55 research outputs found
Comment on Viscous Stability of Relativistic Keplerian Accretion Disks
Recently Ghosh (1998) reported a new regime of instability in Keplerian
accretion disks which is caused by relativistic effects. This instability
appears in the gas pressure dominated region when all relativistic corrections
to the disk structure equations are taken into account. We show that he uses
the stability criterion in completely wrong way leading to inappropriate
conclusions. We perform a standard stability analysis to show that no unstable
region can be found when the relativistic disk is gas pressure dominated.Comment: 9 pages, 4 figures, uses aasms4.sty, submitted for ApJ Letter
Main trends of the quasar main sequence -- effect of viewing angle
We address the effect of the viewing angle of the accretion disk plane and
the geometry of the broad-line region (BLR) with the goal of interpreting the
distribution of quasars along the main sequence (MS). We utilize
photoionization code CLOUDY to model the BLR FeII emission, incorporating the
grossly underestimated role of the form factor (f). We recover the dependence
of the strength of the FeII emission in the optical (R) on
L/L ratio and related observational trends - as a
function of the spectral energy distribution (SED) shape, cloud density,
composition and intra-cloud dynamics, assumed following prior observational
constraints. With this approach, we are now able to explain the diversity of
quasars and the change of the quasar properties along the Main Sequence (MS).
Our approach also explains the rarity of the highest FeII emitters known as the
extreme xA sources and can be used as a predictive tool in future reverberation
mapping studies of Type-1 AGNs. This approach further justifies the use of
quasars as `cosmological probes'.Comment: 15 pages, 5 figures; Contributions of the Astronomical Observatory
Skalnat\'e Pleso (CAOSP) as a Special Issue "Spectral Line Shapes in
Astrophysics and Related Topics
The Wind Dynamics of Super-Eddington Sources in FRADO
We perform non-hydrodynamical 2.5D simulations to study the dynamics of
material above accretion disk based on the disk radiation pressure acting on
dust. We assume a super-accreting underlying disk with the accretion rate of 10
times the Eddington rate with central black hole mass ranging from up to
. Such high accretion rates are characteristic for extreme
sources. We show that for high accretors radiatively dust-driving mechanism
based on FRADO model always leads to a massive outflow from the disk surface,
and the failed wind develops only at larger radii. The outflow rate strongly
depends on the black hole mass, and in optically-thick energy-driven solution
can exceed the accretion rate for masses larger than but
momentum-driven outflow does not exceed the accretion rate even for
super-Eddington accretion, therefore not violating the adopted stationarity of
the disk. However, even in this case the outflow from the disk implies a strong
mechanical feedback.Comment: 13 pages, 3 figures, Accepted for publication in the journal of
Dynamics (MDPI
Spectral Energy Distribution profiles from AGN accretion disc in multi-gap setup
Spectral Energy Distribution (SED) of the broad-band continuum emission from
black-hole accretion discs can serve as a tool to measure parameters of the
central body and constrain the geometry of the inner accretion flow. We focus
on the case of an active galactic nucleus (AGN), with an accretion disc
dominating the UV/optical bands. We parameterize the changes in the thermal and
power-law components, which can reveal the diminution of the emissivity. To
this end we explore the effects of gaps in the accretion disc and the emerging
SED that can be caused by the presence of either (i) the inner, optically thin,
radiatively inefficient hot flow; (ii) a secondary black hole embedded within
the accretion disc; or (iii) a combination of both components. We suggest that
the resulting changes in the SED of the underlying continuum can help us to
understand some departures from the standard-disc scenario. We estimate that
the data required for such a project must be sampled in detail over the far-UV
to soft X-ray bands during the interval of about a month corresponding to the
characteristic variability timescale of an AGN. Detecting a gap at intermediate
radii of a few 100 gravitational radii would require quality photometry with
uncertainties up to 1%. The presence of the central cavity in the
standard disc can be recovered in UV photometric data with an accuracy of 5%
and better. We show the effect of the intrinsic reddening of the source and
demonstrate when it can be disentangled.Comment: 17 pages, 10 figures and 5 tables. Accepted for publication in
Monthly Notices of the Royal Astronomical Societ
Modified models of radiation pressure instability in application to 10, 10, and 10 accreting black holes
Some of the accreting black holes exhibit much stronger variability patterns
than the usual stochastic variations. Radiation pressure instability is one of
the proposed mechanisms which could account for this effect. We aim to model
luminosity changes for objects with black hole mass of 10, 10, and 10
solar masses, using the time-dependent evolution of an accretion disk unstable
due to the dominant radiation pressure. We use a 1-dimensional, vertically
integrated time-dependent numerical scheme which models simultaneous evolution
of the disk and corona, coupled by the vertical mass exchange. We also discuss
the possibility of presence of an inner optically thin flow, namely the
Advection-Dominated Accretion Flow (ADAF). We found that the outburst character
strongly depends on the magnetic field and the outer radius of the disk if this
radius is smaller (due to TDE phenomenon) than the size of the instability zone
in a stationary disk with infinite radius. For microquasars, the dependence on
the magnetic field is monotonic, and the period decreases with the field
strength. For larger black hole masses, the dependence is non-monotonic, and
initial rise of the period is later replaced with the relatively rapid decrease
as the magnetic field continues to rise. Still stronger magnetic field
stabilizes the disk. Our computations confirm that the radiation pressure
instability model can account for heartbeat states in microquasars. Rapid
variability detected in IMBH in the form of Quasi-Periodic Ejection can be
consistent with the model but only if combined with TDE phenomenon. Yearly
repeating variability in Changing Look AGN also requires, in our model, small
outer radius either due to the recent TDE or due to the presence of the gap in
the disk related to the presence of a secondary black hole.Comment: 23 pages, 18 figures, Submitted for publication on Astronomy and
Astrophysics, comments very welcom
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