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Register a new userEccentric Tidal Disruption Event Disks around Supermassive Black Holes: Dynamics and Thermal Emission
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After the Tidal Disruption Event (TDE) of a star around a SuperMassive Black Hole (SMBH), if the stellar debris stream rapidly circularizes and forms a compact disk, the TDE emission is expected to peak in the soft X-ray or far Ultra-Violet (UV). The fact that many TDE candidates are observed to peak in the near UV and optical has challenged conventional TDE emission models. By idealizing a disk as a nested sequence of elliptical orbits which communicate adiabatically via pressure forces, and are heated by energy dissipated during the circularization of the nearly parabolic debris streams, we investigate the dynamics and thermal emission of highly eccentric TDE disks, including the effect of General-Relativistic apsidal precession from the SMBH. We calculate the properties of uniformly precessing, apsidally aligned, and highly eccentric TDE disks, and find highly eccentric disk solutions exist for realistic TDE properties (SMBH and stellar mass, periapsis distance, etc.). Taking into account compressional heating (cooling) near periapsis (apoapsis), we find our idealized eccentric disk model can produce emission consistent with the X-ray and UV/Optical luminosities of many optically bright TDE candidates. Our work attempts to quantify the thermal emission expected from the shock-heating model for TDE emission, and finds stream-stream collisions are a promising way to power optically bright TDEs.
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After the Tidal Disruption Event (TDE) of a star around a SuperMassive Black Hole (SMBH), the bound stellar debris rapidly forms an accretion disk. If the accretion disk is not aligned with the spinning SMBHs equatorial plane, the disk will be driven into Lense-Thirring precession around the SMBHs spin axis, possibly affecting the TDEs light curve. We carry out an eigenmode analysis of such a disk to understand how the disks warp structure, precession, and inclination evolution are influenced by the disks and SMBHs properties. We find an oscillatory warp may develop as a result of strong non-Keplarian motion near the SMBH. The global disk precession frequency matches the Lense-Thirring precession frequency of a rigid disk around a spinning black hole within a factor of a few when the disks accretion rate is high, but deviates significantly at low accretion rates. Viscosity aligns the disk with the SMBHs equatorial plane over timescales of days to years, depending on the disks accretion rate, viscosity, and SMBHs mass. We also examine the effect of fall-back material on the warp evolution of TDE disks, and find that the fall-back torque aligns the TDE disk with the SMBHs equatorial plane in a few to tens of days for the parameter space investigated. Our results place constraints on models of TDE emission which rely on the changing disk orientation with respect to the line of sight to explain observations.
We report results from general relativistic radiation MHD (GRRMHD) simulations of a super-Eddington black hole (BH) accretion disk formed as a result of a tidal disruption event (TDE). We consider the fiducial case of a solar mass star on a mildly penetrating orbit disrupted by a supermassive BH of mass $10^6 , M_odot$, and consider the epoch of peak fall back rate. We post-process the simulation data to compute viewing angle dependent spectra. We perform a parameter study of the dynamics of the accretion disk as a function of BH spin and magnetic flux, and compute model spectra as a function of the viewing angle of the observer. We also consider detection limits based on the model spectra. We find that an accretion disk with a relatively weak magnetic field around the BH (so-called SANE regime of accretion) does not launch a relativistic jet, whether or not the BH is rotating. Such models reasonably reproduce several observational properties of non-jetted TDEs. The same is also true for a non-rotating BH with a strong magnetic field (MAD regime). One of our simulations has a rapidly rotating BH (spin parameter 0.9) as well as a MAD accretion disk. This model launches a powerful relativistic jet, which is powered by the BH spin energy. It reproduces the high energy emission and jet structure of the jetted TDE Swift J1644+57 surprisingly well. Jetted TDEs may thus correspond to the subset of TDE systems that have both a rapidly spinning BH and MAD accretion.
Binary stars that are on close orbits around massive black holes (MBH) such as Sgr A* in the center of the Milky Way are liable to undergo tidal disruption and eject a hypervelocity star. We study the interaction between such a MBH and circular binaries for general binary orientations and penetration depths (i.e. binaries penetrate into the tidal radius around the BH). We show that for very deep penetrators, all binaries are disrupted when the binary rotation axis is roughly oriented toward the BH or it is in the opposite direction. The surviving chance becomes significant when the angle between the binary rotation axis and the BH direction is between pi /4 and 3 pi /4. The surviving chance is as high as $sim$ 20$%$ when the binary rotation axis is perpendicular to the BH direction. The angular dependence is opposite for very shallow penetrators where coplanar prograde orbits have the lowest surviving chance (or equivalently most vulnerable). We provide numerical fits to the disruption probability and energy gain at the the BH encounter as a function of the penetration depth. The latter can be simply rescaled in terms of binary masses, their initial separation and the binary-to-BH mass ratio to evaluate the ejection velocity of a binary members in various systems. We also investigate the disruption of coplanar, eccentric binaries by a MBH. It is shown that for highly eccentric binaries retrograde orbits have a significantly increased disruption probability and ejection velocities compared to the circular binaries.
We use global three dimensional radiation magneto-hydrodynamical simulations to study accretion disks onto a $5times 10^8M_{odot}$ black hole with accretion rates varying from $sim 250L_{Edd}/c^2$ to $1500 L_{Edd}/c^2$. We form the disks with torus centered at $50-80$ gravitational radii with self-consistent turbulence initially generated by the magneto-rotational instability. We study cases with and without net vertical magnetic flux. The inner regions of all disks have radiation pressure $sim 10^4-10^6$ times the gas pressure. Non-axisymmetric density waves that steepen into spiral shocks form as gas flows towards the black hole. In simulations without net vertical magnetic flux, Reynolds stress generated by the spiral shocks are the dominant mechanism to transfer angular momentum. Maxwell stress from MRI turbulence can be larger than the Reynolds stress only when net vertical magnetic flux is sufficiently large. Outflows are formed with speed $sim 0.1-0.4c$. When the accretion rate is smaller than $sim 500 L_{Edd}/c^2$, outflows start around $10$ gravitational radii and the radiative efficiency is $sim 5%-7%$ with both magnetic field configurations. With accretion rate reaching $1500 L_{Edd}/c^2$, most of the funnel region close to the rotation axis becomes optically thick and the outflow only develops beyond $50$ gravitational radii. The radiative efficiency is reduced to $1%$. We always find the kinetic energy luminosity associated with the outflow is only $sim 15%-30%$ of the radiative luminosity. The mass flux lost in the outflow is $sim 15%-50%$ of the net mass accretion rates. We discuss implications of our simulation results on the observational properties of these disks.
The merger rate of stellar-mass black hole binaries (sBHBs) inferred by the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) suggests the need for an efficient source of sBHB formation. Active galactic nucleus (AGN) disks are a promising location for the formation of these sBHBs, as well as binaries of other compact objects, because of powerful torques exerted by the gas disk. These gas torques cause orbiting compact objects to migrate towards regions in the disk where inward and outward torques cancel, known as migration traps. We simulate the migration of stellar mass black holes in an example of a model AGN disk, using an augmented N-body code that includes analytic approximations to migration torques, stochastic gravitational forces exerted by turbulent density fluctuations in the disk, and inclination and eccentricity dampening produced by passages through the gas disk, in addition to the standard gravitational forces between objects. We find that sBHBs form rapidly in our model disk as stellar-mass black holes migrate towards the migration trap. These sBHBs are likely to subsequently merge on short time-scales. The process continues, leading to the build-up of a population of over-massive stellar-mass black holes. The formation of sBHBs in AGN disks could contribute significantly to the sBHB merger rate inferred by LIGO.
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