Time Evolution of the 3-D Accretion Flows: Effects of the Adiabatic Index and Outer Boundary Condition

We study a slightly rotating accretion flow onto a black hole, using the fully three dimensional (3-D)numerical simulations. We consider hydrodynamics of an inviscid flow, assuming a spherically symmetric density distribution at the outer boundary and a small, latitude-dependent angular momentum. We investigate the role of the adiabatic index and gas temperature, and the flow behaviour due to non-axisymmetric effects. Our 3-D simulations confirm axisymmetric results: the material that has too much angular momentum to be accreted forms a thick torus near the equator and the mass accretion rate is lower than the Bondi rate. In our previous study of the 3-D accretion flows, for gamma=5/3, we found that the inner torus precessed, even for axisymmetric conditions at large radii. The present study shows that the inner torus precesses also for other values of the adiabatic index: gamma=4/3, 1.2 and 1.01. However, the time for the precession to set increases with decreasing gamma. In particular, for gamma=1.01 we find that depending on the outer boundary conditions, the torus may shrink substantially due to the strong inflow of the non-rotating matter and the precession will have insufficient time to develop. On the other hand, if the torus is supplied by the continuous inflow of the rotating material from the outer radii, its inner parts will eventually tilt and precess, as it was for the larger gamma's.Comment: 19 pages, 19 figures; accepted to ApJ; version with full resolution figures may be downloaded from http://users.camk.edu.pl/agnes/publ_en.htm

Conditions for the Thermal Instability in the Galactic Centre Mini-spiral region

We explore the conditions for the thermal instability to operate in the mini-spiral region of the Galactic centre (Sgr A*), where both the hot and cold media are known to coexist. The photoionisation Cloudy calculations are performed for different physical states of plasma. We neglect the dynamics of the material and concentrate on the study of the parameter ranges where the thermal instability may operate, taking into account the past history of Sgr A* bolometric luminosity. We show that the thermal instability does not operate at the present very low level of the Sgr A* activity. However, Sgr A* was much more luminous in the past. For the highest luminosity states the two-phase medium can be created up to 1.4 pc from the centre. The presence of dust grains tends to suppress the instability, but the dust is destroyed in the presence of strong radiation field and hot plasma. The clumpiness is thus induced in the high activity period, and the cooling/heating timescales are long enough to preserve later the past multi-phase structure. The instability enhances the clumpiness of the mini-spiral medium and creates a possibility of episodes of enhanced accretion of cold clumps towards Sgr A*. The mechanism determines the range of masses and sizes of clouds; under the conditions of Sgr A*, the likely values come out $1$ - $10^2M_{\oplus}$ for the cloud typical mass.Comment: Accepted for publication in MNRAS, 10 pages, 7 figure

Pair Production in Low Luminosity Galactic Nuclei

Electron-positron pairs may be produced near accreting black holes by a variety of physical processes, and the resulting pair plasma may be accelerated and collimated into a relativistic jet. Here we use a self-consistent dynamical and radiative model to investigate pair production by \gamma\gamma collisions in weakly radiative accretion flows around a black hole of mass M and accretion rate \dot{M}. Our flow model is drawn from general relativistic magnetohydrodynamic simulations, and our radiation field is computed by a Monte Carlo transport scheme assuming the electron distribution function is thermal. We argue that the pair production rate scales as r^{-6} M^{-1} \dot{M}^{6}. We confirm this numerically and calibrate the scaling relation. This relation is self-consistent in a wedge in M, \dot{M} parameter space. If \dot{M} is too low the implied pair density over the poles of the black hole is below the Goldreich-Julian density and \gamma\gamma pair production is relatively unimportant; if \dot{M} is too high the models are radiatively efficient. We also argue that for a power-law spectrum the pair production rate should scale with the observables L_X \equiv X-ray luminosity and M as L_X^2 M^{-4}. We confirm this numerically and argue that this relation likely holds even for radiatively efficient flows. The pair production rates are sensitive to black hole spin and to the ion-electron temperature ratio which are fixed in this exploratory calculation. We finish with a brief discussion of the implications for Sgr A* and M87.Comment: 21 pages, 10 figures, 1 table. Accepted for publication in Ap

