1,132 research outputs found
Outbursts in ultracompact X-ray binaries
Very faint X-ray binaries appear to be transient in many cases with peak
luminosities much fainter than that of usual soft X-ray transients, but their
nature still remains elusive. We investigate the possibility that this
transient behaviour is due to the same thermal/viscous instability which is
responsible for outbursts of bright soft X-ray transients, occurring in
ultracompact binaries for adequately low mass-transfer rates. More generally,
we investigate the observational consequences of this instability when it
occurs in ultracompact binaries. We use our code for modelling the
thermal-viscous instability of the accretion disc, assumed here to be hydrogen
poor. We also take into account the effects of disc X-ray irradiation, and
consider the impact of the mass-transfer rate on the outburst brightness. We
find that one can reproduce the observed properties of both the very faint and
the brighter short transients (peak luminosity, duration, recurrence times),
provided that the viscosity parameter in quiescence is slightly smaller
(typically a factor of between two and four) than in bright soft X-ray
transients and normal dwarf nova outbursts, the viscosity in outburst being
unchanged. This possibly reflects the impact of chemical composition on
non-ideal MHD effects affecting magnetically driven turbulence in poorly
ionized discs.Comment: 9 pages, 7 figures, Astronomy and Astrophysics, in pres
Dwarf nova outbursts in intermediate polars
The disc instability model (DIM) has been very successful in explaining the
dwarf nova outbursts observed in cataclysmic variables. When, as in
intermediate polars (IP), the accreting white dwarf is magnetized, the disc is
truncated at the magnetospheric radius, but for mass-transfer rates
corresponding to the thermal-viscous instability such systems should still
exhibit dwarf-nova outbursts. Yet, the majority of intermediate polars in which
the magnetic field is not large enough to completely disrupt the accretion
disc, seem to be stable, and the rare observed outbursts, in particular in
systems with long orbital periods, are much shorter than normal dwarf-nova
outbursts. We investigate the predictions of the disc instability model for
intermediate polars in order to determine which of the observed properties of
these systems can be explained by the DIM. We use our numerical code for the
time evolution of accretion discs, modified to include the effects of the
magnetic field, with constant or variable mass transfer from the secondary
star. We show that intermediate polars have mass transfer low enough and
magnetic fields large enough to keep the accretion disc stable on the cold
equilibrium branch. We show that the infrequent and short outbursts observed in
long period systems, such as e.g., TV Col, cannot be attributed to the
thermal-viscous instability of the accretion disc, but instead have to be
triggered by an enhanced mass-transfer from the secondary, or, more likely, by
some instability coupling the white dwarf magnetic field with that generated by
the magnetorotational instability operating in the accretion disc. Longer
outbursts (a few days) could result from the disc instability.Comment: 7 pages, 5 figures; submitted to Astronomy & Astrophysic
Radio Loudness of AGNs: Host Galaxy Morphology and the Spin Paradigm
We investigate how the total radio luminosity of AGN-powered radio sources
depends on their accretion luminosity and the central black hole mass. We find
that AGNs form two distinct and well separated sequences on the radio-loudness
- Eddington-ratio plane. We argue that these sequences 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 another parameter in addition
to the accretion rate must play a role in determining the jet production
efficiency in active galactic nuclei, and that this parameter is related to
properties of the host galaxy. The revealed host-related radio dichotomy breaks
down at high accretion rates where the dominant fraction of luminous quasars
hosted by elliptical galaxies is radio quiet. We argue that the huge difference
between the radio-loudness reachable by AGNs in disc and elliptical galaxies
can be explained by the scenario according to which the spin of a black hole
determines the outflow's power, and central black holes can reach large spins
only in early type galaxies (following major mergers), and not (in a
statistical sense) in spiral galaxies.Comment: 7 pages, 4 figures included. Proceedings of the Workshop
`Extragalactic Jets: Theory and Observation from Radio to Gamma Ray',
Girdwood, May 200
Hot white dwarfs and the UV delay in dwarf novae
We calculate the effect of illumination of dwarf nova accretion discs by
radiation from a hot, central, white dwarf. We show that only for very hot
white dwarfs (Teff ~ 40 000$ K) the inner region of quiescent dwarf nova discs
are partially depleted so that the delay between the rise to outburst of the
optical and UV fluxes would be increased as suggested recently by King (1997).
