Special Issue "Accretion Disks, Jets, Gamma-Ray Bursts and Related Gravitational Waves"

A special issue of Universe (ISSN 2218-1997).

Deadline for manuscript submissions: closed (28 February 2019)

Special Issue Editor

Guest Editor
Prof. Dr. Banibrata Mukhopadhyay

Department of Physics, Indian Institute of Science, Bangalore 560012, India
Website | E-Mail
Interests: physics of astrophysical compact objects including accretion disks and outflows/jets; astrophysical fluid dynamics; nuclear astrophysics; field theory in curved spacetime; general relativity and gravitation

Special Issue Information

Dear Colleagues,

The accretion flow is ubiquitous in astrophysics. In fact, the accretion disk plays a tremendous role in identifying a black hole in the universe and its characteristics, namely mass and spin. An accretion flow is often observed to be associated with outflows and/or jets, when a part of infalling matter comes out off near vicinity of the underlying black hole (or central object in general). In order to understand the complete inflow–outflow process, which is still not well-understood, one needs to explore (general) relativistic magnetohydrodynamics (MHD).

Importantly, molecular viscosity in accretion flows is not adequate in order to explain observed luminosity. Hence, it is assumed that transport takes place via turbulence, hence it is the turbulent transport that is responsible for matter infall therein. However, an accretion disk, in particular its Keplerian angular momentum profile, is Rayleigh stable and hence it is difficult to explain the origin of turbulence. However in the presence of weak magnetic fields, via magnetorotational instability (MRI), linear instability and turbulence can be explained. Nevertheless, many accretion systems are cold and hence neutral in charge so MRI is expected to be sluggish. Therefore, the origin of turbulence and transport in accretion flows is still an open question, which needs to be explored, based on the idea of fluid and plasma physics.

A short-duration accretion disk is also expected to form during the formation of stellar mass black holes and neutron stars, called a collapsar disk. Such a disk may often produce jets by means of gamma-ray bursts, which we observe. Due to their very high density and temperature, a collapsar disk is expected to produce a huge neutron flux and hence is often called a neutrino-dominated accretion flow. This is a topic in the interface of relativistic astrophysics and high energy physics. Blending the two branches of physics is useful to understand the formation of compact objects and gamma-ray bursts.

An accretion flow is also responsible to understand blazars which correspond to disk-jet systems around a supermassive black hole. In general, the accretion flow around a supermassive black hole is another important branch of relativistic astrophysics with a great deal of physics to uncover, e.g., luminosity dichotomy of FSRQs and BL lacs, possibility of QPOs in AGNs, etc.

Last, but not least, a new branch of astrophysics, Gravitational Wave Astronomy (GWA), is emerging, with confirmed detection of gravitational waves. Already, many black holes have been identified with respective masses in GWA. On the other hand, black hole masses are determined by X-ray astronomy too. One topic is to check the consistency of the mass of black holes using both branches of astronomy. In general, it is very important to explore GWA in the context of black holes and accretion physics. In a related context, the spin of various identified black holes need to be confirmed, while, theoretically, they may have Kerr parameters between –1 to +1. Some techniques are in the literature to measure the spin of black holes from data, of which the results, however, often do not tally each other. This needs to be enlightened.

All the above topics are primarily planned to consider in this special issue.

Prof. Dr. Banibrata Mukhopadhyay
Guest Editor

Manuscript Submission Information

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Keywords

  • Accretion and accretion disks
  • Black holes and in general compact objects
  • Jets and gamma-ray bursts
  • AGN
  • MHD
  • General relativity
  • QPO
  • Gravitational waves
  • Linear instability
  • Turbulence in accretion flows

Published Papers (7 papers)

