243 research outputs found
Extreme AGN variability: evidence of magnetically elevated accretion?
Rapid, large amplitude variability at optical to X-ray wavelengths is now
seen in an increasing number of Seyfert galaxies and luminous quasars. The
variations imply a global change in accretion power, but are too rapid to be
communicated by inflow through a standard thin accretion disc. Such discs are
long known to have difficulty explaining the observed optical/UV emission from
active galactic nuclei. Here we show that alternative models developed to
explain these observations have larger scale heights and shorter inflow times.
Accretion discs supported by magnetic pressure in particular are geometrically
thick at all luminosities, with inflow times as short as the observed few year
timescales in extreme variability events to date. Future time-resolved,
multi-wavelength observations can distinguish between inflow through a
geometrically thick disc as proposed here, and alternative scenarios of extreme
reprocessing of a central source or instability-driven limit cycles.Comment: 5 pages, 2 figures, submitted to MNRAS letter
Self-regulated black hole accretion, the M-sigma relation, and the growth of bulges in galaxies
We argue that the velocity dispersions and masses of galactic bulges and
spheroids are byproducts of the feedback that regulates rapid black hole growth
in protogalaxies. We suggest that the feedback energy liberated by accretion
must pass through the accreting material, in an energy-conserving flux close-in
and a momentum-conserving flux further out. If the inflowing gas dominates the
gravitational potential outside the Bondi radius, feedback from
Eddington-limited accretion drives the density profile of the gas to that of a
singular isothermal sphere. We find that the velocity dispersion associated
with the isothermal potential, sigma, increases with time as the black hole
mass M grows, in such a way that M is proportional to sigma^4. The coefficient
of this proportionality depends on the radius at which the flow switches from
energy conserving to momentum conserving, and gives the observed M-sigma
relation if the transition occurs at ~100 Schwarzschild radii. We associate
this transition with radiative cooling and show that bremsstrahlung, strongly
boosted by inverse Compton scattering in a two-temperature (T_p >> T_e) plasma,
leads to a transition at the desired radius.
According to this picture, bulge masses M_b are insensitive to the virial
masses of their dark matter haloes, but correlate linearly with black hole
mass. Our analytic model also explains the M_b-sigma (Faber-Jackson) relation
as a relic of black hole accretion. The model naturally explains why the
M-sigma relation has less scatter than either the M-M_b (Magorrian) or the
Faber-Jackson relation. It suggests that the M-sigma relation could extend down
to very low velocity dispersions, and predicts that the relation should not
evolve with redshift.Comment: 6 pages, no figures, submitted to Monthly Notices of the Royal
Astronomical Societ
Hyperaccretion during tidal disruption events: weakly bound debris envelopes and jets
After the destruction of the star during a tidal disruption event (TDE), the
cataclysmic encounter between a star and the supermassive black hole (SMBH) of
a galaxy, approximately half of the original stellar debris falls back onto the
hole at a rate that can initially exceed the Eddington limit by orders of
magnitude. We argue that the angular momentum of this matter is too low to
allow it to attain a disk-like configuration with accretion proceeding at a
mildly super-Eddington rate, the excess energy being carried away by a
combination of radiative losses and radially distributed winds. Instead, we
propose that the infalling gas traps accretion energy until it inflates into a
weakly-bound, quasi-spherical structure with gas extending nearly to the poles.
We study the structure and evolution of such "Zero-Bernoulli accretion" flows
(ZEBRAs) as a model for the super-Eddington phase of TDEs. We argue that such
flows cannot stop extremely super-Eddington accretion from occurring, and that
once the envelope is maximally inflated, any excess accretion energy escapes
through the poles in the form of powerful jets. We compare the predictions of
our model to Swift J1644+57, the putative super-Eddington TDE, and show that it
can qualitatively reproduce some of its observed features. Similar models,
including self-gravity, could be applicable to gamma-ray bursts from collapsars
and the growth of supermassive black hole seeds inside quasi-stars.Comment: 19 pages, 14 figures. Accepted for publication in Ap
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