This depletion, however, must create several small outbursts between main
outbursts, contrary to observations. Lower white dwarf temperatures may cause
the outburts to be of the `inside-out' type removing the UV delay. We conclude
that white dwarf irradiation of dwarf nova discs is not very efficient for
example because the UV radiation from the hot white dwarf does not penetrate
deep enough in the disc atmosphere. The total ablation of the inner disc by
e.g. evaporation (possibly related to illumination) appears to be a very
promising possibility, accounting for both the EUV delay and the general
lightcurves properties.Comment: 6 pages, 8 figures; accepted for publication in MNRA
Mass of a Black Hole Firewall
Quantum entanglement of Hawking radiation has been supposed to give rise to a
Planck density "firewall" near the event horizon of old black holes. We show
that Planck density firewalls are excluded by Einstein's equations for black
holes of mass exceeding the Planck mass. We find an upper limit of
to the surface density of a firewall in a Schwarzschild black hole of mass ,
translating for astrophysical black holes into a firewall density smaller than
Planck density by more than 30 orders of magnitude. A strict upper limit on the
firewall density is given by the Planck density times the ratio .Comment: 6 pages, version published in Phys. Rev. Let
Advection Dominated Accretion Flows Around Kerr Black Holes
We derive all relevant equations needed for constructing a global general
relativistic model of advectively cooled, very hot, optically thin accretion
disks around black holes and present solutions which describe advection
dominated flows in the gravitational field of a Kerr black hole.Comment: ApJ submitte
Are Quasar Jets Matter or Poynting Flux Dominated?
If quasar jets are accelerated by magnetic fields but terminate as matter
dominated, where and how does the transition occur between the
Poynting-dominated and matter-dominated regimes? To address this question, we
study constraints which are imposed on the jet structure by observations at
different spatial scales. We demonstrate that observational data are consistent
with a scenario where the acceleration of a jet occurs within 10^{3-4} R_g. In
this picture, the non-thermal flares -- important defining attributes of the
blazar phenomenon - are produced by strong shocks formed in the region where
the jet inertia becomes dominated by matter. Such shocks may be formed due to
collisions between the portions of a jet accelerated to different velocities,
and the acceleration differentiation is very likely to be related to global MHD
instabilities.Comment: to appear in "Astrophysical Sources of High Energy Particles and
Radiation", AIP Proceedings Series, eds. T. Bulik, G. Madejski, and B. Rudak
(20-24 June 2005, Torun, Poland
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. Since 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 since the correct formula describing irradiation is not well
known.Comment: to be published in MNRAS, uses epsfig.st
Extracting black-hole rotational energy: The generalized Penrose process
In the case involving particles the necessary and sufficient condition for
the Penrose process to extract energy from a rotating black hole is absorption
of particles with negative energies and angular momenta. No torque at the
black-hole horizon occurs. In this article we consider the case of arbitrary
fields or matter described by an unspecified, general energy-momentum tensor
and show that the necessary and sufficient condition for
extraction of a black hole's rotational energy is analogous to that in the
mechanical Penrose process: absorption of negative energy and negative angular
momentum. We also show that a necessary condition for the Penrose process to
occur is for the Noether current (the conserved energy-momentum density vector)
to be spacelike or past directed (timelike or null) on some part of the
horizon. In the particle case, our general criterion for the occurrence of a
Penrose process reproduces the standard result. In the case of relativistic
jet-producing "magnetically arrested disks" we show that the negative energy
and angular-momentum absorption condition is obeyed when the Blandford-Znajek
mechanism is at work, and hence the high energy extraction efficiency up to
found in recent numerical simulations of such accretion flows
results from tapping the black hole's rotational energy through the Penrose
process. We show how black-hole rotational energy extraction works in this case
by describing the Penrose process in terms of the Noether current.Comment: 24 pages, 14 figures, version published in Phys. Rev.
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