ABSTRACT Stars that pass within the Roche radius of a supermassive black hole will be tidally disrupted, yielding a sudden injection of gas close to the black hole horizon which produces an ...electromagnetic flare. A few dozen of these flares have been discovered in recent years, but current observations provide poor constraints on the bolometric luminosity and total accreted mass of these events. Using images from the Wide-field Infrared Survey Explorer, we have discovered transient 3.4 m emission from several previously known tidal disruption flares. The observations can be explained by dust heated to its sublimation temperature due to the intense radiation of the tidal flare. From the break in the infrared light curve we infer that this hot dust is located ∼0.1 pc from the supermassive black hole. Since the dust has been heated by absorbing UV and (potentially) soft X-ray photons of the flare, the reprocessing light curve yields an estimate of the bolometric flare luminosity. For the flare PTF-09ge, we infer that the most likely value of the luminosity integrated over frequencies at which dust can absorb photons is erg s−1, with a factor of 3 uncertainty due to the unknown temperature of the dust. This bolometric luminosity is a factor ∼10 larger than the observed blackbody luminosity. Our work is the first to probe dust in the nuclei of non-active galaxies on sub-parsec scales. The observed infrared luminosity implies a covering factor ∼1% for the nuclear dust in the host galaxies.
Magnetically Driven Jets in the Kerr Metric Hawley, John F; Krolik, Julian H
Astrophysical journal/The Astrophysical journal,
04/2006, Volume:
641, Issue:
1
Journal Article
Peer reviewed
Open access
We compute a series of three-dimensional general relativistic magnetohydrodynamic simulations of accretion flows in the Kerr metric and investigate the properties of the resulting unbound outflows. ...The strength of the outflows increases sharply with increasing black hole rotation rate. Several generic features are observed. The mass in the outflow in concentrated in a hollow cone whose opening angle is largely determined by the effective potential for matter with specific angular momentum comparable to that of the innnermost stable circular orbit. The pressure in the accretion disk's corona provides the dominant force accelerating the matter outward. The principle element shaping the outflow is therefore the centrifugal barrier preventing accreting matter from coming close to the rotation axis. The funnel inside the centrifugal barrier contains very little matter and is dominated by electromagnetic fields that rotate at a rate tied closely to the rotation of the black hole, even when the black hole spins in a sense opposite to the rotation of the accretion flow. These fields carry an outward-going Poynting flux whose immediate energy source is the rotating spacetime of the Kerr black hole. When the spin parameter a/M of the black hole exceeds 0.9, the energy carried to infinity by these outflows can be comparable to the nominal radiative efficiency predicted in the Novikov-Thorne model. Similarly, the expelled angular momentum can be comparable to that accreted by the black hole. Both the electromagnetic and the matter outflows contribute significantly to the energy and angular momentum of the outflow.
ABSTRACT
When a star gets too close to a supermassive black hole, it is torn apart by the tidal forces. Roughly half of the stellar mass becomes unbound and flies away at tremendous velocities – ...around 104 km s−1. In this work, we explore the idea that the shock produced by the interaction of the unbound debris with the ambient medium gives rise to the synchrotron radio emission observed in several tidal disruption event (TDE). We use a moving mesh numerical simulation to study the evolution of the unbound debris and the bow shock around it. We find that as the periapse distance of the star decreases, the outflow becomes faster and wider. A TDE whose periapse distance is a factor of 7 smaller than the tidal radius can account for the radio emission observed in ASASSN-14li. This model also allows us to obtain a more accurate estimate for the gas density around the centre of the host galaxy of ASASSN-14li.
We have carried out fully relativistic numerical simulations of accretion disks in the Kerr metric. In this paper we focus on the unbound outflows that emerge self-consistently from the accretion ...flow. These outflows are found in the axial funnel region and consist of two components: a hot, fast, tenuous outflow in the axial funnel proper and a colder, slower, denser jet along the funnel wall. The funnel-wall jet is excluded from the axial funnel by elevated angular momentum and is also pressure-confined by a magnetized corona. Inside the funnel, a large-scale poloidal magnetic field spontaneously arises from the coupled dynamics of accretion and outflow, although there was no large-scale field in the initial state. Black hole rotation is not required to produce these unbound outflows, but their strength is enhanced by black hole spin. When the black hole spins rapidly, the energy ejected can be tens of percent of the accreted rest mass. At low spin, kinetic energy and enthalpy of the matter dominate the outflow energetics; at high spin, the balance shifts to Poynting flux. We compare the outflows observed in our simulations with those seen in other simulations.
We calculate the vertical structure of a local patch of an accretion disk in which heating by dissipation of MRI-driven MHD turbulence is balanced by radiative cooling. Heating, radiative transport, ...and cooling are computed self-consistently with the structure by solving the equations of radiation MHD in the shearing-box approximation. Using a fully three-dimensional and energy-conserving code, we compute the structure of this disk segment over a span of more than five cooling times. After a brief relaxation period, a statistically steady state develops. Measuring height above the midplane in units of the scale height predicted by a Shakura-Sunyaev model, we find that magnetic pressure causes the disk atmosphere to stretch upward, with the photosphere rising to 7H, in contrast to the 3H predicted by conventional analytic models. This more extended structure, as well as fluctuations in the height of the photosphere, may lead to departures from Planckian form in the emergent spectra. Dissipation is distributed across the region within 3H of the midplane but is very weak at greater altitudes. As a result, the temperature deep in the disk interior is less than that expected when all heat is generated in the midplane. With only occasional exceptions, the gas temperature stays very close to the radiation temperature, even above the photosphere. Because fluctuations in the dissipation are particularly strong away from the midplane, the emergent radiation flux can track dissipation fluctuations with a lag that is only 0.1-0.2 times the mean cooling time of the disk. Long-timescale asymmetries in the dissipation distribution can also cause significant asymmetry in the flux emerging from the top and bottom surfaces of the disk. Radiative diffusion dominates Poynting flux in the vertical energy flow throughout the disk.
