Search

A recent article in Astronomy discusses the challenges of finding black holes and highlights a new discovery made using the Gaia satellite. The Gaia satellite was used to identify two stars with a "wobble" in their orbit, indicating the presence of unseen black hole companions. This led to the discovery of the closest known black hole to Earth, which is unique because it is dormant and does not have an accretion disk. The article also discusses the author's collaboration with Kareem to observe another candidate black hole using telescopes in New Mexico and South Africa. The absence of radio waves suggests that the stellar wind particles never get close enough to the black hole's event horizon to generate radio waves. The article concludes by mentioning that more black holes are expected to be discovered in future data releases from the Gaia mission. [Extracted from the article]

The article titled "THE ROOKIES" discusses a platform called The Rookies that aims to help digital artists gain recognition and launch successful careers. The platform provides support and visibility to emerging artists, allowing them to showcase their work without competing with professionals. The article also features an artist profile of Jonas Nathanael Gali Kristensen, a Danish artist who studied computer graphic arts and has a passion for concept art and illustration. The profile includes a link to his portfolio on The Rookies website. [Extracted from the article]

Local studies of accretion disks and laboratory magnetized plasmas employ analytical coordinate mappings that introduce sheared boundary conditions (BCs). We present a discontinuous Galerkin (DG) algorithm to apply such BCs based on projections and quadrature-free integration. The procedure is high-order accurate, preserves moments exactly and works in multiple dimensions. Tests of increasing complexity are provided, beginning with translations of one and two dimensional fields, followed by 3D and 5D simulations with sheared (twist-shift) BCs. Results show that the algorithm is (p + 1) -order accurate in the DG representation and (p + 2) -order accurate in the cell averages, with p being the order of the polynomial basis. Quantification of the algorithm's hyperdiffusion and discussion of aliasing errors are given. This technique enables conservative local simulations of plasma turbulence with DG, not possible until now. • Invent the first discontinuous Galerkin (DG) algorithm for sheared boundary conditions. • This method is quadrature-free and exactly conservative. • Enable DG simulations of plasmas in core fusion flux-tubes and accretion disk shearing-boxes. [ABSTRACT FROM AUTHOR]

The article present a research on how the twisting of space caused by spinning supermassive black holes affects accretion discs, leading to faster feeding rates. It discusses the wobbling and splitting of accretion discs, along with the Lense-Thirring effect, create a turbulent environment around the black holes, ultimately speeding up the consumption of matter; black hole dynamics and their influence on galaxy evolution; and the quick brightening and dimming observed in some quasars.

Protoplanetary disks around young stars are the birth sites of planetary systems like our own. Disks represent the gaseous dusty matter left after the formation of their central stars. The mass and luminosity of the star, initial disk mass and angular momentum, and gas viscosity govern disk evolution and accretion. Protoplanetary disks are the cosmic nurseries where microscopic dust grains grow into pebbles, planetesimals, and planets. [ABSTRACT FROM AUTHOR]

The astronomers who took the first ever photo of a black hole have won a $3 million Breakthrough Prize. Leah Crane spoke to the team's leader [ABSTRACT FROM AUTHOR]

Accretion disks around black holes radiate a significant fraction of the rest mass of the accreting material in the form of thermal radiation from within a few gravitational radii of the black hole (r≲20GM/c²). In addition, the accreting matter may also be illuminated by hard x rays from the surrounding plasma which adds fluorescent transition lines to the emission. This radiation is emitted by matter moving along geodesics in the metric; therefore the strong Doppler and gravitational redshifts observed in the emission encode information about the strong gravitational field around the black hole. In this paper the possibility of using the x-ray emission as a strong field test of general relativity is explored by calculating the spectra for both the transition line and thermal emission from a thin accretion disk in a series of parametrically deformed Kerr metrics. In addition the possibility of constraining a number of known black hole spacetimes in alternative theories of gravity is also considered. [ABSTRACT FROM AUTHOR]

We explore thick accretion disks around rotating attractors. We detail the configurations analyzing the fluid angular momentum and finally provide a characterization of the disk morphology and different possible topologies. Investigating the properties of orbiting disks, a classification of attractors, possibly identifiable in terms of their spin-mass ratio, is introduced; then an attempt to characterize dynamically a series of different disk topologies is discussed, showing that some of the toroidal configuration features are determined by the ratio of the angular momentum of the orbiting matter and the spin-mass ratio of the attractor. Then we focus on "multistructured" disks constituted by two or more rings of matter orbiting the same attractor, and we prove that some structures are constrained in the dimension of rings, spacing, location, and an upper limit of ring number is provided. Finally, assuming a polytropic equation of state, we study some specific cases. [ABSTRACT FROM AUTHOR]