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Research

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Open AccessArticle
Self-Similar Solution of Hot Accretion Flow with Anisotropic Pressure
Received: 13 March 2019 / Revised: 3 April 2019 / Accepted: 4 April 2019 / Published: 8 April 2019
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Abstract
For the accretion flow in extremely low-luminosity active galactic nuclei, such as our Galactic center (Sgr A*) and M 87, the collisional mean-free path of ions may be much larger than its gyroradius. In this case, the pressure parallel to the magnetic field [...] Read more.
For the accretion flow in extremely low-luminosity active galactic nuclei, such as our Galactic center (Sgr A*) and M 87, the collisional mean-free path of ions may be much larger than its gyroradius. In this case, the pressure parallel to the magnetic field is different from that perpendicular to the field; therefore, the pressure is anisotropic. We study the effects of anisotropic pressure on the dynamics of accretion flow by assuming the flow is radially self-similar. We find that in the case where the outflow is present, the radial and rotational velocities, the sound speed, and the Bernoulli parameter of the accretion flow are all increased when the anisotropic pressure is taken into account. This result suggests that it becomes easier for the accretion flow to generate outflow in the presence of anisotropic pressure. Full article
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Review

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Open AccessReview
Accretion into Black Hole, and Formation of Magnetically Arrested Accretion Disks
Universe 2019, 5(6), 146; https://doi.org/10.3390/universe5060146
Received: 28 March 2019 / Revised: 29 May 2019 / Accepted: 31 May 2019 / Published: 11 June 2019
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Abstract
The exact time-dependent solution is obtained for a magnetic field growth during a spherically symmetric accretion into a black hole (BH) with a Schwarzschild metric. Magnetic field is increasing with time, changing from the initially uniform into a quasi-radial field. Equipartition between magnetic [...] Read more.
The exact time-dependent solution is obtained for a magnetic field growth during a spherically symmetric accretion into a black hole (BH) with a Schwarzschild metric. Magnetic field is increasing with time, changing from the initially uniform into a quasi-radial field. Equipartition between magnetic and kinetic energies in the falling gas is supposed to be established in the developed stages of the flow. Estimates of the synchrotron radiation intensity are presented for the stationary flow. The main part of the radiation is formed in the relativistic region r 7 r g , where r g is a BH gravitational radius. The two-dimensional stationary self-similar magnetohydrodynamic solution is obtained for the matter accretion into BH, in a presence of a large-scale magnetic field, under assumption, that the magnetic field far from the BH is homogeneous and its influence on the flow is negligible. At the symmetry plane perpendicular to the direction of the distant magnetic field, the dense quasi-stationary disk is formed around BH, which structure is determined by dissipation processes. Solutions of the disk structure have been obtained for a laminar disk with Coulomb resistivity and for a turbulent disk. Parameters of the shock forming due to matter infall onto the disk are obtained. The radiation spectrum of the disk and the shock are obtained for the 10 M BH. The luminosity of such object is about the solar one, for a characteristic galactic gas density, with possibility of observation at distances less than 1 kpc. The spectra of a laminar and a turbulent disk structure around BH are very different. The laminar disk radiates mainly in the ultraviolet, the turbulent disk emits a large part of its flux in the infrared. It may occur that some of the galactic infrared star-like sources are a single BH in the turbulent accretion state. The radiative efficiency of the magnetized disk is very high, reaching 0.5 M ˙ c 2 . This model of accretion was called recently as a magnetically arrested disk (MAD). Numerical simulations of MAD and its appearance during accretion into neutron stars, are considered and discussed. Full article
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Open AccessReview
Slim Accretion Disks: Theory and Observational Consequences
Universe 2019, 5(5), 131; https://doi.org/10.3390/universe5050131
Received: 15 April 2019 / Revised: 20 May 2019 / Accepted: 21 May 2019 / Published: 26 May 2019
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Abstract
The concept of slim accretion disks emerged over 30 years ago as an answer to several unsolved problems. Since that time there has been a tremendous increase in the amount of observational data where this model applies. However, many critical issues on the [...] Read more.