This paper continues the analysis of a set of general relativistic three-dimensional MHD simulations of accreting tori in the Kerr metric with different black hole spins. We focus on bound matter ...inside the initial pressure maximum, where the time-averaged motion of gas is inward and an accretion disk forms. We use the flows of mass, angular momentum, and energy in order to understand dynamics in this region. The sharp reduction in accretion rate with increasing black hole spin reported in the first paper of this series is explained by a strongly spin-dependent outward flux of angular momentum conveyed electromagnetically; when a/M . 0.9, this flux can be comparable to the inward angular momentum flux carried by the matter. In all cases, there is outward electromagnetic angular momentum flux throughout the flow; in other words, contrary to the assertions of traditional accretion disk theory, there is in general no "stress edge," no surface within which the stress is zero. The retardation of accretion in the inner disk by electromagnetic torques also alters the radial distribution of surface density, an effect that may have consequences for observable properties, such as Compton reflection. The net accreted angular momentum is sufficiently depressed by electromagnetic effects that in the most rapidly spinning black holes mass growth can lead to spin-down. Spinning black holes also lose energy by Poynting flux; this rate is also a strongly increasing function of black hole spin, rising to 10% of the rest-mass accretion rate at very high spin. As the black hole spins faster, the path of the Poynting flux changes from being predominantly within the accretion disk to being predominantly within the funnel outflow.
We analyze observations obtained with the Chandra X-Ray Observatory of bright Compton-thick active galactic nuclei (AGNs), those with column densities in excess of 1.5 x 10 super(24) cm super(-2) ...along the lines of sight. We therefore view the powerful central engines only indirectly, even at X-ray energies. Using high spatial resolution and considering only galaxies that do not contain circumnuclear starbursts, we reveal the variety of emission AGNs alone may produce. Approximately 1% of the continuum's intrinsic flux is detected in reflection in each case. The only hard X-ray feature is the prominent Fe Ka fluorescence line, with equivalent width greater than 1 keV in all sources. The Fe line luminosity provides the best X-ray indicator of the unseen intrinsic AGN luminosity. In detail, the morphologies of the extended soft X-ray emission and optical line emission are similar, and line emission dominates the soft X-ray spectra. Thus, we attribute the soft X-ray emission to material that the central engines photoionize. Because the resulting spectra are complex and do not reveal the AGNs directly, crude analysis techniques, such as hardness ratios, would misclassify these galaxies as hosts of intrinsically weak, unabsorbed AGNs and would fail to identify the luminous, absorbed nuclei that are present. We demonstrate that a three-band X-ray diagnostic can correctly classify Compton-thick AGNs, even when significant soft X-ray line emission is present. The active nuclei produce most of the galaxies' total observed emission over a broad spectral range, and much of their light emerges at far-infrared wavelengths. Stellar contamination of the infrared emission can be severe, however, making long-wavelength data alone unreliable indicators of the buried AGN luminosity.
We have compiled a large sample of low-redshift active galactic nuclei (AGNs) identified via their emission-line characteristics from the spectroscopic data of the Sloan Digital Sky Survey. Since ...emission lines are often contaminated by stellar absorption lines, we developed an objective and efficient method of subtracting the stellar continuum from every galaxy spectrum before making emission-line measurements. The distribution of the measured Ha FWHM values of emission-line galaxies is strongly bimodal, with two populations separated at about 1200 km s-1. This feature provides a natural separation between narrow-line and broad-line AGNs. The narrow-line AGNs are identified using standard emission-line ratio diagnostic diagrams. There are 1317 broad-line and 3074 narrow-line AGNs identified from about 100,000 galaxy spectra selected over 1151 deg2. This sample is used in a companion paper to determine the emission-line luminosity function of AGNs.
Explicit 2D axisymmetric solutions are found to the hydrostatic equilibrium, energy balance, and photon diffusion equations within obscuring tori around active galactic nuclei (AGNs). These solutions ...demonstrate that infrared radiation pressure can support geometrically thick structures in AGN environments subject to certain constraints: the bolometric luminosity must be roughly similar to 0.03-1 times the Eddington luminosity; and the Compton optical depth of matter in the equatorial plane should be similar to 1, with a tolerance of about an order of magnitude up or down. Both of these constraints are at least roughly consistent with observations. In addition, angular momentum must be redistributed so that the fractional rotational support against gravity rises from the inner edge of the torus to the outer in a manner specific to the detailed shape of the gravitational potential. This model also predicts that the column densities observed in obscured AGNs should range from similar to 10 super(22) to similar to 10 super(24) cm super(-2).
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