Now, researchers have used additional image processing to remove the broader glow of the accretion disk (depicted here by blue contours) and reveal a narrow, bright ring of light, called the photon ring. It is the first time the photon ring, a feature predicted by general relativity, has been seen. In 2019, the Event Horizon Telescope (EHT) released its first image of a black hole: M87*. [Extracted from the article]

We evaluate general-relativistic effects in the motion of stationary self-gravitating accretion disks around a Schwarzschild black hole, assuming the first post-Newtonian (1PN) approximation. There arises an integrability condition that leads to the emergence of two types of general-relativistic corrections to a Newtonian rotation curve. The well-known geometric dragging of frames accelerates rotation, but the hitherto unknown dynamic term, that reflects the disk structure, decelerates rotation. The net result can diminish the Newtonian angular velocity of rotation in a central disk zone, but the geometric dragging of frames dominates in the disk boundary zone. Both effects are nonlinear in nature, and they disappear in the limit of test fluids. Dust disks can only be geometrically dragged, while uniformly rotating gaseous disks are untouched at the 1PN order. General-relativistic contributions can strongly affect rotation periods in Keplerian motion for compact systems. [ABSTRACT FROM AUTHOR]

The spins of a number of supermassive and stellar-mass black holes have been measured based on detections of thermal continuum emission and relativistically broadened iron lines in their x-ray spectra. Likewise, quasiperiodic variability has been observed in several sources. Such measurements commonly make the assumption that black holes are described by the Kerr metric, which according to the no-hair theorem characterizes black holes uniquely in terms of their masses and spins. This fundamental property of black holes can be tested observationally by measuring potential deviations from the Kerr metric introduced by a parametrically deformed Kerr-like spacetime. Thermal spectra, iron lines, and variability have already been studied extensively in several such metrics, which usually depend on only one particular type of deviation or contain unphysical regions outside of the compact object. In this paper, I study these x-ray probes in the background of a new Kerr-like metric which depends on four independent deviation functions and is free of pathological regions outside of the event horizon. I show that the observed signals depend significantly on primarily two types of deviations and that the strong correlation between the spin and the deviation parameters found previously in other Kerr-like metrics is partially broken for rapidly spinning black holes. This suggests that high-spin sources are the best candidates for tests of the no-hair theorem with x rays and I obtain first constraints on such deviations from the stellar-mass black hole Cygnus X-l. [ABSTRACT FROM AUTHOR]

The construction of a generic parametrization to describe the spacetime geometry around astrophysical black hole candidates is an important step to test the Kerr black hole hypothesis. In the last few years, the Johannsen-Psaltis metric has been the most common framework to study possible deviations from the Kerr solution with present and near-future observations. Recently, Cardoso, Pani, and Rico have proposed a more general parametrization. The aim of the present paper is to study this new metric in a specific context, namely, the thermal spectrum of geometrically thin and optically thick accretion disks. The most relevant finding is that the spacetime geometry around objects that look like very-fastrotating Kerr black holes may still have large deviations from the Kerr solution. This is not the case with the Johannsen-Psaltis metric, which means the latter is missing an important class of non-Kerr spacetimes. [ABSTRACT FROM AUTHOR]

In this paper we report on a search for short-duration gravitational wave bursts in the frequency range 64 Hz-1792 Hz associated with gamma-ray bursts (GRBs), using data from GEO 600 and one of the LIGO or Virgo detectors. We introduce the method of a linear search grid to analyze GRB events with large sky localization uncertainties, for example the localizations provided by the Fermi Gamma-ray Burst Monitor (GBM). Coherent searches for gravitational waves (GWs) can be computationally intensive when the GRB sky position is not well localized, due to the corrections required for the difference in arrival time between detectors. Using a linear search grid we are able to reduce the computational cost of the analysis by a factor of 0(10) for GBM events. Furthermore, we demonstrate that our analysis pipeline can improve upon the sky localization of GRBs detected by the GBM, if a high-frequency GW signal is observed in coincidence. We use the method of the linear grid in a search for GWs associated with 129 GRBs observed satellitebased gamma-ray experiments between 2006 and 2011. The GRBs in our sample had not been previously analyzed for GW counterparts. A fraction of our GRB events are analyzed using data from GEO 600 while the detector was using squeezed-light states to improve its sensitivity; this is the first search for GWs using data from a squeezed-light interferometric observatory. We find no evidence for GW signals, either with any individual GRB in this sample or with the population as a whole. For each GRB we place lower bounds on the distance to the progenitor, under an assumption of a fixed GW emission energy of 10-2M⊙c², with a median exclusion distance of 0.8 Mpc for emission at 500 Hz and 0.3 Mpc at 1 kHz. The reduced computational cost associated with a linear search grid will enable rapid searches for GWs associated with Fermi GBM events once the advanced LIGO and Virgo detectors begin operation. [ABSTRACT FROM AUTHOR]

