arXiv subjects

Kristen Menou

Publications and source records attributed to Kristen Menou.

119 records · Page 7Linked to original sources

arXiv2000

Viscosity Mechanisms in Accretion Disks

The self-sustained turbulence which develops in magnetized accretion disks is suppressed in the weakly-ionized, quiescent disks of close binary stars. Because accretion still proceeds during quiescence, another viscosity mechanism operates in these systems. An anticorrelation of the recurrence times of SU UMa dwarf novae with their mass ratio supports spiral waves or shock-waves tidally induced by the companion star as the main process responsible for accretion in the quiescent disks. Other weakly-ionized gaseous disks in systems lacking a massive companion have to rely on yet another transport mechanism or they could be essentially passive.

astro-ph↗

arXiv2000

Advection-Dominated Accretion Onto Weakly-Magnetized White Dwarfs

The boundary layers of weakly-magnetized white dwarfs (WDs) accreting at rates <=10^16 g/s are radially extended, hot, optically-thin, and they advect some of their internally-dissipated energy (Narayan & Popham 1993). Motivated by this, I construct here idealized spectral models of an Advection-Dominated Accretion Flow (ADAF) around a WD, for application to quiescent Dwarf Novae (DN). The Bremsstrahlung cooling of the gas in the ADAF, with temperatures ranging from a few keV to a few tens of keV, can account for the X-ray emission properties of quiescent DN. If the energy advected by the flow is thermalized in the WD atmosphere, the resulting emission from the entire stellar surface (blackbody of temperature T_eff ~ 5 eV) outshines the X-ray luminosity substantially. This extreme-UV component provides a flux in the 0.055-0.28 keV band which is sufficient to power the strong HeII lambda4686 emission lines of quiescent DN by photoionization of the disk material. Reprocessing of the ADAF X-ray emission by a cold outer thin disk could also lead to an observable iron Kalpha fluorescence emission line, which can be used to probe the geometry of the accretion flow. Existing observational data indicate that the presence of ADAFs in quiescent DN is not ubiquitous, while future observations, in particular with the X-ray satellites Chandra and XMM-Newton, have the potential to detect signatures of the hot flow in promising candidates.

astro-ph↗

arXiv2000

Disc instability models for X-ray transients: evidence for evaporation and low alpha-viscosity ?

We construct time-dependent models of accretion discs around black holes and neutron stars. We investigate the effect of evaporating the disc inner regions during quiescence on the predictions of the Disc Instability Model (DIM) for these systems. We do not include irradiation of the disc in the models. Removing the inner, most unstable parts of the accretion disc increases the predicted recurrence times. However, DIMs with values of the viscosity parameter alpha_hot ~ 0.1 and alpha_cold ~ 0.02 (typical of applications of the DIM to standard dwarf nova outbursts) fail to reproduce the long recurrence times of SXTs (unless we resort to fine-tuning of the parameters) independent of the evaporation strength. We show that models with evaporation and a smaller value of alpha_cold (~ 0.005) do reproduce the long recurrence times and the accretion rates at the level of the Eddington rate observed in outburst. The large difference between the values of alpha_hot and alpha_cold, if confirmed when disc irradiation is included, suggests that several viscosity mechanisms operate in these accretion discs. For some choices of parameters our models predict reflares during the decline from outburst. They are a physical property of the model and result from a heating front forming in the wake of an initial cooling front and subsequent, multiple front reflections. The reflares disappear in low-alpha models where front reflection can not occur.

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arXiv1999

On the Cosmological Evolution of the Luminosity Function and the Accretion Rate of Quasars

We consider a class of models for the redshift evolution (between $0\lsim z \lsim 4$) of the observed optical and X-ray quasar luminosity functions (LFs), with the following assumptions: (i) the mass-function of dark matter halos follows the Press-Schechter theory, (ii) the black hole (BH) mass scales linearly with the halo mass, (iii) quasars have a constant universal lifetime, and (iv) a thin accretion disk provides the optical luminosity of quasars, while the X-ray/optical flux ratio is calibrated from a sample of observed quasars. The mass accretion rate $\dot{M}$ onto quasar BHs is a free parameter of the models, that we constrain using the observed LFs. The accretion rate $\dot M$ inferred from either the optical or X-ray data under these assumptions generally decreases as a function of cosmic time from $z \simeq 4$ to $z \simeq 0$. We find that a comparable accretion rate is inferred from the X-ray and optical LF only if the X-ray/optical flux ratio decreases with BH mass. Near $z\simeq 0$, $\dot M$ drops to substantially sub-Eddington values at which advection-dominated accretion flows (ADAFs) exist. Such a decline of $\dot M$, possibly followed by a transition to radiatively inefficient ADAFs, could explain both the absence of bright quasars in the local universe and the faintness of accreting BHs at the centers of nearby galaxies. We argue that a decline of the accretion rate of the quasar population is indeed expected in cosmological structure formation models.

