ABSTRACT The origin of the broad line region (BLR) in active galaxies remains unknown. It seems to be related to the underlying accretion disk, but an efficient mechanism is required to raise the ...material from the disk surface without giving signatures of the outflow that are too strong in the case of the low ionization lines. We discuss in detail two proposed mechanisms: (1) radiation pressure acting on dust in the disk atmosphere creating a failed wind and (2) the gravitational instability of the underlying disk. We compare the predicted location of the inner radius of the BLR in those two scenarios with the observed position obtained from the reverberation studies of several active galaxies. The failed dusty outflow model well represents the observational data while the predictions of the self-gravitational instability are not consistent with observations. The issue that remains is why do we not see any imprints of the underlying disk instability in the BLR properties.
Most results of the reverberation monitoring of active galaxies showed a universal scaling of the time delay of the Hβ emission region with the monochromatic flux at 5100 , with very small ...dispersion. Such a scaling favored the dust-based formation mechanism of the broad-line region (BLR). Recent reverberation measurements showed that actually a significant fraction of objects exhibit shorter lags than the previously found scaling. Here we demonstrate that these shorter lags can be explained by the old concept of scaling of the BLR size with the ionization parameter. Assuming a universal value of this parameter and a universal value of the cloud density reproduces the distribution of observational points in the time delay-monochromatic flux plane, provided that a range of black hole spins is allowed. However, a confirmation of the new measurements for low/moderate Eddington ratio sources is strongly needed before the dust-based origin of the BLR can be excluded.
Abstract
The close neighborhood of a supermassive black hole contains not only the accreting gas and dust but also stellar-sized objects, such as late-type and early-type stars and compact remnants ...that belong to the nuclear star cluster. When passing through the accretion flow, these objects perturb it by the direct action of stellar winds, as well as their magnetic and gravitational effects. By performing general-relativistic magnetohydrodynamic simulations, we investigate how the passages of a star can influence the supermassive black hole gaseous environment. We focus on the changes in the accretion rate and the emergence of blobs of plasma in the funnel of an accretion torus. We compare results from 2D and 3D numerical computations that have been started with comparable initial conditions. We find that a quasi-stationary inflow can be temporarily inhibited by a transiting star, and the plasmoids can be ejected along the magnetic field lines near the rotation axis. We observe the characteristic signatures of the perturbing motion in the power spectrum of the accretion variability, which provides an avenue for a multi-messenger detection of these transient events. Finally, we discuss the connection of our results to multiwavelength observations of galactic nuclei, with the emphasis on ten promising sources (Sgr A*, OJ 287, J0849+5108, RE J1034+396, 1ES 1927+65, ESO 253–G003, GSN 069, RX J1301.9+2747, eRO-QPE1, and eRO-QPE2).
Abstract According to general relativity, astrophysical black holes are described by a small number of parameters. Apart from the mass of the black hole ( M ), among the most interesting ...characteristics is the spin ( a ), which determines the degree of rotation, i.e., the angular momentum of the black hole. The latter is observationally constrained by the spectral and timing properties of the radiation signal emerging from an accretion disk of matter orbiting near the event horizon. In the case of the planar (standard, equatorial) accretion disk, this is the location of the innermost stable circular orbit that determines the observable radiation characteristics and allows us to measure the spin. In this paper, we discuss a more general case of the innermost stable spherical orbits (ISSOs) extending above and below the equatorial plane. To this end, we study the nonequatorial geodesic motion of particles following inclined, spherical, relativistically precessing trajectories with the aim of exploring the boundary between the regions of stable (energetically bound) and escaping (energetically unbound) motion. The concept of the radius of the ISSO should play a role in determining the inner rim of a tilted or geometrically thick accretion flow. We demonstrate that the region of inclined bound orbits has a complicated structure due to enhanced precession near the inner rim. We also explore the fate of particles launched below the radius of the marginally bound spherical orbit: these may either plunge into the event horizon or escape to radial infinity.
Context. Supermassive black holes reside in cores of galaxies, where they are often surrounded by a nuclear cluster and a clumpy torus of gas and dust. Mutual interactions can set some stars on a ...plunging trajectory towards the black hole. Aims. We model the pericentre passage of a dust-enshrouded star during which the dusty envelope becomes stretched by tidal forces and is affected by the interaction with the surrounding medium. In particular, we explore under which conditions these encounters can lead to periods of enhanced accretion activity. Methods. We discuss different scenarios for such a dusty source. To this end, we employed a modification of the Swift integration package. Elements of the cloud were modelled as numerical particles that represent the dust component that interacts with the optically thin gaseous environment. Results. We determine the fraction of the total mass of the dust component that is diverted from the original path during the passages through the pericentre at ≃103 Schwarzschild radii and find that the main part of the dust (≳90% of its mass) is significantly affected upon the first crossing. The fraction of mass captured at the second passage generally decreases to very low values. Conclusions. As an example, we show predictions for the dusty source evolution assuming the current orbital parameters of the G2 cloud (also known as Dusty S-Cluster Object, DSO) in our Galactic centre. Encounter of a core-less cloud with a supermassive black hole is, most likely, a non-repeating event: the cloud is destroyed. However, in the case of a dust-enshrouded star, part of the envelope survives the pericentre passage. We discuss an offset of ≲0.3 arcsec between the centre of mass of the diverted part and the star along the eccentric orbit. Finally, we examine an interesting possibility of a binary star embedded within a common wind envelope that becomes dispersed at the pericentre passage.