Kilohertz quasi periodic oscillations (QPOs) can be used as a probe of the inner regions of accretion disks in compact stars and hence also of the properties of the central object. Most models of kHz QPOs involve epicyclic frequencies to explain their origin. We compute the epicyclic frequencies of nearly circular orbits around rotating strange quark stars. The MIT bag model is used to model the equation of state of quark matter, and the uniformly rotating stellar configurations are computed in full general relativity. The vertical epicyclic frequency and the related nodal precession rate of inclined orbits are very sensitive to the oblateness of the rotating star. For rotating stellar models of moderate and high-mass strange stars, the sense of the nodal precession changes at a certain rotation rate. At lower stellar rotation rates, the orbital nodal precession is prograde, as it is in the Kerr metric, while at higher rotation rates, the precession is retrograde, as it is for Maclaurin spheroids. Thus, qualitatively, the orbits around rapidly rotating strange quark stars are affected more strongly by the effects of stellar oblateness than by the effects of general relativity. We show that epicyclic and orbital frequencies calculated numerically for small mass strange stars are in very good agreement with analytical formulas for Maclaurin spheroids [ABSTRACT FROM AUTHOR]

We report on simulations in general relativity of magnetized disks accreting onto black hole binaries. We vary the binary mass ratio from 1:1 to 1:10 and evolve the systems when they orbit near the binary-disk decoupling radius. We compare (surface) density profiles, accretion rates (relative to a single, nonspinning black hole), variability, effective a-stress levels and luminosities as functions of the mass ratio. We treat the disks in two limiting regimes: rapid radiative cooling and no radiative cooling. The magnetic field lines clearly reveal jets emerging from both black hole horizons and merging into one common jet at large distances. The magnetic fields give rise to much stronger shock heating than the pure hydrodynamic flows, completely alter the disk structure, and boost accretion rates and luminosities. Accretion streams near the horizons are among the densest structures; in fact, the 1:10 no-cooling evolution results in a refilling of the cavity. The typical effective temperature in the bulk of the disk is ~105(M/108M⊙)-1/4(L/Ledd)1/4 K yielding characteristic thermal frequencies ~1015(M/108M⊙)-1/4(L/Ledd)1/4(1+z)-1 Hz. These systems are thus promising targets for many extragalactic optical surveys, such as the LSST, WFIRST, and PanSTARRS. [ABSTRACT FROM AUTHOR]

The article discusses research on black holes using the Event Horizon Telescope radio antenna array, focusing on planned imaging of the Sagittarius A* massive black hole in the Milky Way as it engulfs a gas cloud expected to approach it 2014. Topics include the formation of matter into accretion disks near black holes, synchrotron radiation of charged particles emitted in supersonic jets from black holes, and supermassive black holes in the Centaurus A and M87 galaxies.

The nature of the supermassive black hole candidates in galactic nuclei can be tested by analyzing the profile of the Kα iron line observed in their x-ray spectrum. In this paper, we consider the possibility that the spacetime in the immediate vicinity of these objects may be described by some nonvacuum exact solutions of Einstein's equations resulting as the end state of gravitational collapse. The vacuum far away portion of the spacetime is described by the Schwarzschild metric, while the interior part may be either regular or have a naked singularity at the center. The iron line generated around this class of objects has specific features, which can be used to distinguish such objects from Kerr black holes. In particular, their iron line cannot have the characteristic low-energy tail of the line generated from accretion disks around fast-rotating Kerr black holes. We can thus conclude that the supermassive black hole candidates whose spin parameter has been estimated to be close to 1 assuming the Kerr background cannot be this kind of objects. [ABSTRACT FROM AUTHOR]

