440 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&
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
X-ray spectra reveal the reawakening of the repeat changing-look AGN NGC 1566
We present simultaneous XMM-Newton and NuSTAR observations of the repeat
changing-look AGN NGC 1566, which dramatically increased in brightness in the
IR to X-ray bands in 2018. The broad-band X-ray spectrum was taken at the peak
of the outburst and is typical of Seyfert 1 AGN. The spectrum shows a soft
excess, Compton hump, warm absorption and reflection, ruling out tidal
disruption as the cause of the outburst and demonstrating that a 'standard'
accretion disk can develop very rapidly. The high resolution grating spectrum
reveals that the outburst has launched a ~ 500 km/s outflow, and shows
photoionised emission lines from rest-frame gas. We discuss possible mechanisms
for the outburst, and conclude that it is most likely caused by a disk
instability.Comment: 5 pages, 2 figures, accepted to MNRAS letter
Characterizing outdoor recreation user groups:a typology of peri-urban recreationists in the Kromme Rijn area, the Netherlands
Increasing urbanization accompanied by a growing demand for recreational use of peri-urban green spaces is likely to cause land-use conflicts. The nature of these conflicts is subject to the heterogeneous preferences of outdoor recreationists. Although the importance of diversifying between recreational user groups has been acknowledged in the literature, most studies - and especially studies on landscape preferences - focus on specific user groups and are not operationalized for creating a recreationist typology. This study presents a typology on case-study level accounting for variations in landscape preferences, visitation behavior, and socio-demographic information. The typology is based on data originating from a structured questionnaire with a total of 200 respondents. It is constructed by employing two commonly used data-driven methods for typology development: PCA of preferences for landscape characteristics and cluster analysis of all data (landscape preferences, visitation behavior and socio-economic variables) to derive alternative typologies. Comparing the results of both types of analysis leads to a consistent picture of the main differences between three distinct outdoor recreation user groups which we refer to as ‘the convenience recreationist’, ‘the day tripper’ and ‘the culture/nature recreationist’. The first user group prefers convenient, short-term recreation, close to home. The second group is characterized by clear preferences for one-day recreational activities and destinations, while the last group is mainly defined by its strong interest in culture and nature. Our study identified large variations in outdoor recreation preferences and recreation needs between these three user groups. Understanding the heterogeneity of recreation preferences can help to articulate effective landscape management strategies, targeted to ensure the multi-functional character of peri-urban landscapes for different types of users
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