Aberrational Effects for Shadows of Black Holes

In this paper, we discuss how the shadow of a Kerr black hole depends on the motion of the observer. In particular, we derive an analytical formula for the boundary curve of the shadow for an observer moving with given four-velocity at given Boyer--Lindquist coordinates. We visualize the shadow for various values of parameters.Comment: 12 pages, 3 figures; Proceedings of the 524. WE-Heraeus-Seminar held at the Physikzentrum, Bad Honnef, Germany, 17.--23.2.201

Observing supermassive black holes in virtual reality

We present a full 360 degree (i.e., 4$\pi$ steradian) general-relativistic ray-tracing and radiative transfer calculations of accreting supermassive black holes. We perform state-of-the-art three-dimensional general relativistic magnetohydrodynamical simulations using the BHAC code, subsequently post-processing this data with the radiative transfer code RAPTOR. All relativistic and general-relativistic effects, such as Doppler boosting and gravitational redshift, as well as geometrical effects due to the local gravitational field and the observer's changing position and state of motion, are therefore calculated self-consistently. Synthetic images at four astronomically-relevant observing frequencies are generated from the perspective of an observer with a full 360-degree view inside the accretion flow, who is advected with the flow as it evolves. As an example, we calculated images based on recent best-fit models of observations of Sagittarius A*. These images are combined to generate a complete 360-degree Virtual Reality movie of the surrounding environment of the black hole and its event horizon. Our approach also enables the calculation of the local luminosity received at a given fluid element in the accretion flow, providing important applications in, e.g., radiation feedback calculations onto black hole accretion flows. In addition to scientific applications, the 360-degree Virtual Reality movies we present also represent a new medium through which to communicate black hole physics to a wider audience, serving as a powerful educational tool.Comment: 25 pages, 11 figures, 1 movie; https://www.youtube.com/watch?v=SXN4hpv977s&t=57

The Jet in the Galactic Center: An Ideal Laboratory for Magnetohydrodynamics and General Relativity

In this paper we review and discuss some of the intriguing properties of the Galactic Center supermassive black hole candidate Sgr A*. Of all possible black hole sources, the event horizon of Sgr A*, subtends the largest angular scale on the sky. It is therefore a prime candidate to study and image plasma processes in strong gravity and it even allows imaging of the shadow cast by the event horizon. Recent mm-wave VLBI and radio timing observations as well as numerical GRMHD simulations now have provided several breakthroughs that put Sgr A* back into the focus. Firstly, VLBI observations have now measured the intrinsic size of Sgr A* at multiple frequencies, where the highest frequency measurements have approached the scale of the black hole shadow. Moreover, measurements of the radio variability show a clear time lag between 22 GHz and 43 GHz. The combination of size and timing measurements, allows one to actually measure the flow speed and direction of magnetized plasma at some tens of Schwarzschild radii. This data strongly support a moderately relativistic outflow, consistent with an accelerating jet model. This is compared to recent GRMHD simulation that show the presence of a moderately relativistic outflow coupled to an accretion flow Sgr A*. Further VLBI and timing observations coupled to simulations have the potential to map out the velocity profile from 5-40 Schwarzschild radii and to provide a first glimpse at the appearance of a jet-disk system near the event horizon. Future submm-VLBI experiments would even be able to directly image those processes in strong gravity and directly confirm the presence of an event horizon.Comment: invited talk to appear in "Jets on All Scales", IAU Symposium 275, G.E. Romero, R.A. Sunyaev & T. Belloni, eds., Cambridge University Press, 9 pages, LaTex, 4 figure

