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We systematically investigate the variability of polarized X-rays on a timescale of a few seconds in the low/hard state of the black hole binary Cygnus X-1. The correlation between polarization degrees and angles with X-ray intensity was analyzed using data collected by the Imaging X-ray Polarimetry Explorer (IXPE) in 2022 June. Given that X-ray variability in the low/hard state of Cygnus X-1 is non-periodic, flux peaks were aggregated to suppress statistical fluctuations. We divided the temporal profiles of these aggregated flux peaks into seven time segments and evaluated the polarization for each segment. The results reveal that the polarization degree was 4.6% |$\pm$| 1.2% and 5.3% |$\pm$| 1.2% before and after the peak, respectively, but decreased to 3.4% |$\pm$| 1.1% and 2.7% |$\pm$| 1.1% in the segments including and immediately following the peak. Furthermore, the polarization angle exhibited a slight shift from approximately 30 |$^{\circ }$| to |${\sim} 40^{\circ }$| before and after the peak. These findings suggest that the accretion disk contracts with increasing X-ray luminosity, and the closer proximity of the X-ray emitting gas to the black hole may lead to reduced polarization. [ABSTRACT FROM AUTHOR]

Regular black holes are crucially important as approaches to solving the singularity problem, and in this paper, the accretion disk of Bardeen and Hayward models has been studied. For this purpose, we calculated the physical properties of black holes, including radiant energy, luminosity derivative, temperature, and conversion efficiency of accretion mass into radiation. The obtained results show that the nonzero-free parameters of regular black holes cause the radius of the innermost stable circular orbit of the disk to shift to smaller values. As a result of this displacement, we saw an increase in the profiles of radiant energy, luminosity derivative, and temperature. We also find that Bardeen and Hayward’s black holes are more efficient in converting mass to radiation than Schwarzschild. Finally, we compared the free parameter of these two black holes with the spin of the rotating black hole and found that the Bardeen and Hayward black holes can mimic the slowly rotating Kerr black hole. [ABSTRACT FROM AUTHOR]

The work is devoted to the construction of a gas-dynamic model of the accretion disk around a neutron star (NS). The developed multidimensional code is used to study the stability of stationary axially symmetrical models by carry out of evolutionary calculations in 3D taking into account viscosity, as well as taking into account the diffusion of radiation in 2D. It is shown that an arbitrary stationary axially symmetrical disk with a monotonic decrease in density with a cylindrical radius transforms, due to viscosity, braking and spreading of matter along the NS, into a new quasi-stationary toroidal configuration. The stability study of the stationary toroidal configuration confirmed the formation of large-scale vortex structures at the initial periodic disturbance of angular velocity in azimuth, now taking into account the "turbulent" viscosity. It turned out that the presence of large-scale structures leads to an acceleration of braking, i.e., an effective increase in viscosity. [ABSTRACT FROM AUTHOR]

The aim of the current paper is to apply Bambi's method (Bambi, 2015) to a source which contains two or more simultaneous triads of variability components. The joint x variable that can be composed in this case, unlike some previous studies, allows the goodness of the fit to be tested. It appears that a good fit requires one of the observation groups to be disregarded. Even then, the model's prediction for the mass of the neutron star in the accreting millisecond pulsar IGR J17511-3057 is way too high to be accepted. [ABSTRACT FROM AUTHOR]

We examine the effect of the variable viscosity parameter (α ) in relativistic, low angular momentum advective accretion flow around rotating black holes. Following the recent simulation studies of the magnetohydrodynamic disk that reveal the radial variation of α (r) , we theoretically investigate the properties of the global transonic accretion flow considering a one-dimensional power-law prescription of the viscosity parameter as α (r) ∝ r θ , where the viscosity exponent θ is a constant. In doing so, we adopt the relativistic equation of state and solve the fluid equations that govern the flow motion inside the disk. We find that depending on the flow parameters, accretion flow experiences centrifugally supported shock transitions and such shocked accretion solutions continue to exist for wide ranges of the flow energy, angular momentum, accretion rate, and viscosity exponent, respectively. Due to shock compression, the hot and dense postshock flow (hereafter PSC) can produce the high-energy radiations after reprocessing the soft photons from the preshock flow via inverse Comptonization. Since PSC is usually described using shock radius ( r s ), compression ratio (R ), and shock strength (S ), we study the role of θ in detemining r s , R , and S , respectively. Moreover, we obtain the parameter space for a shock and find that the possibility of shock formation diminishes as θ is increased. Finally, we compute the limiting value of θ (i.e., θ max ) that admits a shock and find that flow can sustain more viscosity when it accretes onto a rapidly rotating ( a k → 1 ) black hole in comparison to a weakly rotating ( a k → 0 ) black hole. [ABSTRACT FROM AUTHOR]

Astrophysical plasma ejections (jets) are formed and powered by black holes that accrete material from their companion star in binary systems. Black hole X-ray binary systems constitute potential powerful galactic and extragalactic neutrino and gamma-ray sources. After being accelerated to highly relativistic velocities and subjected to various energy-consuming interactions, the lepto-hadronic content of the jets produces secondary particles such as pions and muons that decay to gamma-ray photons and neutrinos heading towards the Earth. In this work, we employ a jet emission model in order to predict the neutrino and gamma-ray integral fluxes emanating from some of the most investigated and prominent stellar black hole X-ray binary systems in the Milky Way, such as GRO J1655-40, Cygnus X-1, SS 433, and GRS 1915+105. For the sake of comparison, we also include an extragalactic system, namely, LMC X-1, located in the Large Magellanic Cloud. For the case of gamma-ray emissions, we also include absorption effects due to X-ray emission from the accretion disk and the black hole corona, as well as ultraviolet (UV) emission from the binary system's companion star. [ABSTRACT FROM AUTHOR]

