Fractal features in accretion discs

Fractal concepts have been introduced in the accretion disc as a new feature. Due to the fractal nature of the flow, its continuity condition undergoes modifications. The conserved stationary fractal flow admits only saddle points and centre-type points in its phase portrait. Completely analytical solutions of the equilibrium point conditions indicate that the fractal properties enable the flow to behave like an effective continuum of lesser density, and facilitate the generation of transonicity. However, strongly fractal flows inhibit multitransonicity from developing. The mass accretion rate exhibits a fractal scaling behaviour, and the entire fractal accretion disc is stable under linearized dynamic perturbations.     

Broad reprocessed Balmer emission from warped accretion discs

Double-peaked broad emission lines in active galactic nuclei are generally considered to be formed in an accretion disc. In this paper, we compute the profiles of reprocessing emission lines from a relativistic, warped accretion disc around a black hole in order to explore the possibility that certain asymmetries in the double-peaked emission-line profile which cannot be explained by a circular Keplerian disc may be induced by disc warping. The disc warping also provides a solution for the energy budget in the emission-line region because it increases the solid angle of the outer disc portion subtended to the inner portion of the disc. We adopted a parametrized disc geometry and a central point-like source of ionizing radiation to capture the main characteristics of the emission-line profile from such discs. We find that the ratio between the blue and red peaks of the line profiles becoming less than unity can be naturally predicted by a twisted warped disc, and a third peak can be produced in some cases. We show that disc warping can reproduce the main features of multipeaked line profiles of four active galactic nuclei from the Sloan Digital Sky Survey.     

A steady-state solution for warped accretion discs

We consider a thin accretion disc warped due to the Bardeen–Petterson effect, presenting both analytical and numerical solutions for the situation in which the two viscosity coefficients vary with radius as a power law, with the two power-law indices not necessarily equal. The analytical solutions are compared with numerical ones, showing that our new analytical solution is more accurate than the previous one, which overestimated the inclination change in the outer disc. Our new analytical solution is appropriate for moderately warped discs, while for extremely misaligned discs only a numerical solution is appropriate.     

Delayed X-ray emission from fallback in compact-object mergers

When double neutron star or neutron star–black hole binaries merge, the final remnant may comprise a central solar-mass black hole surrounded by a  ∼0.01–0.1 M_⊙  torus. The subsequent evolution of this disc may be responsible for short γ-ray bursts (SGRBs). A comparable amount of mass is ejected into eccentric orbits and will eventually fallback to the merger site after ∼0.01 s. In this paper, we investigate analytically the fate of the fallback matter, which may provide a luminous signal long after the disc is exhausted. We find that matter in the eccentric tail returns at a super-Eddington rate and eventually (≳0.1 s) is unable to cool via neutrino emission and accrete all the way to the black hole. Therefore, contrary to previous claims, our analysis suggests that fallback matter is not an efficient source of late-time accretion power and unlikely to cause the late-flaring activity observed in SGRB afterglows. The fallback matter rather forms a radiation-driven wind or a bound atmosphere. In both the cases, the emitting plasma is very opaque and photons are released with a degraded energy in the X-ray band. We therefore suggest that compact binary mergers could be followed by an 'X-ray renaissance', as late as several days to weeks after the merger. This might be observed by the next generation of X-ray detectors.     

A comparative analysis of standard accretion discs spectra: an application to ultraluminous X-ray sources

We compare standard models of accretion discs around black holes (BHs) that include the appropriate zero-torque inner boundary condition and relativistic effects on the emission and propagation of radiation. The comparison is performed adopting the multicolour disc blackbody model (MCD) as reference and looking for the parameter space in which it is in statistical agreement with 'more physical' accretion disc models. We find simple 'recipes' that can be used for adjusting the estimates of the physical inner radius of the disc, the BH mass and the accretion rate inferred using the parameters of the MCD fits. We applied these results to four ultraluminous X-ray sources for which MCD spectral fits of their X-ray soft spectral components have been published and find that, in three cases (NGC 1313 X-1, X-2 and M 81 X-9), the BH masses inferred for a standard disc around a Schwarzschild BH are in the interval  ∼100–200 M_⊙  . Only if the BH is maximally rotating are the masses comparable to the much larger values previously derived in the literature.     

Magnetic connection and current distribution in black hole accretion discs

We discuss one of the possible origins of large-scale magnetic fields based on a continuous distribution of toroidal electric current flowing in the inner region of the disc around a Kerr black hole (BH) in the framework of general relativity. It turns out that four types of configuration of the magnetic connection (MC) are generated, i.e. MC of the BH with the remote astrophysical load (MCHL), MC of the BH with the disc (MCHD), MC of the plunging region with the disc (MCPD) and MC of the inner and outer disc regions (MCDD). It turns out that the Blandford–Znajek process can be regarded as one type of MC, i.e. MCHL. In addition, we propose a scenario for fitting the quasi-periodic oscillations in BH binaries based on MCDD associated with the magnetic reconnection.     

Iron line profiles and self-shadowing from relativistic thick accretion discs

We present Fe Kα line profiles from and images of relativistic discs with finite thickness around a rotating black hole using a novel code. The line is thought to be produced by iron fluorescence of a relatively cold X-ray-illuminated material in the innermost parts of the accretion disc and provides an excellent diagnostic of accretion flows in the vicinity of black holes. Previous studies have concentrated on the case of a thin, Keplerian accretion disc. This disc must become thicker and sub-Keplerian with increasing accretion rates. These can affect the line profiles and in turn can influence the estimation of the accretion disc and black hole parameters from the observed line profiles. We here embark on, for the first time, a fully relativistic computation which offers key insights into the effects of geometrical thickness and the sub-Keplerian orbital velocity on the line profiles. We include all relativistic effects such as frame-dragging, Doppler boost, time dilation, gravitational redshift and light bending. We find that the separation and the relative height between the blue and red peaks of the line profile diminish as the thickness of the disc increases. This code is also well suited to produce accretion disc images. We calculate the redshift and flux images of the accretion disc and find that the observed image of the disc strongly depends on the inclination angle. The self-shadowing effect appears remarkable for a high inclination angle, and leads to the black hole shadow being in this case, completely hidden by the disc itself.