We perform several black-hole binary evolutions using fully nonlinear numerical relativity techniques at separations large enough that low-order post-Newtonian expansions are expected to be accurate. As a case study, we evolve an equal-mass nonspinning black-hole binary from a quasicircular orbit at an initial coordinate separation of D = 100M for three different resolutions. We find that the orbital period of this binary (in the numerical coordinates) is T = 6422M. The orbital motion agrees with post-Newtonian predictions' to within 1%. However,'wè find that the time derivative of the coordinate separation is dominated by a purely gauge'effect leading to an apparent contraction and expansion of the orbit at twice the orbital frequency. Based on these results, we improved our evolution techniques and studied a set of black hole binaries in quasicircular orbits starting at D = 20M, D = 50M, and D = 100M for ~5, 3, and 2 orbits, respectively. We then find good agreement between the numerical results and post-Newtonian predictions for the orbitaj frequency and radial decay rate, radiated energy and angular momentum, and waveform amplitude and phases. The results are relevant for the future computation of long-term waveforms to assist in the detection and analysis of gravitational waves by the next generation of detectors as well as the long-term simulations of black-hole binaries required to accurately model astrophysically realistic circumbinary accretion disks. [ABSTRACT FROM AUTHOR]

According to the no-hair theorem, astrophysical black holes are uniquely described by the Kerr metric. In order to test this theorem with observations in either the electromagnetic or gravitational-wave spectra, several Kerr-like spacetimes have been constructed which describe potential deviations from the Kerr spacetime in parametric form. For electromagnetic tests of the no-hair theorem, such metrics allow for the proper modeling of the accretion flows around candidate black holes and the radiation emitted from them. In many of these models, the location of the inner edge of the accretion disk is of special importance. This inner edge is often taken to coincide with the innermost stable circular orbit, which can serve as a direct probe of the spin and the deviation from the Kerr metric. In certain cases, however, an innermost stable circular orbit does not exist, and the inner edge of an accretion disk is instead determined by an instability against small perturbations in the direction vertical to the disk. In this paper, I analyze the properties of accretion disks in the Kerr-like metric proposed by Johannsen and Psaltis [Phys. Rev. D 83, 124015 (2011)], whose inner edges are located at the radii where this vertical instability occurs. I derive expressions of the energy and axial angular momentum of disk particles that move on circular equatorial orbits and calculate the locations of the inner disk edges. As a possible observable of such accretion disks, I simulate profiles of relativistically broadened iron lines and show that they depend significantly on the values of the spin and the deviation parameter. [ABSTRACT FROM AUTHOR]

We consider a geometrically thin, Keplerian disk in the orbital plane of a binary black hole (BHBH) consisting of a spinning primary and low-mass secondary (mass ratio q ≲ 1). To account for the principle effects of general relativity (GR), we propose a modification of the standard Newtonian evolution equation for the (orbit-averaged) time-varying disk surface density. In our modified equation the viscous torque in the disk is treated in full GR, while the tidal torque is handled in the Newtonian limit. Our GR-hybrid treatment is reasonable because the tidal torque is concentrated near the orbital radius of the secondary and is most important prior to binary-disk decoupling, when the orbital separation is large and resides in the weak-field regime. The tidal torque on the disk diminishes during late merger and vanishes altogether following merger. By contrast, the viscous torque drives the flow into the strong-field region and onto the primary during all epochs. Following binary coalescence, the viscous torque alone governs the time-dependent accretion onto the remnant, as well as the temporal behavior, strength and spectrum of the aftermath electromagnetic radiation from the disk. We solve our GR-hybrid equation for a representative BHBH-disk system, identify several observable EM signatures of the merger, and compare results obtained for the gas and EM radiation with those found with the Newtonian prescription. [ABSTRACT FROM AUTHOR]

The article discusses research regarding the formation of exoplanets, or planets orbiting other stars, with information on the challenges faced by astrophysicists in hypothesizing how exoplanets develop from cosmic dust and primordial gases. Topics include the difficulty in detecting smaller planetesimals due to the limitations of telescopes, the theories of core accretion or gravitational instability in which planets either form around smaller structures or from the breakdown of larger structures, and the role of protoplanetary disks in exoplanet development.