The investigation of thin accretion disc surrounding the Einstein‐ Euler‐ Heisenberg‐ Anti‐ de Sitter black hole is demonstrated. Additionally, the black hole event horizons and compute their effective potential and equations of motion is analyzed. The specific energy, specific angular momentum, and specific angular velocity of the particles that move in circular orbits above the thin accretion disk are obtained. Also, the effects of parameters of the Einstein‐ Euler‐ Heisenberg‐ Anti De sitter black hole on specific angular velocity, specific heat energy, and specific angular momentum, have been discussed in detail. The positions of the innermost stable circular orbits and illustrate the effective potential are displayed. Furthermore, the circular orbits of black hole are obtained numerically and locates the position of the innermost stable circular orbit and event horizon. Also, the Gamma ray burst emitted from the Einstein Euler Heisenberg Anti de sitter black hole is studied. [ABSTRACT FROM AUTHOR]

We study the impact of compressibility on two-dimensional turbulent flows, such as those modeling astrophysical disks. We demonstrate that the direction of cascade undergoes continuous transition as the Mach number Ma increases, from inverse at Ma = 0, to direct at M a = ∞. Thus, at M a ∼ 1 comparable amounts of energy flow from the pumping scale to large and small scales, in accord with previous data. For supersonic turbulence with M a ≫ 1 the cascade is direct, as in three dimensions, which results in multifractal density field. For that regime ( M a ≫ 1) we derive a Kolmogorov-type law for potential forcing and obtain an explicit expression for the third order correlation tensor of the velocity. We further show that all third order structure functions are zero up to first order in the inertial range scales, which is in sharp contrast with incompressible turbulence where the third order structure function, that describes the energy flux associated with the energy cascade is non-zero. The properties of compressible turbulence have significant implications on the amplification of magnetic fields in conducting fluids. We thus demonstrate that imposing external magnetic field on compressible flows of conducting fluids allows to manipulate the flow producing possibly large changes even at small Mach numbers. Thus Zeldovich's antidynamo theorem, by which at Ma = 0 the magnetic field is zero in the steady state, must be used with caution. Real flows have finite Ma and, however small it is, for large enough values of I, the magnetic flux through the disk, the magnetic field changes the flow appreciably, or rearranges it completely. This renders the limit Ma → 0 singular for non-zero values of I. Of particular interest is the effect of the density multifractality, at M a ≫ 1 which is relevant for astrophysical disks. We demonstrate that in that regime, in the presence of non-zero I the magnetic field energy is enhanced by a large factor as compared to its estimates based on the mean field. Finally, based on the insights described above, we propose a novel two-dimensional Burgers' turbulence, whose three-dimensional counterpart is used for studies of the large-scale structure of the Universe, as a model for supersonic two-dimensional magnetohydrodynamic flows. [ABSTRACT FROM AUTHOR]

Massive gravity offers an interesting modification of general relativity by considering a nonzero mass for the graviton. We present a de Rham, Gabadadze and Tolley (dRGT) massive gravity model in the presence of higher order curvature gravity. We obtain a spherical solution for the field equations in this theory. The spherical solution possesses an electric charge and a cosmological constant, and it reduces to the Schwarzschild solution in the limit of a negligible graviton mass and the absence of the higher order term. We study the thermodynamics of this black hole and find that the mass of the graviton and the higher order term of gravity have a significant effect on the thermodynamic properties of the black hole. We also show the entropy of a black hole is independent of the mass of the graviton. Using the observational constraints on the coefficient of the higher order term, we determine an upper limit for the graviton mass. We compute the other thermodynamic quantities, such as heat capacity and Helmholtz free energy. Assuming that the spherical solutions are the modified Schwarzschild, we consider the relativistic thin accretion disk and study the effects of higher order term on thermal properties of the disk at the infrared limit in an asymptotic safety scenario. [ABSTRACT FROM AUTHOR]

The effects of a gravitomagnetic charge on the thermal radiation properties of a thin accretion disk surrounding a non-rotating black hole are studied. The studied system consists of a non-rotating black hole with a non-zero gravitomagnetic charge and a Novikov–Thorne disk that is thin and optically thick. It is found that the gravitomagnetic charge enhances the gravitational field of the central black hole, resulting in an increase in the event horizon and innermost stable circular orbit (ISCO) radii. However, the maximum flux of radiant energy from the accretion disk is reduced and shifted outward from the central object due to the effect of the gravitomagnetic charge. The thermal profile of the accretion disk also exhibits a similar dependence on the radial coordinate and the gravitomagnetic charge of the black hole. The radiative efficiency of the accretion disk decreases from around 6% to approximately 2% with an increase in the value of the gravitomagnetic charge by l ≃ 3 M. The thermal spectra of the accretion disk are also shifted towards lower frequencies, corresponding to the gravitational redshift of electromagnetic radiation coming from the disk, with an increase in the value of the gravitomagnetic charge. One may conclude that the effect of the gravitomagnetic charge is opposite to the effect of black hole spin. [ABSTRACT FROM AUTHOR]

X-ray binary systems (XRBs) exhibit similar dynamics and multimessenger emission mechanisms to active galactic nuclei (AGNs) with the benefit of shorter time scaling. Those systems produce rich spectral energy distributions (SEDs) ranging from the radio band to the very high energy gamma rays. The emission origin varies between the system's accretion disk (X-rays) to the corona and, most notably, to the two twin plasma ejections (jets) that often meet the interstellar medium forming highly observable radio lobes. Modeling of the jets offers an excellent opportunity to understand the intrinsic mechanisms and the jet particles, such as electrons, positrons, and protons. In this work, we employ a lepto-hadronic jet model that assumes particle acceleration through shock waves over separate zonal regions of the jet. The hadronic models consider proton–proton collisions that end up in gamma-ray photons through neutral pion decays. The main leptonic mechanisms involve synchrotron radiation (from both electrons and protons) and inverse Compton scattering of ambient photons (coming from the disk, the corona, and the companion star) on jet electrons. The emissions from the disk, the corona, and the donor star are also included in the SED calculations, along with the photon absorption effects due to their interaction with higher-energy jet photons. We apply the model on a 10 M ⊙ black hole accreting at the Eddington rate out of a 20 M ⊙ companion star. One of our goals is to investigate and determine an optimal frame concerning the values for the free parameters that enter our calculations to produce higher integral fluxes. [ABSTRACT FROM AUTHOR]