The concept of slim accretion disks emerged over 30 years ago as an answer to several unsolved problems. Since that time there has been a tremendous increase in the amount of observational data where this model applies. However, many critical issues on the theoretical side remain unsolved, as they are inherently difficult. This is the issue of the disk stability under radiation pressure, the role of the magnetic field in the energy transfer inside the disk, the formation (or not) of a warm corona, and outflows. Thus the progress has to be done both through further developments of the model and through careful comparison with the observational data. Full article
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Open AccessReview
Fifty Years of Energy Extraction from Rotating Black Hole: Revisiting Magnetic Penrose Process
Universe 2019, 5(5), 125; https://doi.org/10.3390/universe5050125
Received: 21 March 2019 / Revised: 7 May 2019 / Accepted: 10 May 2019 / Published: 22 May 2019
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Abstract
Magnetic Penrose process (MPP) is not only the most exciting and fascinating process mining the rotational energy of black hole but it is also the favored astrophysically viable mechanism for high energy sources and phenomena. It operates in three regimes of efficiency, namely [...] Read more.
Magnetic Penrose process (MPP) is not only the most exciting and fascinating process mining the rotational energy of black hole but it is also the favored astrophysically viable mechanism for high energy sources and phenomena. It operates in three regimes of efficiency, namely low, moderate and ultra, depending on the magnetization and charging of spinning black holes in astrophysical setting. In this paper, we revisit MPP with a comprehensive discussion of its physics in different regimes, and compare its operation with other competing mechanisms. We show that MPP could in principle foot the bill for powering engine of such phenomena as ultra-high-energy cosmic rays, relativistic jets, fast radio bursts, quasars, AGNs, etc. Further, it also leads to a number of important observable predictions. All this beautifully bears out the promise of a new vista of energy powerhouse heralded by Roger Penrose half a century ago through this process, and it has today risen in its magnetically empowered version of mid 1980s from a purely thought experiment of academic interest to a realistic powering mechanism for various high-energy astrophysical phenomena. Full article
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Open AccessReview
Induced Gravitational Collapse, Binary-Driven Hypernovae, Long Gramma-ray Bursts and Their Connection with Short Gamma-ray Bursts
Universe 2019, 5(5), 110; https://doi.org/10.3390/universe5050110
Received: 25 February 2019 / Revised: 2 May 2019 / Accepted: 5 May 2019 / Published: 9 May 2019
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Abstract
There is increasing observational evidence that short and long Gamma-ray bursts (GRBs) originate in different subclasses, each one with specific energy release, spectra, duration, etc, and all of them with binary progenitors. The binary components involve carbon-oxygen cores (COcore), neutron stars [...] Read more.
There is increasing observational evidence that short and long Gamma-ray bursts (GRBs) originate in different subclasses, each one with specific energy release, spectra, duration, etc, and all of them with binary progenitors. The binary components involve carbon-oxygen cores (CO core ), neutron stars (NSs), black holes (BHs), and white dwarfs (WDs). We review here the salient features of the specific class of binary-driven hypernovae (BdHNe) within the induced gravitational collapse (IGC) scenario for the explanation of the long GRBs. The progenitor is a CO core -NS binary. The supernova (SN) explosion of the CO core , producing at its center a new NS ( ν NS), triggers onto the NS companion a hypercritical, i.e., highly super-Eddington accretion process, accompanied by a copious emission of neutrinos. By accretion the NS can become either a more massive NS or reach the critical mass for gravitational collapse with consequent formation of a BH. We summarize the results on this topic from the first analytic estimates in 2012 all the way up to the most recent three-dimensional (3D) smoothed-particle-hydrodynamics (SPH) numerical simulations in 2018. Thanks to these results it is by now clear that long GRBs are richer and more complex systems than thought before. The SN explosion and its hypercritical accretion onto the NS explain the X-ray precursor. The feedback of the NS accretion, the NS collapse and the BH formation produce asymmetries in the SN ejecta, implying the necessity of a 3D analysis for GRBs. The newborn BH, the surrounding matter and the magnetic field inherited from the NS, comprises the inner engine from which the GRB electron-positron ( e + e ) plasma and the high-energy emission are initiated. The impact of the e + e on the asymmetric ejecta transforms the SN into a hypernova (HN). The dynamics of the plasma