It has been proposed that the supermassive black hole candidates at the centers of galaxies might be wormholes formed in the early Universe and connecting our Universe with other sister universes. The analysis of the profile of the relativistic Kα iron line is currently the only available approach to probe the spacetime geometry around these objects. In this paper, we compute the expected Kα iron line in some wormhole spacetimes and we compare the results with the line produced around Kerr black holes. The line produced in accretion disks around nonrotating or very slow-rotating wormholes is relatively similar to the one expected around Kerr black holes with mid or high value of spin parameter and current observations are still marginally compatible with the possibility that the supermassive black hole candidates in galactic nuclei are these objects. For wormholes with spin parameter a* ≥ 0.02, the associated Kα iron line is instead quite different from the one produced around Kerr black holes, and their existence may already be excluded. [ABSTRACT FROM AUTHOR]

Black-hole-neutron-star mergers resulting in the disruption of the neutron star and the formation of an accretion disk and/or the ejection of unbound material are prime candidates for the joint detection of gravitational-wave and electromagnetic signals when the next generation of gravitational-wave detectors comes online. However, the disruption of the neutron star and the properties of the postmerger remnant are very sensitive to the parameters of the binary (mass ratio, black-hole spin, neutron star radius). In this paper, we study the impact of the radius of the neutron star and the alignment of the black-hole spin on black-hole-neutron-star mergers within the range of mass ratio currently deemed most likely for field binaries (MBH ~ 7MNS) and for black-hole spins large enough for the neutron star to disrupt (JBH/M²QH = 0.9). We find that (i) In this regime, the merger is particularly sensitive to the radius of the neutron star, with remnant masses varying from 0.3MNS to 0.1MNS for changes of only 2 km in the NS radius; (ii) 0.01M⊙-0.05M⊙ of unbound material can be ejected with kinetic energy ≳ 1051 ergs, a significant increase compared to low mass ratio, low spin binaries. This ejecta could power detectable postmerger optical and radio afterglows, (iii) Only a small fraction of the Advanced LIGO events in this parameter range have gravitational-wave signals which could offer constraints on the equation of state of the neutron star (at best ~3% of the events for a single detector at design sensitivity), (iv) A misaligned black-hole spin works against disk formation, with less neutron-star material remaining outside of the black hole after merger, and a larger fraction of that material remaining in the tidal tail instead of the forming accretion disk, (v) Large kicks μkick 300 km/s can be given to the final black hole as a result of a precessing black-hole-neutron-star merger, when the disruption of the neutron star occurs just outside or within the innermost stable spherical orbit. [ABSTRACT FROM AUTHOR]

Abstract: Current explicit integration techniques in fluid dynamics are deeply limited by the Courant–Friedrichs–Lewy condition of the time step progression, based on the adopted spatial resolution coupled with the maximum value between the kinetic velocity or the signal transmission speed in the computational domain. Eulerian implicit integration techniques, even though more time consuming, can allow us to perform stable computational fluid dynamics paying the price of a relatively larger inaccuracy in the calculations, without suffering such a strict temporal limitation. In this paper, we present a simple and effective scheme to perform free Lagrangian Smooth Particle Hydrodynamics (SPH) implicit integrations in the semi-Lagrangian approach without any Jacobian matrix inversion operations for viscous Navier–Stokes flows. Applications to SPH accretion disc simulation around a massive black hole (MBH) in a binary stellar system are shown, together with the comparison to the same results obtained according to the traditional explicit integration techniques. Some 1D and 2D critical tests are also discussed to check the validity of the technique. [Copyright &y& Elsevier]

We analyze the configuration of a thin rotating accretion disk, which is embedded in a magnetic field inducing a backreaction in the gravitating plasma. The aim of this study is to determine the conditions under which the gaseous accretion model of Shakura can be reconciled with the magnetohydrodynamical picture requested to trigger the underlying turbulent behavior. We focus our attention on the generalized Ohm equation in order to understand if the plasma backreaction is able to provide the proper toroidal current, allowing a nonzero Mailing velocity. In the limit of linear plasma backreaction, this analysis shows how the Shakura profile of accretion turns out to be inconsistent. In particular, comparing the azimuthal and the generalized Ohm equilibrium equations, we argue that it is not possible to maintain a constant rate of accretion. A nonstationary scenario for the disk configuration is then outlined and it results in a transient process which is however associated to a vanishing accretion rate. [ABSTRACT FROM AUTHOR]