Superoutbursts in WZ Sge-type dwarf novae (DNe) are characterized by both early superhumps and ordinary superhumps originating from the 2 : 1 and 3 : 1 resonances, respectively. However, some WZ Sge-type DNe show a superoutburst lacking early superhumps; it is not well established how these differ from superoutbursts with an early superhump phase. We report time-resolved photometric observations of the WZ Sge-type DN V627 Peg during its 2021 superoutburst. The detection of ordinary superhumps before the superoutburst peak highlights that this 2021 superoutburst of V627 Peg, like that in 2014, did not feature an early superhump phase. The duration of stage B superhumps was slightly longer in the 2010 superoutburst accompanied by early superhumps than that in the 2014 and 2021 superoutbursts, which lacked early superhumps. This result suggests that an accretion disk experiencing the 2 : 1 resonance may have a larger mass at the inner part of the disk and hence needs more time for the inner disk to become eccentric. The presence of a precursor outburst in the 2021 superoutburst suggests that the maximum disk radius should be smaller than that of the 2014 superoutburst, even though the duration of quiescence was longer than that before the 2021 superoutburst. This could be accomplished if the 2021 superoutburst was triggered as an inside-out outburst or if the mass transfer rate in quiescence changes by a factor of two, suggesting that the outburst mechanism and quiescence state of WZ Sge-type DNe may have more variety than ever thought. [ABSTRACT FROM AUTHOR]

In recent years, unprecedented progress has been achieved regarding black holes’ observation through the electromagnetic channel. The images of the supermassive black holes M87* and Sgr A* released by the Event Horizon Telescope (EHT) Collaboration provided direct visual evidence for their existence, which has stimulated further studies on various aspects of the compact celestial objects. Moreover, the information stored in these images provides a new way to understand the pertinent physical processes that occurred near the black holes, to test alternative theories of gravity, and to furnish insight into fundamental physics. In this review, we briefly summarize the recent developments on the topic. In particular, we elaborate on the features and formation mechanism of black hole shadows, the properties of black hole images illuminated by the surrounding thin accretion disk, and the corresponding polarization patterns. The potential applications of the relevant studies are also addressed. [ABSTRACT FROM AUTHOR]

Contrary to science-fiction movies such as Interstellar, planets orbiting around black holes cannot be stable. Because due to the extreme instability in the accretion disk and the risk of collision of radiations and hot energetic matters in the disk, the existence of the planet will also be endangered, and the question about the habitability of black hole exoplanets (or Miller's planets) seems meaningless. But in this paper, we have considered an ideal situation in which life could form on a black hole exoplanet. It seems that even in an ideal situation, although it would be possible to create simple microbial life on the surface of Miller's planet, this young life would not be sustainable, and would eventually die out. [ABSTRACT FROM AUTHOR]

In this paper, we study spin effects in the neutrino gravitational scattering by a supermassive black hole with a magnetized accretion disk having a finite thickness. We exactly describe the propagation of ultrarelativistic neutrinos on null geodesics and solve the spin precession equation along each neutrino trajectory. The interaction of neutrinos with the magnetic field is due to the nonzero diagonal magnetic moment. Additionally, neutrinos interact with plasma of the accretion disk electroweakly within the Fermi approximation. These interactions are obtained to change the polarization of incoming neutrinos, which are left particles. The fluxes of scattered neutrinos, proportional to the survival probability of spin oscillations, are derived for various parameters of the system. In particular, we are focused on the matter influence on the outgoing neutrinos flux. The possibility to observe the predicted effects for astrophysical neutrinos is briefly discussed. [ABSTRACT FROM AUTHOR]

The work reviews the investigation of electromagnetic, optical, and energetic properties of astrophysical and galactic black holes and surrounding matter. The astrophysical applications of the theoretical models of black hole environment to the description of various observed phenomena, such as cosmic rays of the ultra-high-energy, black hole shadow, gravitational lensing, quasinormal modes, jets showing relativistic effects such as the Doppler beaming, thermal radiation from the accretion discs, quasiperiodic oscillations are discussed. It has been demonstrated that the observational data strongly depends on the structure and evolution of the accretion disk surrounding the central black hole. It has been shown that the simulated images of supermassive black holes obtained are in agreement with the observational images obtained by event horizon telescope collaboration. High energetic activity from supermassive black holes due to the magnetic Penrose process discussed in the work is in agreement with the highly energetic cosmic rays observed. The astronomical observation of black holes provides rich fundamental physics laboratories for experimental tests and verification of various models of black hole accretion and different theories of gravity in the regime of strong gravity. [ABSTRACT FROM AUTHOR]

Recent VLBI monitoring has found transverse motions of the M87 jet. However, due to the limited cadence of previous observations, details of the transverse motion have not been fully revealed yet. We have regularly monitored the M87 jet at KVN and VERA Array (KaVA) 22 GHz from December 2013 to June 2016. The average time interval of the observation is ∼0.1 year, which is suitable for tracking short-term structural changes. From these observations, the M87 jet is well represented by double ridge lines in the region 2–12 mas from the core. We found that the ridge lines exhibit transverse oscillations in all observed regions with an average period of 0.94 ± 0.12 years. When the sinusoidal fit is performed, we found that the amplitude of this oscillation is an order of ∼0.1 mas, and the oscillations in the northern and southern limbs are almost in phase. Considering the amplitude, it does not originate from Earth's parallax. We propose possible scenarios of the transverse oscillation, such as the propagation of jet instabilities or magneto-hydrodynamic (MHD) waves or perturbed mass injection around magnetically dominated accretion flows. [ABSTRACT FROM AUTHOR]

