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Register a new userThe dynamics of twisted disc formed after the tidal disruption of a star by a rotating black hole
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We consider misaligned accretion discs formed after tidal disruption events occurring when a star encounters a supermassive rotating black hole. We use the linear theory of warped accretion discs to find the disc shape when the stream produced by the disrupted star provides a source of mass and angular momentum that is misaligned with the black hole. The evolution of the surface density and aspect ratio is found from a one dimensional vertically averaged model. We extend previous work which assumed a quasi-stationary disc to allow unrestricted dynamical propagation of disc tilt and twist through time dependent backgrounds. We consider a smaller value of the viscosity parameter, $alpha =0.01,$ finding the dynamics varies significantly. At early times the disc inclination is found to be nearly uniform at small radii where the aspect ratio is large. However, since torques arise from the Lense-Thirring effect and the stream there is non uniform precession. We propose a simple model for this requiring only the background surface density and aspect ratio. At these times the $alpha sim 0.01$ disc exhibits a new feature. An inclined hot inner region joins an outer low inclination cool region via a thin transition front propagating outwards with a speed exceeding that of bending waves in the cool region. These waves accumulate where the propagation speeds match producing an inclination spike separating inner and outer discs. At late times a sequence of quasi-stationary configurations approximates disc shapes at small radii. We discuss observational implications of our results.
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We investigate misaligned accretion discs formed after tidal disruption events that occur when a star encounters a supermassive black hole. We employ the linear theory of warped accretion discs to find the shape of a disc for which the stream arising from the disrupted star provides a source of angular momentum that is misaligned with that of the black hole. For quasi-steady configurations we find that when the warp diffusion or propagation time is large compared to the local mass accretion time and/or the natural disc alignment radius is small, misalignment is favoured. These results have been verified using SPH simulations. We also simulated 1D model discs including gas and radiation pressure. As accretion rates initially exceed the Eddington limit the disc is initially advection dominated. Assuming the $alpha$ model for the disc, where it can be thermally unstable it subsequently undergoes cyclic transitions between high and low states. During these transitions the aspect ratio varies from $sim 1$ to $sim 10^{-3}$ which is reflected in changes in the degree of disc misalignment at the stream impact location. For maximal black hole rotation and sufficiently large values of viscosity parameter $alpha > sim 0.01-0.1$ the ratio of the disc inclination to that of the initial stellar orbit is estimated to be $0.1-0.2$ in the advection dominated state, while reaching of order unity in the low state. Misalignment descreases with decrease of $alpha$, but increases as the black hole rotation parameter decreases. Thus, it is always significant when the latter is small.
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 $propto t^{-5/3}$, would stop at a time $T_{rm tr} simeq eta T_{rm b}$. Here, $eta sim0.25$ and $T_{rm b}$ is the orbital period of the SMBHB. After a period of interruption, the accretion recurs discretely at time $T_{rm r} simeq xi T_b$, where $xi sim 1$. Both $eta$ and $xi$ 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.
Galaxy mergers produce supermassive black hole binaries, which emit gravitational waves prior to their coalescence. We perform three-dimensional hydrodynamic simulations to study the tidal disruption of stars by such a binary in the final centuries of its life. We find that the gas stream of the stellar debris moves chaotically in the binary potential and forms accretion disks around both black holes. The accretion light curve is modulated over the binary orbital period owing to relativistic beaming. This periodic signal allows to detect the decay of the binary orbit due to gravitational wave emission by observing two tidal disruption events that are separated by more than a decade.
We study the final stages of the evolution of a binary system consisted of a black hole and a white dwarf star. We implement the quantum hydrodynamic equations and carry out numerical simulations. As a model of a white dwarf star, we consider a zero temperature droplet of attractively interacting degenerate atomic bosons and spin-polarized atomic fermions. Such mixtures are investigated experimentally nowadays. We find that the white dwarf star is stripped off its mass while passing the periastron. Due to nonlinear effects, the accretion disk originated from the white dwarf becomes fragmented and the onset of a quantum turbulence with giant quantized vortices present in the bosonic component of the accretion disk is observed. The binary system ends its life in a spectacular way, revealing quantum features underlying the white dwarf stars structure. We find a charged mass, falling onto a black hole, could be responsible for recently discovered ultraluminous X-ray bursts. The simulations show that the final passage of a white dwarf near a black hole can cause a gamma-ray burst.
Aims: A strong, hard X-ray flare was discovered (IGR J12580+0134) by INTEGRAL in 2011, and is associated to NGC 4845, a Seyfert 2 galaxy never detected at high-energy previously. To understand what happened we observed this event in the X-ray band on several occasions. Methods: Follow-up observations with XMM-Newton, Swift, and MAXI are presented together with the INTEGRAL data. Long and short term variability are analysed and the event wide band spectral shape modelled. Results: The spectrum of the source can be described with an absorbed (N_H ~ 7x10^22 cm^{-2}) power law (Gamma simeq 2.2), characteristic of an accreting source, plus a soft X-ray excess, likely to be of diffuse nature. The hard X-ray flux increased to maximum in a few weeks and decreased over a year, with the evolution expected for a tidal disruption event. The fast variations observed near the flare maximum allowed us to estimate the mass of the central black hole in NGC 4845 as ~ 3x10^5 Msun. The observed flare corresponds to the disruption of about 10% of an object with a mass of 14-30 Jupiter. The hard X-ray emission should come from a corona forming around the accretion flow close to the black hole. This is the first tidal event where such a corona has been observed.
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