An interplay of magnetic fields and gravitation drives accretion and outflows near black holes. However, a specific mechanism is still a matter of debate; it is very likely that different processes ...dominate under various conditions. In particular, for the acceleration of particles and their collimation in jets, an ordered component of the magnetic field seems to be essential. Here we discuss the role of large-scale magnetic fields in transporting the charged particles and dust grains from the bound orbits in the equatorial plane of a rotating (Kerr) black hole and the resulting acceleration along trajectories escaping the system in a direction parallel to the symmetry axis (perpendicular to the accretion disk). We consider a specific scenario of destabilization of circular geodesics of initially neutral matter by charging (e.g., due to photoionization). Some particles may be set on escaping trajectories and attain relativistic velocity. The case of charged particles differs from charged dust grains by their charge-to-mass ratio, but the acceleration mechanism operates in a similar manner. It appears that the chaotic dynamics controls the outflow and supports the formation of near-horizon escape zones. We employ the technique of recurrence plots to characterize the onset of chaos in the outflowing medium. We investigate the system numerically and construct the basin-boundary plots, which show the location and the extent of the escape zones. The effects of black hole spin and magnetic field strength on the formation and location of escape zones are discussed, and the maximal escape velocity is computed.
We have explored the ideas that parametric resonance affects nearly geodesic motion around a black hole or a neutron star, and that it may be relevant to the high-frequency (twin) quasi-periodic ...oscillations that occur in some low-mass X-ray binaries. We have assumed the particles or fluid elements of an accretion disc to be subject to an isotropic perturbation having a hypothetical but rather general form. We find that the parametric resonance is indeed excited close to the radius where epicyclic frequencies of the radial and meridional oscillations are in a
$2:3$
ratio. The location and frequencies of the highest amplitude excitation vary with the strength of the perturbation. These results agree with actual frequency ratios of twin kHz QPOs that have been reported in some black hole candidates, and they may be consistent also with correlation of the twin peaks in Sco X-1.
ABSTRACT
Spectral energy distribution (SED) of the broad-band continuum emission from black-hole accretion discs can serve as a tool to measure parameters of the central body and constrain the ...geometry of the inner accretion flow. We focus on the case of an active galactic nucleus (AGN), with an accretion disc dominating the UV/optical bands. We parametrize the changes in the thermal and power-law components, which can reveal the diminution of the emissivity. To this end we explore the effects of gaps in the accretion disc and the emerging SED that can be caused by the presence of either (i) the inner, optically thin, radiatively inefficient hot flow; (ii) a secondary black hole embedded within the accretion disc; or (iii) a combination of both components. We suggest that the resulting changes in the SED of the underlying continuum can help us understand some departures from the standard-disc scenario. We estimate that the data required for such a project must be sampled in detail over the far-UV to soft X-ray bands during the interval of about a month corresponding to the characteristic variability time-scale of an AGN. Detecting a gap at intermediate radii of a few 100 gravitational radii would require quality photometry with uncertainties up to $\sim 1~{{\ \rm per\ cent}}$. The presence of the central cavity in the standard disc can be recovered in UV photometric data with an accuracy of 5 per cent and better. We show the effect of the intrinsic reddening of the source and demonstrate when it can be disentangled.
ABSTRACT
High-sensitivity X-ray polarimetric observations of black hole X-ray binaries, which will soon become available with the launches of space-borne X-ray observatories with sensitive X-ray ...polarimeters, will be able to put independent constraints on the black hole as well as the accretion flow, and possibly break degeneracies that cannot be resolved by spectral/timing observations alone. In this work, we perform a series of general relativistic Monte Carlo radiative transfer simulations to study the expected polarization properties of X-ray radiation emerging from lamp-post coronae in black hole X-ray binaries. We find that the polarization degree of the coronal emission of black hole X-ray binaries is sensitive to the spin of the black hole, the height of the corona, and the dynamics of the corona.
Abstract
Observations of the near-infrared excess object G2/DSO increased attention toward the Galactic center and its vicinity. The predicted flaring event in 2014 and the outcome of the intense ...monitoring of the supermassive black hole in the center of our Galaxy did not fulfill all predictions about a significantly enhanced accretion event. Subsequent observations addressed the question concerning the nature of the object because of its compact shape, especially during its periapse in 2014. Theoretical approaches have attempted to answer the contradictory behavior of the object, resisting the expected dissolution of a gaseous cloud due to tidal forces in combination with evaporation and hydrodynamical instabilities. However, assuming that the object is instead a dust-enshrouded young stellar object seems to be in line with the predictions of several groups and observations presented in numerous publications. Here we present a detailed overview and analysis of the observations of the object that have been performed with SINFONI (VLT) and we provide a comprehensive approach to clarify the nature of G2/DSO. We show that the tail emission consists of two isolated and compact sources with different orbital elements for each source rather than an extended and stretched component as it appeared in previous representations of the same data. Considering our recent publications, we propose that the monitored dust-enshrouded objects are remnants of a dissolved young stellar cluster whose formation was initiated in the circumnuclear disk. This indicates a shared history, which agrees with our analysis of the D- and X-sources.
PDF
