507 research outputs found
Interruption of Tidal Disruption Flares By Supermassive Black Hole Binaries
Supermassive black hole binaries (SMBHBs) are products of galaxy mergers, and
are important in testing Lambda cold dark matter cosmology and locating
gravitational-wave-radiation sources. A unique electromagnetic signature of
SMBHBs in galactic nuclei is essential in identifying the binaries in
observations from the IR band through optical to X-ray. Recently, the flares in
optical, UV, and X-ray caused by supermassive black holes (SMBHs) tidally
disrupting nearby stars have been successfully used to observationally probe
single SMBHs in normal galaxies. In this Letter, we investigate the accretion
of the gaseous debris of a tidally disrupted star by a SMBHB. Using both
stability analysis of three-body systems and numerical scattering experiments,
we show that the accretion of stellar debris gas, which initially decays with
time , would stop at a time . Here, and is the orbital period of the SMBHB.
After a period of interruption, the accretion recurs discretely at time , where . Both and sensitively
depend on the orbital parameters of the tidally disrupted star at the tidal
radius and the orbit eccentricity of SMBHB. The interrupted accretion of the
stellar debris gas gives rise to an interrupted tidal flare, which could be
used to identify SMBHBs in non-active galaxies in the upcoming transient
surveys.Comment: 13 pages, including one color figure; typos corrected; appeared in
ApJ Letters (November 20 issue
A large accretion disk of extreme eccentricity in the TDE ASASSN-14li
In the canonical model for tidal disruption events (TDEs), the stellar debris
circularizes quickly to form an accretion disk of size about twice the orbital
pericenter of the star. Most TDEs and candidates discovered in the optical/UV
have broad optical emission lines with complex and diverse profiles of puzzling
origin. Liu et al. recently developed a relativistic elliptical disk model of
constant eccentricity in radius for the broad optical emission lines of TDEs
and well reproduced the double-peaked line profiles of the TDE candidate
PTF09djl with a large and extremely eccentric accretion disk. In this paper, we
show that the optical emission lines of the TDE ASASSN-14li with radically
different profiles are well modelled with the relativistic elliptical disk
model, too. The accretion disk of ASASSN-14li has an eccentricity 0.97 and
semimajor axis of 847 times the Schwarzschild radius (r_S) of the black hole
(BH). It forms as the consequence of tidal disruption of a star passing by a
massive BH with orbital pericenter 25r_S. The optical emission lines of
ASASSN-14li are powered by an extended X-ray source of flat radial distribution
overlapping the bulk of the accretion disk and the single-peaked asymmetric
line profiles are mainly due to the orbital motion of the emitting matter
within the disk plane of inclination about 26\degr and of pericenter
orientation closely toward the observer. Our results suggest that modelling the
complex line profiles is powerful in probing the structures of accretion disks
and coronal X-ray sources in TDEs.Comment: 10 pages, 8 figures, accepted for publication in the MNRA
Interaction of Recoiling Supermassive Black Holes with Stars in Galactic Nuclei
Supermassive black hole binaries (SMBHBs) are the products of frequent galaxy
mergers. The coalescence of the SMBHBs is a distinct source of gravitational
wave (GW) radiation. The detections of the strong GW radiation and their
possible electromagnetic counterparts are essential. Numerical relativity
suggests that the post-merger supermassive black hole (SMBH) gets a kick
velocity up to 4000 km/s due to the anisotropic GW radiations. Here we
investigate the dynamical co-evolution and interaction of the recoiling SMBHs
and their galactic stellar environments with one million direct N-body
simulations including the stellar tidal disruption by the recoiling SMBHs. Our
results show that the accretion of disrupted stars does not significantly
affect the SMBH dynamical evolution. We investigate the stellar tidal
disruption rates as a function of the dynamical evolution of oscillating SMBHs
in the galactic nuclei. Our simulations show that most of stellar tidal
disruptions are contributed by the unbound stars and occur when the oscillating
SMBHs pass through the galactic center. The averaged disruption rate is
~10^{-6} M_\odot yr^{-1}, which is about an order of magnitude lower than that
by a stationary SMBH at similar galactic nuclei. Our results also show that a
bound star cluster is around the oscillating SMBH of about ~ 0.7% the black
hole mass. In addition, we discover a massive cloud of unbound stars following
the oscillating SMBH. We also investigate the dependence of the results on the
SMBH masses and density slopes of the galactic nuclei.Comment: 38 pages, 10 figues; accepted for publication in Ap
Transient X-ray Emission from Normal Galactic Nuclei
X-ray transients appeared in optically non-active galactic nuclei have been
observed in recent years. The most popular model explaining this kind of
phenomena is the conventional tidal disruption model. In this model, when a
star moves within the tidal radius of a black hole, part of the star materials
will fall into the black hole through an accretion disk, which gives rise to
the luminous flare. We propose that the X-ray emission may not necessarily come
from radiation of the accretion disk alone. Instead, it may be related to a
jet. As the jet travels in the interstellar medium, a shock is produced and
synchrotron radiation is expected. We compared the model light curve and the
synchrotron radiation spectrum with the observed data, and find that our model
explains the observed light curve and late-time spectrum well. Our model
predicts that these transient active galactic nuclei could be sources of the
future gamma-ray satellites, e.g. GLAST and the emission region will be
expanding with time.Comment: 5 figures, accepted by A&
Four dual AGN candidates observed with the VLBA
According to hierarchical structure formation models, merging galaxies are
expected to be seen in different stages of their coalescence. However,
currently there are no straightforward observational methods neither to select
nor to confirm a large number of dual active galactic nuclei (AGN) candidates.