This work proposes a methodology for testing phenomenologically motivated emission processes that account for the flux and polarization distribution and global structure of the 230 GHz sources imaged by the Event Horizon Telescope (EHT): Messier (M)87* and Sagittarius (Sgr) A*. We introduce into general relativistic magnetohydrodynamic (GRMHD) simulations some novel models to bridge the largely uncertain mechanisms by which high-energy particles in jet/accretion flow/black hole (JAB) system plasmas attain billion-degree temperatures and emit synchrotron radiation. The "Observing" JAB Systems methodology then partitions the simulation to apply different parametric models to regions governed by different plasma physics—an advance over methods in which one parametrization is used over simulation regions spanning thousands of gravitational radii from the central supermassive black hole. We present several classes of viewing-angle-dependent morphologies and highlight signatures of piecewise modeling and positron effects, including a MAD/SANE dichotomy in which polarized maps appear dominated by intrinsic polarization in the MAD case and by Faraday effects in the SANE case. The library of images thus produced spans a wide range of morphologies awaiting discovery by the groundbreaking EHT instrument and its yet more sensitive, higher-resolution next-generation counterpart, ngEHT. [ABSTRACT FROM AUTHOR]

Astrophysical plasma ejections (jets) are formed and powered by black holes that accrete material from their companion star in binary systems. Black hole X-ray binary systems constitute potential powerful galactic and extragalactic neutrino and gamma-ray sources. After being accelerated to highly relativistic velocities and subjected to various energy-consuming interactions, the lepto-hadronic content of the jets produces secondary particles such as pions and muons that decay to gamma-ray photons and neutrinos heading towards the Earth. In this work, we employ a jet emission model in order to predict the neutrino and gamma-ray integral fluxes emanating from some of the most investigated and prominent stellar black hole X-ray binary systems in the Milky Way, such as GRO J1655-40, Cygnus X-1, SS 433, and GRS 1915+105. For the sake of comparison, we also include an extragalactic system, namely, LMC X-1, located in the Large Magellanic Cloud. For the case of gamma-ray emissions, we also include absorption effects due to X-ray emission from the accretion disk and the black hole corona, as well as ultraviolet (UV) emission from the binary system’s companion star.

The recent image of our galaxy's supermassive black hole Sgr A* derived from the 7 April 2017 data of the Event Horizon Telescope Collaboration shows multiple hot spots in its accretion disk. Using the analytical framework, we demonstrate that the observed hot spots may not be disjoint elements but causally linked components ("petals") of one rotating quasi-stationary macro-structure formed in the thermo-vorticial field within the accretion disk. [ABSTRACT FROM AUTHOR]

Tidal disruption events (TDEs) around supermassive black holes (SMBHs) are a potential laboratory to study super-Eddington accretion disks and sometimes result in powerful jets or outflows which may shine in the radio and sub-millimeter bands. In this work, we modeled the thermal synchrotron emission of jets by general relativistic radiation magneto-hydrodynamics (GRRMHD) simulations of a BH accretion disk/jet system which assumed the TDE resulted in a magnetized accretion disk around a BH accreting at ∼12–25 times the Eddington accretion rate. Through synthetic observations with the Next Generation Event Horizon Telescope (ngEHT) and an image reconstruction analysis, we demonstrate that TDE jets may provide compelling targets within the context of the models explored in this work. In particular, we found that jets launched by a SANE super-Eddington disk around a spin a * = 0.9 reach the ngEHT detection threshold at large distances (up to 100 Mpc in this work). A two-temperature plasma in the jet or weaker jets, such as a spin a * = 0 model, requires a much closer distance, as we demonstrate detection at 10 Mpc for limiting cases of a * = 0 , R = 1 or a * = 0.9 , R = 20 . We also demonstrate that TDE jets may appear as superluminal sources if the BH is rapidly rotating and the jet is viewed nearly face on. [ABSTRACT FROM AUTHOR]

A three-dimensional numerical model has been developed to study the flow structure in close binary systems with a magnetic field. The model uses a system of equations of modified magnetic hydrodynamics, which allows describing all the main dynamic effects associated with the magnetic field. It takes into account the processes of radiation heating and cooling, heating due to current dissipation, as well as magnetic field diffusion. The model allows calculations in a wide range of magnetic field values. Comparison of the calculation results with observational data confirms the reliability and high efficiency of the model. The paper presents the calculation results of the flow structure in a typical intermediate polar. It is shown that an accretion disk is formed in such a binary system, which has the following characteristic features: "hot line", tidal shock waves, precession density wave, magnetospheric region, and accretion columns. In this case, the magnetic field in the disk is predominantly toroidal. The paper also presents the results of calculations for typical polars. In such systems, instead of an accretion disk, a collimated stream of matter is formed, moving along the magnetic field lines to the magnetic poles of the white dwarf. It is shown that in synchronous polars, variations of the mass transfer rate lead to a change in the spatial configuration of the flow. In asynchronous polars, changes in the flow structure for different phases of the beat period are observed as well as the processes of switching the flow between the magnetic poles of the accretor. Numerical calculations of the asynchronous system are performed under the assumption of the dipole configuration of the magnetic field for different values of the dipole offset relative to the center of the white dwarf. The paper presents a method for estimating this offset from observational light curves. [ABSTRACT FROM AUTHOR]

The blazar PKS 0208-512 was in the lowest γ -ray brightness state during the initial 10 years of observations with the Fermi Gamma-ray Space Telescope (Fermi), which was an order of magnitude lower than its flux state during the EGRET era (1991–2000). The weekly averaged maximum γ -ray flux of this source during the first 10 years of Fermi observation is nearly a factor of 3 lower than the highest flux observed by EGRET in a single epoch. During the period 2018–2020, the source showed a large γ -ray flare, with the average brightness similar to the period 1991–2000. We observed the source with AstroSat, during its low and high activity states, respectively. We carried out broad-band spectral energy distribution (SED) modeling of the source using a one-zone leptonic emission model during its various brightness states. From the SED modeling, we found that there was an inefficient conversion from the bulk energy to the particle energy during the long-term low-activity states as compared to the high flux state during the EGRET era and the later part of Fermi observation. [ABSTRACT FROM AUTHOR]