Black hole accretion disks can form through the collapse of rotating massive stars. These disks produce large numbers of neutrinos and antineutrinos of electron flavor that can influence energetics and nucleosynthesis. Neutrinos are produced in sufficient numbers that, after they are emitted, they can undergo flavor transformation facilitated by the neutrino self interaction. We show that some of the neutrino flavor transformation phenomenology for accretion disks is similar to that of the supernova case, but also, we find the disk geometry lends itself to different transformation behaviors. These trans-formations strongly influence the nucleosynthetic outcome of disk winds. [ABSTRACT FROM AUTHOR]

We study here circular timelike geodesies in the Janis-Newman-Winicour and Gamma metric space-times which contain a strong curvature naked singularity and reduce to the Schwarzschild metric for a specific value of one of the parameters. We show that for both the metrics the range of allowed parameters can be divided into three regimes where structure of the circular geodesies is qualitatively different. It follows that the properties of the accretion disks around such naked singularities can be significantly different from those of disks around black holes. This adds to previous studies showing that if naked singularities exist in nature, their observational signature would be significantly different from that of the black hole. [ABSTRACT FROM AUTHOR]

We numerically solve the one-dimensional Boltzmann equation of the neutrino and antineutrino transport in accretion disks and obtain the fully energy-dependent and direction-dependent neutrino-and antineutrino-emitting spectra, under the condition that the distribution of the mass density, temperature and chemical components are given. Then, we apply the resulting neutrino- and antineutrino-emitting spectra to calculate the corresponding annihilation rate of neutrino pairs above the neutrino-dominated accretion disk and find that the released energy resulting from the annihilation of neutrino pairs cannot provide sufficient energy for the most energetic short gamma ray bursts whose isotropic luminosity can be as high as 1052 ergs/s unless the high-temperature zone, where the temperature is beyond 10 MeV, can stretch over 200 km in the disk. We also compare the resulting luminosity of neutrinos and antineutrinos with the results from the two commonly used approximate treatments of the neutrino and antineutrino luminosity: the Fermi-Dirac black-body limit and a simplified model of neutrino transport, i.e., the gray-body model, and find that both of them overestimate the neutrino/antineutrino luminosity and their annihilation rate greatly. Additionally, as did in Sawyer (2003), we also check the validity of the two-stream approximation, and find that it is a good approximation to high accuracy. [ABSTRACT FROM AUTHOR]

When the jets collide with clouds of gas and dust surrounding the baby star, a Herbig-Haro object like HH111 is born. QUANTUM GRAVITY EVERYTHING YOU NEED TO KNOW ABOUT THE UNIVERSE THIS MONTH SNAPSHOT The Orion B molecular cloud hides HH111, a prime example of a rare celestial phenomenon generated by protostars - very young stars still in the process of coalescing. To capture this image of HH111, the Hubble Space Telescope used its Wide Field Camera 3, which can observe at the infrared wavelengths, to reveal the object. [Extracted from the article]

We compare the optical appearance of a thin accretion disk in the Simpson-Visser spacetime to the Schwarzschild black hole case in this paper. We calculate and illustrate the redshift and observed flux distributions as viewed by distant observers at various inclination angles. Simpson-Visser family of metrics creates Novikov-Thorne (NT) accretion disks images that nearly look like a Schwarzschild black hole's NT accretion disk. We have studied also the embedding diagram, the electromagnetic properties of the accretion disk such as the temperature and the radiation flux of the energy by the accretion disk and the accretion efficiency. Compared to the Schwarzschild black hole, we find that the temperature, radiation flux of the energy, and the luminosity of the accretion disk increase by increasing the regularization parameter l. We conclude that, based on astrophysical observational signatures in the properties of the electromagnetic spectrum, we can distinguish the wormhole geometries from the regular black holes (black-bounce) and the Schwarzschild black hole. [ABSTRACT FROM AUTHOR]