Magnetized Accretion Flows: Effects of Gas Pressure

We study how axisymmetric magnetohydrodynamical (MHD) accretion flows depend on gamma adiabatic index in the polytropic equation of state. This work is an extension of Moscibrodzka & Proga (2008), where we investigated the gamma dependence of 2-D Bondi-like accretion flows in the hydrodynamical (HD) limit. Our main goal is to study if simulations for various gamma can give us insights into to the problem of various modes of accretion observed in several types of accretion systems such as black hole binaries (BHB), active galactic nuclei (AGN), and gamma-ray bursts (GRBs). We find that for gamma >~ 4/3, the fast rotating flow forms a thick torus that is supported by rotation and gas pressure. As shown before for gamma=5/3, such a torus produces a strong, persistent bipolar outflow that can significantly reduce the polar funnel accretion of a slowly rotating flow. For low gamma, close to 1, the torus is thin and is supported by rotation. The thin torus produces an unsteady outflow which is too weak to propagate throughout the polar funnel inflow. Compared to their HD counterparts, the MHD simulations show that the magnetized torus can produce an outflow and does not exhibit regular oscillations. Generally, our simulations demonstrate how the torus thickness affects the outflow production. They also support the notion that the geometrical thickness of the torus correlates with the power of the torus outflow. Our results, applied to observations, suggest that the torus ability to radiatively cool and become thin can correspond to a suppression of a jet as observed in the BHB during a transition from a hard/low to soft/high spectral state and a transition from a quiescent to hard/low state in AGN.Comment: 17 pages, 7 figures, accepted for publication in MNRA

RAPTOR I: Time-dependent radiative transfer in arbitrary spacetimes

Observational efforts to image the immediate environment of a black hole at the scale of the event horizon benefit from the development of efficient imaging codes that are capable of producing synthetic data, which may be compared with observational data. We aim to present RAPTOR, a new public code that produces accurate images, animations, and spectra of relativistic plasmas in strong gravity by numerically integrating the equations of motion of light rays and performing time-dependent radiative transfer calculations along the rays. The code is compatible with any analytical or numerical spacetime. It is hardware-agnostic and may be compiled and run both on GPUs and CPUs. We describe the algorithms used in RAPTOR and test the code's performance. We have performed a detailed comparison of RAPTOR output with that of other radiative-transfer codes and demonstrate convergence of the results. We then applied RAPTOR to study accretion models of supermassive black holes, performing time-dependent radiative transfer through general relativistic magneto-hydrodynamical (GRMHD) simulations and investigating the expected observational differences between the so-called fast-light and slow-light paradigms. Using RAPTOR to produce synthetic images and light curves of a GRMHD model of an accreting black hole, we find that the relative difference between fast-light and slow-light light curves is less than 5%. Using two distinct radiative-transfer codes to process the same data, we find integrated flux densities with a relative difference less than 0.01%. For two-dimensional GRMHD models, such as those examined in this paper, the fast-light approximation suffices as long as errors of a few percent are acceptable. The convergence of the results of two different codes demonstrates that they are, at a minimum, consistent.Comment: 18 pages, 14 figures, 5 table

Visibility of black hole shadows in low-luminosity AGN

Accreting black holes tend to display a characteristic dark central region called the black hole shadow, which depends only on space–time/observer geometry and which conveys information about the black hole’s mass and spin. Conversely, the observed central brightness depression, or image shadow, additionally depends on the morphology of the emission region. In this paper, we investigate the astrophysical requirements for observing a meaningful black hole shadow in GRMHD-based models of accreting black holes. In particular, we identify two processes by which the image shadow can differ from the black hole shadow: evacuation of the innermost region of the accretion flow, which can render the image shadow larger than the black hole shadow, and obscuration of the black hole shadow by optically thick regions of the accretion flow, which can render the image shadow smaller than the black hole shadow, or eliminate it altogether. We investigate in which models the image shadows of our models match their corresponding black hole shadows, and in which models the two deviate from each other. We find that, given a compact and optically thin emission region, our models allow for measurement of the black hole shadow size to an accuracy of 5 per cent. We show that these conditions are generally met for all MAD simulations we considered, as well as some of the SANE simulations