The thermal radiation properties of the accretion disk around a non-rotating black hole with a perfect fluid dark matter (PFDM) environment are investigated. A non-rotating black hole surrounded by perfect fluid dark matter together with a classical geometrically thin but optically thick Novikov–Thorne disk is selected as a system to be analyzed. It is observed that the perfect fluid dark matter strengthens the gravitational field, which leads to both the increase of the radii of the event horizon and the innermost stable circular orbit (ISCO). However, for the flux of the radiant energy over the accretion disk, the maximum flux is reduced and shifted outwards the central object under the influence of the perfect fluid dark matter. The dependence of the thermal profile of the disk on the radial coordinate and the intensity of perfect fluid dark matter shows analogous behavior. It has been demonstrated that the radiative efficiency of the accretion disk is increased from ∼6% up to ∼20% with the increase in the intensity of the surrounding perfect fluid dark matter. The thermal spectra of the accretion disk has also been explored, which is shifted towards the lower frequencies (corresponding to the gravitational redshift of the electromagnetic radiation coming from the disk) with the increase in the intensity of the perfect fluid dark matter. [ABSTRACT FROM AUTHOR]

In this paper, we have investigated the motion of photons as well as the frequency shift of photons emitted by geodesic particles influenced by the central black hole (BH) described by Bardeen solution surrounded by perfect fluid dark matter (PFDM). It has been shown that in general there are two stable photon spheres formed due to the gravitational attraction of photons by the central gravitating compact object. It has been obtained that with the increase of the space–time parameters these photon spheres come close to each other until they merge forming unique photon sphere around central BH. Investigation of the frequency shift of photons radiated by the particles geodesically moving in the equatorial plane has shown that the former becomes stronger for the bigger values of the intensity of PFDM and smaller values of the magnetic charge of the Bardeen solution. [ABSTRACT FROM AUTHOR]

X-ray binary systems (XRBs) exhibit similar dynamics and multimessenger emission mechanisms to active galactic nuclei (AGNs) with the benefit of shorter time scaling. Those systems produce rich spectral energy distributions (SEDs) ranging from the radio band to the very high energy gamma rays. The emission origin varies between the system’s accretion disk (X-rays) to the corona and, most notably, to the two twin plasma ejections (jets) that often meet the interstellar medium forming highly observable radio lobes. Modeling of the jets offers an excellent opportunity to understand the intrinsic mechanisms and the jet particles, such as electrons, positrons, and protons. In this work, we employ a lepto-hadronic jet model that assumes particle acceleration through shock waves over separate zonal regions of the jet. The hadronic models consider proton–proton collisions that end up in gamma-ray photons through neutral pion decays. The main leptonic mechanisms involve synchrotron radiation (from both electrons and protons) and inverse Compton scattering of ambient photons (coming from the disk, the corona, and the companion star) on jet electrons. The emissions from the disk, the corona, and the donor star are also included in the SED calculations, along with the photon absorption effects due to their interaction with higher-energy jet photons. We apply the model on a 10M⊙ black hole accreting at the Eddington rate out of a 20M⊙ companion star. One of our goals is to investigate and determine an optimal frame concerning the values for the free parameters that enter our calculations to produce higher integral fluxes.

In this paper, we investigate the black hole (BH) spin contribution to jet power, especially for the magnetic arrested disk (MAD), where only inner accretion disk luminosity is closely coupled with the spin-jet power, and try to explain the 'outliers' track of the radio L R to X-ray luminosity L X in two black hole X-ray binaries (BHXBs). Our results suggest that the BZ-jet and the inner-disk coupling could account for the 'outliers' track of the radio/X-ray correlation in two BHXBs, H1743-322 and MAXI J1348-630. Although the accretion disk of H1743-322 in the outburst could be in the MAD state, there is a lower probability that MAXI J1348-630 is in the MAD state due to its low jet production efficiency. The difference in the inner-disk bolometric luminosity ratio of the two sources implies that these two BHXBs are in different inner-disk accretion states. We further investigate the phase-changing regime of MAXI J1348-630 and find that there is a phase transition around L X / L Edd ∼ 10 − 3 . The assumption of sub-MAD is discussed as well. [ABSTRACT FROM AUTHOR]

H 1743-322 is a well-known black hole X-ray binary (BH XRBs) that has been observed in several outbursts over the past. In this work, we have performed the spectral and timing analysis of H 1743-322 during the "faint" 2005 outburst for the first time with the RXTE/PCA data. In this outburst, the spectral and timing parameters (e.g., T in , Γ , R in , r m s and QPOs, etc.) presented an obvious change and a q-like pattern was found in the Hardness Intensity Diagram (HID), which often named as the hysteresis effect of BH XRBs. The radius of the innermost stable circular orbit was constrained as R ISCO ∼3.50 R g , which predicts that H 1743-322 is a lower-spin black hole. We further explored the correlation between timing and spectral properties. The relation of photon index Γ and X-ray flux, F 3 – 25 keV , presented a transition between negative and positive correlation when the X-ray luminosity, L 3 – 25 keV , is above and below a critical X-ray luminosity, L X , crit ≃ 2.55 × 10 − 3   L Edd , which can be well explained by the Shakura-Sunyaev disk–corona model (SSD-corona) and advection-dominated accretion flow (ADAF). We also found the tight linear, negative correlation between photon index Γ and the total fractional r m s . Since the amount of soft photons from the accretion disk seems invariable, an increase of the number of soft photons will dilute the variability from the harder photons. Therefore, the softer the X-ray spectra will result in the smaller total fractional r m s . The above results suggested that the 2005 outburst of H 1743-322 was a normal outburst and H 1743-322 represented similar properties with other black hole X-ray binaries. [ABSTRACT FROM AUTHOR]

In the general accretion disk model theory, the accretion disk surrounding an astronomical object comprises fluid rings obeying Keplerian motion. However, we should consider relativistic and rotational effects as we close in toward the center of accretion disk surrounding spinning compact massive objects such as a black hole or a neutron star. In this study, we explore the geometry of the inner portion of the accretion disk in the context of Mukhopadhyay’s pseudo-Newtonian potential approximation for the full general relativity theory. We found that the shape of the accretion disk “puffs up” or becomes thicker and the luminosity of the disk could exceed the Eddington luminosity near the surface of the compact spinning black hole.