The feature of the observed shadows and rings of an astrophysical black hole (BH) may depend on its accretion flows and magnetic charge. We find that the shadow radii and critical impact parameters of the Hayward BH are decreased with the increase of the magnetic charge. Comparing the Schwarzschild BH with the Hayward BH using the ray-tracing method, we show that the density and deflection of lights increase with the magnetic charge, and the BH singularity does not affect the generation of the shadow. Based on three optically thin accretion flow models, the two-dimensional shadows in celestial coordinates are derived. It is found that the shadow and photon ring luminosities of a Hayward BH surrounded by infalling spherical accretion flow are dimmer than that of a static spherical accretion flow. Taking three kinds of inner radii at which the accretion flow stops radiating, we find that the observed luminosity of a Hayward BH surrounded by a thin disk accretion flow is dominated by the direct emission, and the photon ring emission has a weak influence on it. These results suggest that the size of the observed shadow is related to the space-time geometry, and the luminosities of both the shadows and rings are affected by the accretion flow property and the BH magnetic charge. [ABSTRACT FROM AUTHOR]

Axionlike particles (ALPs) could mix with photons in the presence of astrophysical magnetic fields. Searching for this effect in gamma-ray observations of blazars has provided some of the strongest constraints on ALP parameter space so far. Previously, photon-photon dispersion of gamma rays off of the cosmic microwave background has been shown to be important for these calculations and is universally included in ALP-photon mixing models. Here, we assess the effects of dispersion off of other photon fields within the blazar (produced by the accretion disk, the broad line region, the dust torus, starlight, and the synchrotron field) by modeling the jet and fields of the flat spectrum radio quasar 3C454.3 and propagating ALPs through the model both with and without the full dispersion calculation. We find that the full dispersion calculation can strongly affect the mixing, particularly at energies above 100 GeV--often reducing the ALP-photon conversion probability. This could have implications for future searches planned with, e.g., the Cherenkov Telescope Array, particularly those looking for a reduced opacity of the Universe at the highest energies. [ABSTRACT FROM AUTHOR]

The observation of the shadow of the supermassive black hole M87* by the Event Horizon Telescope (EHT) is sensitive to the spacetime geometry near the circular photon orbit and beyond, and it thus has the potential to test general relativity in the strong field regime. Obstacles to this program, however, include degeneracies between putative deviations from general relativity and both the description of the accretion flow and the uncertainties on "calibration parameters," such as, e.g., the mass and spin of the black hole. In this work, we introduce a formalism, based on a principal component analysis, capable of reconstructing the black hole metric (i.e., the "signal") in an agnostic way, while subtracting the "foreground" due to the uncertainties in the calibration parameters and the modeling of the accretion flow. We apply our technique to simulated mock data for spherically symmetric black holes surrounded by a thick accretion disk. We show that separation of signal and foreground may be possible with next generation EHT-like experiments. [ABSTRACT FROM AUTHOR]

Recently, a distinct shadow mechanism was proposed by Wang et al. from the asymmetric thin-shell wormhole (ATW) [X. Wang et al., Phys. Lett. B 811, 135930 (2020)]. On the other hand, Gralla, Holz, and Wald's work [Phys. Rev. D 100, 024018 (2019)] represented a nice description of photon rings in the presence of an accretion disk around a black hole. In this paper, we are inspired to thoroughly investigate the observational appearance of an accretion disk around the ATW. Although the spacetime outside an ATW with a throat could be identical to that containing a black hole with its event horizon, we show evident additional photon rings from the ATW spacetime. Moreover, a potential lensing band between two highly demagnified photon rings is found. Our analysis provides an optically observational signature to distinguish ATWs from black holes. [ABSTRACT FROM AUTHOR]

We study stable circular orbits (SCO) around static spherically symmetric configuration of general relativity with a nonlinear scalar field (SF). The configurations are described by solutions of the Einstein-SF equations with monomial SF potential V(ϕ)=|ϕ|2n, n>2, under the conditions of the asymptotic flatness and behavior of SF ϕ∼1/r at spatial infinity. We proved that under these conditions, the solution exists and is uniquely defined by the configuration mass M>0 and scalar "charge" Q. The solutions and the space-time geodesics have been investigated numerically in the range n≤40, |Q|≤60, M≤60. We focus on how nonlinearity of the field affects properties of SCO distributions (SCOD), which, in turn, affect topological form of the thin accretion disk around the configuration. Maps are presented showing the location of possible SCOD types for different M, Q, n. We found a lot of differences from the Fisher-Janis-Newman-Winicour metric (FJNW) dealing with the linear SF, though basic qualitative properties of the configurations have much in common with the FJNW case. For some values of n, a topologically new SCOD type was discovered that is not available for the FJNW metric. Like FJNW, all images of accretion disks have a dark spot in the center (mimicking the ordinary black hole), either because there is no SCO near the center or due to the strong deflection of photons near the singularity, although fine features other than a black hole can appear with a special choice of M, Q. [ABSTRACT FROM AUTHOR]

