Jean-Pierre Lasota

The disc instability model of dwarf-novae and low-mass x-ray binary transients

Abstract The disc instability model which is supposed to describe outbursts of dwarf nova and low-mass X-ray binary transient systems is presented and reviewed in detail. Various deficiencies of the model are pointed out and various remedies and generalizations are presented and discussed.

Thermal equilibria of accretion disks

We show that most of hot, optically thin accretion disk models which ignore advective cooling are not self-consistent. We have found new types of optically thin disk solutions where cooling is dominated by radial advection of heat. These new solutions are thermally and viscously stable.

Radio Loudness of Active Galactic Nuclei: Observational Facts and Theoretical Implications

We investigate how the total radio luminosity of AGN-powered radio sources depends on their accretion luminosity and the central black hole mass. Our studies cover about 7 orders of magnitude in accretion luminosity (expressed in Eddington units, i.e., as Eddington ratios) and the full range of AGN black hole masses. We find that AGNs form two distinct and well-separated sequences on the radio-loudness-Eddington-ratio plane. The upper sequence is formed by radio-selected AGNs, and the lower sequence contains mainly optically selected objects. Whereas an apparent gap between the two sequences may be an artifact of selection effects, the sequences themselves mark the real upper bounds of radio loudness of two distinct populations of AGNs: those hosted respectively by elliptical and disk galaxies. Both sequences show the same dependence of the radio loudness on the Eddington ratio (an increase with decreasing Eddington ratio), which suggests that the normalization of this dependence is determined by the black hole spin. This implies that central black holes in giant elliptical galaxies have (on average) much larger spins than black holes in spiral/disk galaxies. This galaxy-morphology-related radio dichotomy breaks down at high accretion rates where the dominant fraction of luminous quasars hosted by elliptical galaxies is radio quiet. This led to speculations in the literature that formation of powerful jets at high accretion rates is intermittent and related to switches between two disk accretion modes, as directly observed in some black hole X-ray binaries. We argue that such intermittency can be reconciled with the spin paradigm, provided that successful formation of relativistic jets by rotating black holes requires collimation by MHD outflows from accretion disks.

X-RAY IRRADIATION IN LOW-MASS BINARY SYSTEMS

We calculate self-consistent models of X-ray-irradiated accretion discs in close binary systems. We show that a point X-ray source powered by accretion and located in the disc plane cannot modify the disc structure, mainly because of the self-screening by the disc of its outer regions. As observations show that the emission of the outer disc regions in low-mass X-ray binaries is dominated by the reprocessed X-ray flux, accretion discs in these systems must be either warped or irradiated by a source above the disc plane, or both. We analyse the thermal--viscous stability of irradiated accretion discs and derive the stability criteria of such systems. We find that, contrary to the usual assumptions, the critical accretion rate below which a disc is unstable is rather uncertain because the correct formula describing irradiation is not well known.

Black Hole and Neutron Star Transients in Quiescence

We consider the X-ray luminosity difference between neutron star and black hole soft X-ray transients (NS and BH SXTs) in quiescence. The current observational data suggest that BH SXTs are significantly fainter than NS SXTs. The luminosities of quiescent BH SXTs are consistent with the predictions of binary-evolution models for the mass transfer rate if (1) accretion occurs via an advection-dominated accretion flow (ADAF) in these systems and (2) the accreting compact objects have event horizons. The luminosities of quiescent NS SXTs are not consistent with the predictions of ADAF models when combined with binary-evolution models, unless most of the mass accreted in the ADAF is prevented from reaching the neutron star surface. We consider the possibility that mass accretion is reduced in quiescent NS SXTs because of an efficient propeller and develop a model of the propeller effect that accounts for the observed luminosities. We argue that modest winds from ADAFs are consistent with the observations, while strong winds are probably not.

Is the Accretion Flow in NGC 4258 Advection Dominated

The mass of the central black hole in the active galaxy NGC 4258 (M106) has been measured to be

Unified description of accretion flows around black holes

M=3.6\times10^7\Msun

Black Hole Spin and Galactic Morphology

(Miyoshi et al. 1995). The Eddington luminosity corresponding to this mass is

Are Quasar Jets Dominated by Poynting Flux

L_E=4.5\times10^{45}

Advection-dominated Flows around Black Holes and the X-Ray Delay in the Outburst of GRO J1655–40

erg s