The accretion disk around a black hole and its emissions play an essential role in theoretical analysis of the black hole image. In the literature, two analytical toy models of accretions are widely adopted: the spherical model and the thin disk model. They are different geometrically but both thin optically. We polish them for free-falling accretions around static spherical black holes. As an application, we investigate the images of a class of nonsingular black holes conformally related to the Schwarzschild black hole. These black holes are vacuum solutions of a family of conformal gravity theories. Results are compared with the Schwarzschild black hole of the same mass. Our results indicate that the conformal factor does not affect the shadow radius seen by distant observers, but it leaves an imprint on the intensity image of black hole. • The shadow and intensity image are studied for black holes in conformal gravity. • It is shown that the shadow radius is not affected by the distribution of emissions. • Spherical and thin-disk models of accretion around static black holes are polished. • Doppler boosting and deboosting effects on black hole images are clarified. [ABSTRACT FROM AUTHOR]

We investigate the observational appearance of spherically symmetric black holes in perfect fluid dark matter (PFDM), with the celestial sphere and an accretion disk being light sources. For the celestial sphere model, we show the variations in black holes' appearance from different angles. And for an optically thin accretion disk, the intensity observed can be characterized by the redshift factor and the transfer functions. Then we investigate the angular dependencies for a resting and rotating accretion disk, respectively. It is found that as the observation angle increases, the blue-shift effect of black holes in rotating accretion disks becomes more pronounced. Our study may provide new insights into the physical characteristics of PFDM black holes and their surrounding environment. [ABSTRACT FROM AUTHOR]

We present the properties of relativistic, inviscid, low angular momentum, advective accretion flow in a f (R) gravity theory that satisfactorily mimics the asymptotically flat vacuum solutions of the Einstein's equations. With this, we solve the governing equations describing the accretion flow and obtain the global transonic accretion solutions in terms of flow energy (E), angular momentum (λ) and gravity parameter (A) that determines the effect of f (R) gravity. We observe that depending on the model parameters, flow may contain either single or multiple critical points. We separate the effective domain of the parameter space in λ − E plane that admits accretion solutions possessing multiple critical points and observe that solution of this kind continues to form for wide range of the flow parameters. We examine the modification of the parameter space and reveal that it gradually shrinks with the decrease of A , and ultimately disappears for A = − 2. 34. Finally, we calculate the disk luminosity (L) considering bremsstrahlung emission process and find that global accretion solutions passing through the inner critical point are more luminous compared to the outer critical point solutions. [ABSTRACT FROM AUTHOR]

We perform a comparative spectro-temporal analysis on the variability classes of GRS 1915+105 and IGR J17091-3624 to draw inferences regarding the underlying accretion flow mechanism. The ν , as well as C2 class Rossi X-ray Timing Explorer observation, have been considered for analysis. We investigate the intensity variation of the source in different energy domains that correspond to different components of the accretion flow and infer the relative dominance of these flow components during the dip/flare events. We correlate the dependence of the dynamic photon index (Θ) with intensities in different energy bands and comment on the transition of the source to hard/soft phases during soft dips/flares. We also report the presence of sharp QPOs at ∼ 7.1 Hz corresponding to both softer and harder domain in the case of ν variability class of GRS 1915+105 and discuss the possible accretion flow configuration it suggests. Sharp QPO around ∼ 20 mHz is observed in ν and C2 classes of IGR J17091-3624 in low and mid energy band (2.0–6.0 keV and 6.0–15.0 keV), but remains undetected in high energy (15.0–60.0 keV). The 2.5–25.0 keV background-subtracted spectra have also been fitted with TCAF along with a Compton reflection component. A plausible accretion flow mechanism in order to explain the observed variability has been proposed. [ABSTRACT FROM AUTHOR]

Abstract It is well known that quantum effects may lead to removal of the intrinsic singularity point of back holes. Also, the quintessence scalar field is a candidate model for describing late-time acceleration expansion. Accordingly, Kazakov and Solodukhin considered the existence of back-reaction of the spacetime due to the quantum fluctuations of the background metric to deform a Schwarzschild black hole, which led to a change of the intrinsic singularity of the black hole to a 2-sphere with a radius of the order of the Planck length. Also, Kiselev rewrote the Schwarzschild metric by taking into account the quintessence field in the background. In this study, we consider the quantum-corrected Schwarzschild black hole inspired by Kazakov–Solodukhin’s work, and the Schwarzschild black hole surrounded by quintessence deduced by Kiselev to study the mutual effects of quantum fluctuations and quintessence on the accretion onto the black hole. Consequently, the radial component of the 4-velocity and the proper energy density of the accreting fluid have a finite value on the surface of its central 2-sphere due to the presence of quantum corrections. Also, by comparing the accretion parameters in different kinds of black holes, we infer that the presence of a point-like electric charge in the spacetime is somewhat similar to some quantum fluctuations in the background metric.