in the asymmetric ejecta leads to signatures depending on the viewing angle. This explains the ultrarelativistic prompt emission in the MeV domain and the mildly-relativistic flares in the early afterglow in the X-ray domain. The feedback of the ν NS pulsar-like emission on the HN explains the X-ray late afterglow and its power-law regime. All of the above is in contrast with a simple GRB model attempting to explain the entire GRB with the kinetic energy of an ultrarelativistic jet extending through all of the above GRB phases, as traditionally proposed in the “collapsar-fireball” model. In addition, BdHNe in their different flavors lead to ν NS-NS or ν NS-BH binaries. The gravitational wave emission drives these binaries to merge producing short GRBs. It is thus established a previously unthought interconnection between long and short GRBs and their occurrence rates. This needs to be accounted for in the cosmological evolution of binaries within population synthesis models for the formation of compact-object binaries. Full article
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Open AccessReview
The Unique Blazar OJ 287 and Its Massive Binary Black Hole Central Engine
Universe 2019, 5(5), 108; https://doi.org/10.3390/universe5050108
Received: 26 March 2019 / Revised: 1 May 2019 / Accepted: 6 May 2019 / Published: 8 May 2019
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Abstract
The bright blazar OJ 287 is the best-known candidate for hosting a nanohertz gravitational wave (GW) emitting supermassive binary black hole (SMBBH) in the present observable universe. The binary black hole (BBH) central engine model, proposed by Lehto and Valtonen in 1996, was [...] Read more.
The bright blazar OJ 287 is the best-known candidate for hosting a nanohertz gravitational wave (GW) emitting supermassive binary black hole (SMBBH) in the present observable universe. The binary black hole (BBH) central engine model, proposed by Lehto and Valtonen in 1996, was influenced by the two distinct periodicities inferred from the optical light curve of OJ 287. The current improved model employs an accurate general relativistic description to track the trajectory of the secondary black hole (BH) which is crucial to predict the inherent impact flares of OJ 287. The successful observations of three predicted impact flares open up the possibility of using this BBH system to test general relativity in a hitherto unexplored strong field regime. Additionally, we briefly describe an ongoing effort to interpret observations of OJ 287 in a Bayesian framework. Full article
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Open AccessReview
Approaching the Black Hole by Numerical Simulations
Received: 2 February 2019 / Revised: 23 April 2019 / Accepted: 23 April 2019 / Published: 29 April 2019
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Abstract
Black holes represent extreme conditions of physical laws. Predicted about a century ago, they are now accepted as astrophysical reality by most of the scientific community. Only recently has more direct evidence of their existence been found—the detection of gravitational waves from black [...] Read more.
Black holes represent extreme conditions of physical laws. Predicted about a century ago, they are now accepted as astrophysical reality by most of the scientific community. Only recently has more direct evidence of their existence been found—the detection of gravitational waves from black hole mergers and of the shadow of a supermassive black hole in the center of a galaxy. Astrophysical black holes are typically embedded in an active environment which is affected by the strong gravity. When the environmental material emits radiation, this radiation may carry imprints of the black hole that is hosting the radiation source. In order to understand the physical processes that take place in the close neighborhood of astrophysical black holes, numerical methods and simulations play an essential role. This is simply because the dynamical evolution and the radiative interaction are far too complex in order to allow for an analytic solution of the physical equations. A huge progress has been made over the last decade(s) in the numerical code development, as well as in the computer power that is needed to run these codes. This review tries to summarize the basic questions and methods that are involved in the undertaking of investigating the astrophysics of black holes by numerical means. It is intended for a non-expert audience interested in an overview over this broad field. The review comes along without equations and thus without a detailed expert discussion of the underlying physical processes or numerical specifics. Instead, it intends to illustrate the richness of the field and to motivate further reading. The review puts some emphasis on magneto-hydrodynamic simulations but also touches radiation transfer and merger simulations, in particular pointing out differences in these approaches. Full article
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