources from the system (primary, secondary, hot spot). We show that although it is marginally possible for the accretion disc in the low-state to stay on the hot stable branch, the required mass-transfer rate in the normal state would then have to be extremely high, of the order of 10$^{19}$ gs$^{-1}$ or even larger. This would make the system so intrinsically bright that its distance should be much larger than allowed by all estimates. We show that observations of FO Aqr are well accounted for by the same mechanism that we have suggested as explaining the absence of outbursts during low states of VY Scl stars: during the decay, the magnetospheric radius exceeds the circularization radius, so that the disc disappears before it enters the instability strip for dwarf nova outbursts. Our results are unaffected, and even reinforced, if accretion proceeds both via the accretion disc and directly via the stream during some intermediate stages; the detailed process through which the disc disappears still needs investigations.Comment: 6 pages, 5 figures. Accepted for publication in A&