The work reviews the investigation of electromagnetic, optical, and energetic properties of astrophysical and galactic black holes and surrounding matter. The astrophysical applications of the theoretical models of black hole environment to the description of various observed phenomena, such as cosmic rays of the ultra-high-energy, black hole shadow, gravitational lensing, quasinormal modes, jets showing relativistic effects such as the Doppler beaming, thermal radiation from the accretion discs, quasiperiodic oscillations are discussed. It has been demonstrated that the observational data strongly depends on the structure and evolution of the accretion disk surrounding the central black hole. It has been shown that the simulated images of supermassive black holes obtained are in agreement with the observational images obtained by event horizon telescope collaboration. High energetic activity from supermassive black holes due to the magnetic Penrose process discussed in the work is in agreement with the highly energetic cosmic rays observed. The astronomical observation of black holes provides rich fundamental physics laboratories for experimental tests and verification of various models of black hole accretion and different theories of gravity in the regime of strong gravity.

The recent image of our galaxy’s supermassive black hole Sgr A* derived from the 7 April 2017 data of the Event Horizon Telescope Collaboration shows multiple hot spots in its accretion disk. Using the analytical framework, we demonstrate that the observed hot spots may not be disjoint elements but causally linked components (“petals”) of one rotating quasi-stationary macro-structure formed in the thermo-vorticial field within the accretion disk.

Purpose: Studying the accretion disc structure in the Q2237+0305 gravitationally lensed quasar in optical wavelengths; estimation of parameters of the matter accretion regime onto a central black hole. Design/methodology/approach: Measuring the time delays between the quasar brightness variations in two spectral ranges allows to obtain direct estimates of distances between the quasar’s zones, which radiate in the selected wavelengths (the reverberation mapping method). Findings: New estimates of the time delays between the Q2237+0305 light curves in the R and V spectral bands have been obtained from the observations of 2004, as well as more accurate estimates for the light curves of the 2005 season reported earlier. The time delay value averaged over the two years is ΔtR-V ≈ 6.7 ± 2.4 days in the observer’s coordinate system, or Δt̃R-V ≈ 2.49 ± 0.9 days in the rest frame. With this delay, a distance between the accretion disk annular zones responsible for the radiation in V and R is RR-RV = Δt̃R-V · c ≈ 6.46·1015 sm. This indicates an accretion disk size far exceeding that one predicted by the standard thin accretion disc model by Shakura and Sunyaev. In this work, a suggestion is checked that thus large disk size can be a consequence of a super-critical (super-Eddington) accretion regime considered in the classical work by Shakura and Sunyaev, where the supercritical accretion regime has been shown to results in formation of an extended optically thick scattering envelope on the disk periphery. Conclusions: Analytical expressions for the radius and temperature of such an envelope derived by Shakura and Sunyaev have been used in this work to calculate the RR and RV envelope dimensions in the R and V spectral bands expected from the super-critical accretion regime scenario in the Q2237+0305 quasar. The calculations have been made for three values of the black hole mass covering the whole range of the existing estimates for Q2237+0305, from MВH = 2·108 Mʘ to MВH = 20·108 Mʘ. The accretion rate was assumed to be ṁ = 17. The calculated envelope radii in spectral bands R and V are well consistent with the inter-band time delay value of 2.49 days obtained in the present work from the data of observations. Thus, it can be argued that the black hole in the Q2237+0305 quasar is accreting the matter in a moderately super-critical regime, which results in creating an extended optically thick scattering envelope, and this is just in this envelope that the reverberation responses examined in this work arise. Parameter α for the black hole- (efficiency of the angular momentum transport) is varying within 0.005 0.006 minimal mass, and from 0.029 to 0.033 for the maximal one, while parameter A (ratio of the energy losses in Compton scattering to those in free-free transitions) is within 50 to 100.

Tidal disruption events (TDEs) around supermassive black holes (SMBHs) are a potential laboratory to study super-Eddington accretion disks and sometimes result in powerful jets or outflows which may shine in the radio and sub-millimeter bands. In this work, we modeled the thermal synchrotron emission of jets by general relativistic radiation magneto-hydrodynamics (GRRMHD) simulations of a BH accretion disk/jet system which assumed the TDE resulted in a magnetized accretion disk around a BH accreting at ∼12–25 times the Eddington accretion rate. Through synthetic observations with the Next Generation Event Horizon Telescope (ngEHT) and an image reconstruction analysis, we demonstrate that TDE jets may provide compelling targets within the context of the models explored in this work. In particular, we found that jets launched by a SANE super-Eddington disk around a spin a*=0.9 reach the ngEHT detection threshold at large distances (up to 100 Mpc in this work). A two-temperature plasma in the jet or weaker jets, such as a spin a*=0 model, requires a much closer distance, as we demonstrate detection at 10 Mpc for limiting cases of a*=0,R=1 or a*=0.9,R=20. We also demonstrate that TDE jets may appear as superluminal sources if the BH is rapidly rotating and the jet is viewed nearly face on.

A three-dimensional numerical model has been developed to study the flow structure in close binary systems with a magnetic field. The model uses a system of equations of modified magnetic hydrodynamics, which allows describing all the main dynamic effects associated with the magnetic field. It takes into account the processes of radiation heating and cooling, heating due to current dissipation, as well as magnetic field diffusion. The model allows calculations in a wide range of magnetic field values. Comparison of the calculation results with observational data confirms the reliability and high efficiency of the model. The paper presents the calculation results of the flow structure in a typical intermediate polar. It is shown that an accretion disk is formed in such a binary system, which has the following characteristic features: “hot line”, tidal shock waves, precession density wave, magnetospheric region, and accretion columns. In this case, the magnetic field in the disk is predominantly toroidal. The paper also presents the results of calculations for typical polars. In such systems, instead of an accretion disk, a collimated stream of matter is formed, moving along the magnetic field lines to the magnetic poles of the white dwarf. It is shown that in synchronous polars, variations of the mass transfer rate lead to a change in the spatial configuration of the flow. In asynchronous polars, changes in the flow structure for different phases of the beat period are observed as well as the processes of switching the flow between the magnetic poles of the accretor. Numerical calculations of the asynchronous system are performed under the assumption of the dipole configuration of the magnetic field for different values of the dipole offset relative to the center of the white dwarf. The paper presents a method for estimating this offset from observational light curves.