astro-ph↗

arXiv1999

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 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.

astro-ph↗

arXiv1999

Disc instability models, evaporation and radius variations

We show that the outcome of disc instability models is strongly influenced by boundary conditions such as the position of the inner and outer disc edges. We discuss other sources of uncertainties, such as the tidal torque, and we conclude that disc illumination, disk size variations and a proper prescription for the tidal torque must be included in models if one wishes to extract meaningful physical information on e.g. viscosity from the comparison of predicted and observed lightcurves.

astro-ph↗

arXiv1998

Structure and properties of transition fronts in accretion discs

We use high-resolution time-dependent numerical simulations of accretion discs around white dwarfs to study the structure and properties of transition fronts in the context of the thermal-viscous disc instability model. The thermal structure of cooling and heating fronts is dominated by radiative cooling and viscous heating, respectively, except in a very narrow precursor region in heating fronts where advection and radial transport of energy dominate. Cooling fronts are much broader than heating fronts, but the widths of both types of fronts scale with the local vertical scale height of the disc. We confirm that during a fair fraction of the propagation time of a cooling front, the structure of the inner disc is close to self-similar. The speed of heating fronts is ~ a few km/s, while the speed of cooling fronts is ~ a fraction of a km/s. We show that direct measurements of the speed of transition fronts probably cannot discriminate between various prescriptions proposed for the viscosity parameter alpha. A natural prediction of the disc instability model is that fronts decelerate as they propagate in the disc, independent of the prescription for alpha. Observation of this effect would confirm that dwarf nova outbursts are driven by the thermal-viscous instability. Most of our results also apply to low mass X-ray binaries in which the accreting object is a neutron star or a black hole.

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arXiv1998

A Population of Faint Non-Transient Low Mass Black Hole Binaries

We study the thermal and viscous stability of accretion flows in Low Mass Black Hole Binaries (LMBHBs). We consider a model in which an inner advection-dominated accretion flow (ADAF) is surrounded by a geometrically thin accretion disk, the transition between the two zones occurring at a radius R_tr. In all the known LMBHBs, R_tr appears to be such that the outer disks could suffer from a global thermal-viscous instability. This instability is likely to cause the transient behavior of these systems. However, in most cases, if R_tr were slightly larger than the estimated values, the systems would be globally stable. This suggests that a population of faint persistent LMBHBs with globally stable outer disks could be present in the Galaxy. Such LMBHBs would be hard to detect because they would lack large amplitude outbursts, and because their ADAF zones would have very low radiative efficiencies, making the systems very dim. We present model spectra of such systems covering the optical and X-ray bands.

astro-ph↗

arXiv1998

Accretion disc outbursts: a new version of an old model

We have developed 1D time-dependent numerical models of accretion discs, using an adaptive grid technique and an implicit numerical scheme, in which the disc size is allowed to vary with time. The code fully resolves the cooling and heating fronts propagating in the disc. We show that models in which the radius of the outer edge of the disc is fixed produce incorrect results, from which probably incorrect conclusions about the viscosity law have been inferred. In particular we show that outside-in outbursts are possible when a standard bimodal behaviour of the Shakura-Sunyaev viscosity parameter alpha is used. We also discuss to what extent insufficient grid resolutions have limited the predictive power of previous models. We find that the global properties (magnitudes, etc. ...) of transient discs can be addressed by codes using a high, but reasonable, number of fixed grid points. However, the study of the detailed physical properties of the transition fronts generally requires resolutions which are out of reach of fixed grid codes. It appears that most time-dependent models of accretion discs published in the literature have been limited by resolution effects, improper outer boundary conditions, or both.

astro-ph↗

arXiv1998

Astrophysical Evidence for Black Hole Event Horizons

Astronomers have discovered many potential black holes in X-ray binaries and galactic nuclei. These black holes are usually identified by the fact that they are too massive to be neutron stars. Until recently, however, there was no convincing evidence that the objects identified as black hole candidates actually have event horizons. This has changed with extensive applications of a class of accretion models for describing the flow of gas onto compact objects; for these solutions, called advection-dominated accretion flows (ADAFs), the black hole nature of the accreting star, specifically its event horizon, plays an important role. We review the evidence that, at low luminosities, accreting black holes in both X-ray binaries and galactic nuclei contain ADAFs rather than the standard thin accretion disk.

gr-qc↗

arXiv1997

On the origin of episodic accretion in Dwarf Novae

We show that dwarf nova disks in quiescence have rather low magnetic Reynolds number, of order 10^3. Numerical simulations of magnetized accretion disks suggest that under these conditions magnetohydrodynamic turbulence and the associated angular momentum transport is sharply reduced. This could be the physical origin of episodic accretion in dwarf nova disks. If so, the standard disk instability model needs to be revised.

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