16 research outputs found
Line Shifts, Broad-Line Region Inflow, and the Feeding of AGNs
Velocity-resolved reverberation mapping suggests that the broad-line regions
(BLRs) of AGNs can have significant net inflow. We use the STOKES radiative
transfer code to show that electron and Rayleigh scattering off the BLR and
torus naturally explains the blueshifted profiles of high-ionization lines and
the ionization dependence of the blueshifts. This result is insensitive to the
geometry of the scattering region. If correct, this model resolves the
long-standing conflict between the absence of outflow implied by
velocity-resolved reverberation mapping and the need for outflow if the
blueshifting is the result of obscuration. The accretion rate implied by the
inflow is sufficient to power the AGN. We suggest that the BLR is part of the
outer accretion disk and that similar MHD processes are operating. In the
scattering model the blueshifting is proportional to the accretion rate so
high-accretion-rate AGNs will show greater high-ionization line blueshifts as
is observed. Scattering can lead to systematically too high black hole mass
estimates from the C IV line. We note many similarities between narrow-line
region (NLR) and BLR blueshiftings, and suggest that NLR blueshiftings have a
similar explanation. Our model explains the higher blueshifts of broad
absorption line QSOs if they are more highly inclined. Rayleigh scattering from
the BLR and torus could be more important in the UV than electron scattering
for predominantly neutral material around AGNs. The importance of Rayleigh
scattering versus electron scattering can be assessed by comparing line
profiles at different wavelengths arising from the same emission-line region.Comment: 10 pages, 7 figures, Astrophysical Journal in press. The only changes
from the previous version are to include some additional discussion of the
plausibility of supersonic inflow velocities (see section 5.2) and some
additional reference
Extreme Coronal Line Emitters: Tidal Disruption of Stars by Massive Black Holes in Galactic Nuclei?
Tidal disruption of stars by supermassive black holes at the centers of
galaxies is expected to produce unique emission line signatures, which have not
yet been explored adequately. Here we report the discovery of extremely strong
coronal lines from [Fe X] up to [Fe XIV] in a sample of seven galaxies
(including two recently reported cases), that we interpret as such signatures.
This is the first systematic search for objects of this kind, by making use of
the immense database of the Sloan Digital Sky Survey. The galaxies, which are
non-active as evidenced by the narrow-line ratios, show broad emission lines of
complex profiles in more than half of the sample. Both the high ionization
coronal lines and the broad lines turn out to be fading on time scales of years
in objects observed with spectroscopic follow-ups, suggesting their transient
nature. Variations of inferred non-stellar continua, which have absolute
magnitudes of at least -16 to -18 mag in the g band, are also detected in more
than half of the sample. These extreme coronal line emitters reside in sub-L_*
disk galaxies (-21.3 < M_i < -18.5) with small stellar velocity dispersions.
The sample seems to form two distinct types based on the presence or absence of
the [Fe VII] lines, with the latter having relatively low luminosities of [O
III], [Fe XI], and the host galaxies. These characteristics can most naturally
be understood in the context of transient accretion onto intermediate mass
black holes at galactic centers following tidal disruption of stars in a
gas-rich environment. We estimate the incidence of such events to be around
10^-5 per year for a galaxy with -21.5 < M_i < -18.5.Comment: 32 pages, 12 figures, ApJ accepted, typos correcte
The growth of supermassive black holes fed by accretion disks
Supermassive black holes are probably present in the centre of the majority
of the galaxies. There is a consensus that these exotic objects are formed by
the growth of seeds either by accreting mass from a circumnuclear disk and/or
by coalescences during merger episodes.
The mass fraction of the disk captured by the central object and the related
timescale are still open questions, as well as how these quantities depend on
parameters like the initial mass of the disk or the seed or on the angular
momentum transport mechanism. This paper is addressed to these particular
aspects of the accretion disk evolution and of the growth of seeds.
The time-dependent hydrodynamic equations were solved numerically for an
axi-symmetric disk in which the gravitational potential includes contributions
both from the central object and from the disk itself. The numerical code is
based on a Eulerian formalism, using a finite difference method of
second-order, according to the Van Leer upwind algorithm on a staggered mesh.