Most attempts involve the better understanding of double-peaked narrow emission
line sources, to distinguish the objects where the emission lines originate
from narrow-line kinematics or jet-driven outflows from those which might
harbour dual AGN. We observed four such candidate sources with the Very Long
Baseline Array (VLBA) at 1.5 GHz with 10 milli-arcsecond angular
resolution where spectral profiles of AGN optical emission suggested the
existence of dual AGN. In SDSS J210449.13-000919.1 and SDSS J23044.82-093345.3,
the radio structures are aligned with the optical emission features, thus the
double-peaked emission lines might be the results of jet-driven outflows. In
the third detected source SDSS J115523.74+150756.9, the radio structure is less
extended and oriented nearly perpendicular to the position angle derived from
optical spectroscopy. The fourth source remained undetected with the VLBA but
it has been imaged with the Very Large Array at arcsec resolution a few months
before our observations, suggesting the existence of extended radio structure.
In none of the four sources did we detect two radio-emitting cores, a
convincing signature of duality.Comment: 35 pages, 3 figures, 2 tables, accepted for publication in Ap
Tidal Disruption of Stellar Objects by Hard Supermassive Black Hole Binaries
Supermassive black hole binaries (SMBHBs) are expected by the hierarchical
galaxy formation model in CDM cosmology. There is some evidence in the
literature for SMBHBs in AGNs, but there are few observational constraints on
the evolution of SMBHBs in inactive galaxies and gas-poor mergers. On the
theoretical front, it is unclear how long is needed for a SMBHB in a typical
galaxy to coalesce. In this paper we investigate the tidal interaction between
stars and binary BHs and calculate the tidal disruption rates of stellar
objects by the BH components of binary. We derive the interaction cross
sections between SMBHBs and stars from intensive numerical scattering
experiments with particle number and calculate the tidal disruption
rates by both single and binary BHs for a sample of realistic galaxy models,
taking into account the general relativistic effect and the loss cone refilling
because of two-body interaction. We estimate the frequency of tidal flares for
different types of galaxies using the BH mass function in the literature. We
find that because of the three-body slingshot effect, the tidal disruption rate
in SMBHB system is more than one order of magnitude smaller than that in single
SMBH system. The difference is more significant in less massive galaxies and
does not depend on detailed stellar dynamical processes. Our calculations
suggest that comparisons of the calculated tidal disruption rates for both
single and binary BHs and the surveys of X-ray or UV flares at galactic centers
could tell us whether most SMBHs in nearby galaxies are single and whether the
SMBHBs formed in gas-poor galaxy mergers coalesce rapidly.Comment: 43 pages, 12 figures, 2 tables; accepted for publication in Ap
Super-solar N/C in the NLS1 Galaxy Markarian 1044
Narrow-Line Seyfert 1s (NLS1s) are known to have extreme values of a number
of properties compared to Active Galactic Nuclei (AGN) as a class. In
particular, previous emission-line studies have suggested that NLS1s are
unusually metal rich compared to broad-line AGN of comparable luminosity. We
present low- and medium-resolution spectroscopic observations of the NLS1
Markarian 1044 with the Hubble Space Telescope Imaging Spectrometer (STIS). We
identify two blueshifted intrinsic absorption systems at -1145 and -295 km/s
relative to the systemic velocity of the galaxy. Using a simple photoionization
model of the absorbing gas, we find that the strongest and best-measured of the
absorption systems has N/C approximately 6.96 times the solar value. We also
report on the discovery of three new Ly-alpha forest lines with neutral
Hydrogen column density log greater than 12.77 in the log. This number is
consistent with the 2.6 expected in the path length to the source redshift of
Mrk 1044.Comment: Submitted to ApJ. 21 pages including 4 figures & 5 table
GRB 110328A/Swift J164449.3+573451: The Tidal Obliteration of a Deeply Plunging Star?
We examine the tidal disruption event scenario to explain Sw 1644+57, a
powerful and persistent X-ray source which suddenly became active as GRB
110328A. The precise localization at the center of a z=0.35 galaxy argues for
activity of the central engine as the underlying cause. We look at the
suggestion by Bloom et al of the possibility of a tidal disruption event (TDE).
We argue that Sw 1644+57 cannot be explained by the traditional TDE model in
which the periastron distance is close to the tidal disruption radius - three
independent lines of argument indicate the orbit must be deeply plunging or
else the powerful jet we are observing could not be produced. These arguments
stem from (i) comparing the early X-ray light curve to the expected theoretical
fallback rate, (ii) looking at the time of transition to disk-dominated decay,
and (iii) considering the TDE rate. Due to the extreme excess in the tidal
force above that which would be required minimally to disrupt the star in a
deeply plunging orbit at periastron, we suggest this scenario might be referred
to more descriptively as a TOE (tidal obliteration event) rather than a TDE.Comment: 7 pages, 2 figures, accepted by the Astrophysical Journal, major
revisions since vers. [1]; corrupted file in vers. [2] replace
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