This work proposes a methodology for testing phenomenologically motivated emission processes that account for the flux and polarization distribution and global structure of the 230 GHz sources imaged by the Event Horizon Telescope (EHT): Messier (M)87* and Sagittarius (Sgr) A*. We introduce into general relativistic magnetohydrodynamic (GRMHD) simulations some novel models to bridge the largely uncertain mechanisms by which high-energy particles in jet/accretion flow/black hole (JAB) system plasmas attain billion-degree temperatures and emit synchrotron radiation. The “Observing” JAB Systems methodology then partitions the simulation to apply different parametric models to regions governed by different plasma physics—an advance over methods in which one parametrization is used over simulation regions spanning thousands of gravitational radii from the central supermassive black hole. We present several classes of viewing-angle-dependent morphologies and highlight signatures of piecewise modeling and positron effects, including a MAD/SANE dichotomy in which polarized maps appear dominated by intrinsic polarization in the MAD case and by Faraday effects in the SANE case. The library of images thus produced spans a wide range of morphologies awaiting discovery by the groundbreaking EHT instrument and its yet more sensitive, higher-resolution next-generation counterpart, ngEHT.

We performed the timing and spectral studies of a Z source GX 17+2 observed from Astrosat LAXPC instrument. Cross-Correlation Function (CCF) was performed using soft (3–5 keV) and hard (16–40 keV) X-ray bands across the hardness intensity diagram and found correlated/anti-correlated hard and soft lags which seems to be a common feature in these sources. We performed spectral analysis for few of these observations and found no consistent variation in the spectral parameters during the lags, however 10–40% change was noticed in diskbb and power-law components in few of observations. For the first time, we report the detection of HBOs around ∼ 25 Hz and ∼ 33 Hz along with their harmonics using AstroSat LAXPC data. On comparison with spectral results of HB and other branches, we found that inner disk front is close to the last stable orbit and as such no systematic variations are observed. We suggest that the detected lags are readjustment time scales of corona close to the NS and constrained its height to be around few tens to hundreds of km. The detected lags and no significant variation of inner disk front across the HID strongly indicate that structural variation in corona is the most possible cause of Z track in HID. [ABSTRACT FROM AUTHOR]

The blazar PKS 0208-512 was in the lowest γ-ray brightness state during the initial 10 years of observations with the Fermi Gamma-ray Space Telescope (Fermi), which was an order of magnitude lower than its flux state during the EGRET era (1991–2000). The weekly averaged maximum γ-ray flux of this source during the first 10 years of Fermi observation is nearly a factor of 3 lower than the highest flux observed by EGRET in a single epoch. During the period 2018–2020, the source showed a large γ-ray flare, with the average brightness similar to the period 1991–2000. We observed the source with AstroSat, during its low and high activity states, respectively. We carried out broad-band spectral energy distribution (SED) modeling of the source using a one-zone leptonic emission model during its various brightness states. From the SED modeling, we found that there was an inefficient conversion from the bulk energy to the particle energy during the long-term low-activity states as compared to the high flux state during the EGRET era and the later part of Fermi observation.

The thermal radiation properties of the accretion disk around a non-rotating black hole with a perfect fluid dark matter (PFDM) environment are investigated. A non-rotating black hole surrounded by perfect fluid dark matter together with a classical geometrically thin but optically thick Novikov–Thorne disk is selected as a system to be analyzed. It is observed that the perfect fluid dark matter strengthens the gravitational field, which leads to both the increase of the radii of the event horizon and the innermost stable circular orbit (ISCO). However, for the flux of the radiant energy over the accretion disk, the maximum flux is reduced and shifted outwards the central object under the influence of the perfect fluid dark matter. The dependence of the thermal profile of the disk on the radial coordinate and the intensity of perfect fluid dark matter shows analogous behavior. It has been demonstrated that the radiative efficiency of the accretion disk is increased from ∼6% up to ∼20% with the increase in the intensity of the surrounding perfect fluid dark matter. The thermal spectra of the accretion disk has also been explored, which is shifted towards the lower frequencies (corresponding to the gravitational redshift of the electromagnetic radiation coming from the disk) with the increase in the intensity of the perfect fluid dark matter.

p { margin-bottom: 0.1in; direction: ltr; line-height: 120%; text-align: left; }a:link { } In this paper, the evolution of accretion disks in the post-Newtonian limit has been investigated. These disks are formed around gravitational compact objects such as black holes, neutron stars, or white dwarfs. Although most analytical researches have been conducted in this context in the framework of Newtonian dynamics and gravity, it is necessary to consider the effects of relativity on the structure of disks near the central body. To this end, by adding the post-Newtonian corrections to the hydrodynamic equations of the fluids, the equations for the time transformation of the accretion disks at the post-Newtonian limit were obtained; by using this equation, the surface density of the disk, which is dependent on radius and time, is got quasi- analytically. Finally, we compare the time evolution of the accretion disks in Newtonian and post-Newtonian gravity.

In this paper, we investigate the black hole (BH) spin contribution to jet power, especially for the magnetic arrested disk (MAD), where only inner accretion disk luminosity is closely coupled with the spin-jet power, and try to explain the ‘outliers’ track of the radio LR to X-ray luminosity LX in two black hole X-ray binaries (BHXBs). Our results suggest that the BZ-jet and the inner-disk coupling could account for the ‘outliers’ track of the radio/X-ray correlation in two BHXBs, H1743-322 and MAXI J1348-630. Although the accretion disk of H1743-322 in the outburst could be in the MAD state, there is a lower probability that MAXI J1348-630 is in the MAD state due to its low jet production efficiency. The difference in the inner-disk bolometric luminosity ratio of the two sources implies that these two BHXBs are in different inner-disk accretion states. We further investigate the phase-changing regime of MAXI J1348-630 and find that there is a phase transition around LX/LEdd∼10−3. The assumption of sub-MAD is discussed as well.