The present simulations indicate that seeds capture about a half of the
initial disk mass, a result weakly dependent on model parameters. The
timescales required for accreting 50% of the disk mass are in the range 130-540
Myr, depending on the adopted parameters. These timescales permit to explain
the presence of bright quasars at z ~ 6.5. Moreover, at the end of the disk
evolution, a "torus-like" geometry develops, offering a natural explanation for
the presence of these structures in the central regions of AGNs, representing
an additional support to the unified model.Comment: 10 pages, 7 figures. Accepted for publication by Astronomy and
Astrophysic
The Tidal Disruption of Giant Stars and Their Contribution to the Flaring Supermassive Black Hole Population
Sun-like stars are thought to be regularly disrupted by supermassive black
holes (SMBHs) within galactic nuclei. Yet, as stars evolve off the main
sequence their vulnerability to tidal disruption increases drastically as they
develop a bifurcated structure consisting of a dense core and a tenuous
envelope. Here we present the first hydrodynamic simulations of the tidal
disruption of giant stars and show that the core has a substantial influence on
the star's ability to survive the encounter. Stars with more massive cores
retain large fractions of their envelope mass, even in deep encounters.
Accretion flares resulting from the disruption of giant stars should last for
tens to hundreds of years. Their characteristic signature in transient searches
would not be the decay typically associated with tidal disruption
events, but a correlated rise over many orders of magnitude in brightness on
months to years timescales. We calculate the relative disruption rates of stars
of varying evolutionary stages in typical galactic centers, then use our
results to produce Monte Carlo realizations of the expected flaring event
populations. We find that the demographics of tidal disruption flares are
strongly dependent on both stellar and black hole mass, especially near the
limiting SMBH mass scale of . At this black hole mass, we
predict a sharp transition in the SMBH flaring diet beyond which all observable
disruptions arise from evolved stars, accompanied by a dramatic cutoff in the
overall tidal disruption flaring rate. Black holes less massive than this
limiting mass scale will show observable flares from both main sequence and
evolved stars, with giants contributing up to 10% of the event rate. The
relative fractions of stars disrupted at different evolutionary states can
constrain the properties and distributions of stars in galactic nuclei other
than our own.Comment: 18 pages, 18 figures, submitted to Ap
Evolution and precession of accretion disk in tidal disruption events
In a supermassive black hole (BH) tidal disruption event (TDE), the tidally disrupted star feeds the BH via an accretion disk. Most often it is assumed that the accretion rate history, hence the emission light curve, tracks the rate at which new debris mass falls back onto the disk, notably the tâ5/3 power law. But this is not the case when the disk evolution due to viscous spreading - the driving force for accretion - is carefully considered. We construct a simple analytical model that comprehensively describes the accretion rate history across 4 different phases of the disk evolution, in the presence of mass fallback and disk wind loss. Accretion rate evolves differently in those phases which are governed by how the disk heat energy is carried away, early on by advection and later by radiation. The accretion rate can decline as steeply as tâ5/3 only if copious disk wind loss is present during the early advection-cooled phase. Later, the accretion rate history is tâ8/7 or shallower. These have great implications on the TDE flare light curve. A TDE accretion disk is most likely misaligned with the equatorial plane of the spinning BH. Moreover, in the TDE the accretion rate is super- or near-Eddington thus the disk is geometrically thick, for which case the BHâs frame dragging effect may cause the disk precess as a solid body, which may manifest itself as quasi-periodic signal in the TDE light curve. Our disk evolution model predicts the disk precession period increases with time, typically as â t. The results are applied to the recently jetted TDE flare Swift transient J1644 + 57 which shows numerous, quasi-periodic dips in its long-term X-ray light curve. As the current TDE sample increases, the identification of the disk precession signature provides a unique way of measuring BH spin and studying BH accretion physics
Tidal disruption events from the first XMM-Newton slew survey
Observations over the past decade have revealed that supermassive black holes (SMBHs) likely reside at the centres of most or all bulge galaxies. Confirmation of their dormant presence in non-active galaxies is difficult to obtain. An unavoidable consequence of the existence of remnant SMBHs is the detection of a tidal disruption event. This is discovered as flaring radiation produced when a star is tidally disrupted and subsequently accreted by the black hole. Two of these exceptional events have been discovered by XMM-Newton in the first slew catalogue, NGCâ3599 and SDSSâJ132341.97+482701.3. Here we show their evolution up to four years after the peak of the outburst including a detailed analysis of NGCâ3599, for which novel follow-up observations are presented here