Motivated by electromagnetic-field confinement due to plasma near accreting black holes, we continue our exploration of the linear dynamics of an electromagnetic field propagating in curved spacetime in the presence of plasma by including three effects that were neglected in our previous analysis: collisions in the plasma, thermal corrections, and the angular momentum of the background black hole spacetime. We show that: (i) the plasma-driven long-lived modes survive in a collisional plasma except when the collision timescale is unrealistically small; (ii) thermal effects, which might be relevant for accretion disks around black holes, do not affect the axial long-lived modes; (iii) in the case of a spinning black hole the plasma-driven modes become superradiantly unstable at the linear level; (iv) the polar sector in the small-frequency regime admits a reflection point due to the resonant properties of the plasma. Dissipative effects such as absorption, formation of plasma waves, and nonlinear dynamics play a crucial role in the vicinity of this resonant point. [ABSTRACT FROM AUTHOR]

We present new equilibrium solutions of stationary models of magnetized thick disks (or tori) around Kerr black holes with synchronized scalar hair. The models reported here largely extend our previous results based on constant radial distributions of the specific angular momentum along the equatorial plane. We introduce a new way to prescribe the distribution of the disk's angular momentum based on a combination of two previous proposals and compute the angular momentum distribution outside the equatorial plane by resorting to the construction of von Zeipel cylinders. We find that the effect of the scalar hair on the black hole spacetime can yield significant differences in the disk morphology and properties compared to what is found if the spacetime is purely Kerr. Some of the tori built within the most extreme, background hairy black hole spacetime of our sample exhibit the appearance of two maxima in the gravitational energy density which impacts the radial profile distributions of the disk's thermodynamical quantities. The models reported in this paper can be used as initial data for numerical evolutions with general relativistic magnetohydrodynamic codes to study their stability properties. Moreover, they can be employed as illuminating sources to build shadows of Kerr black holes with scalar hair which might help further constrain the no-hair hypothesis as new observational data are collected. [ABSTRACT FROM AUTHOR]

We consider the circular motion of test particles in the gravitational field of a static and axially symmetric compact object described by the q metric. To this end, we calculate orbital parameters of test particles on accretion disks such as angular velocity (Ω), total energy (E), angular momentum (L), and radius of the innermost stable circular orbit (r ISCO) as functions of the mass (m) and quadrupole (q) parameters of the source. The radiative flux, differential, and spectral luminosity of the accretion disk, which are quantities that can be experimentally measured, are then explored in detail. The obtained results are compared with the corresponding ones for the Schwarzschild and Kerr black holes in order to establish whether black holes may be distinguished from the q metric via observations of the accretion disk's spectrum. [ABSTRACT FROM AUTHOR]

In this paper, we constructed the magnetized thick disk model analytically around the static black hole in the presence of an external distribution of matter up to the quadrupole moment. This space-time is a solution to Einstein's field equation describing the exterior of a static and axially symmetric object locally. This work aims to study this space-time and the effects of quadrupole moments via studying the properties of the equilibrium sequences of magnetized, non-self-gravitating disks in this space-time. We discussed a procedure to build the thick disk model based on a combination of two approaches previously considered in the literature. We have examined different angular momentum distributions, and we have shown the properties of this relativistic accretion disk model in this background. [ABSTRACT FROM AUTHOR]

Recently, the image of a Schwarzschild black hole with an accretion disk has been revisited, and it showed that the "photon ring," defined as highly bent light rays that intersect the disk plane more than twice, is extremely narrow and makes a negligible contribution to the total brightness. In this paper, we investigate the observational appearance of an optically and geometrically thin accretion disk around a hairy black hole in an Einstein-Maxwell-scalar model. Intriguingly, we find that in a certain parameter regime, due to an extra maximum or an "anklelike" structure in the effective potential for photons, the photon ring can be remarkably wide, thus making a notable contribution to the flux of the observed image. In particular, there appears a wide and bright annulus, which comprises multiple concentric bright thin rings with different luminosity, in the high resolution image. [ABSTRACT FROM AUTHOR]

Today we have quite stringent constraints on possible violations of the weak-equivalence principle from the comparison of the acceleration of test-bodies of different composition in Earth's gravitational field. In the present paper, we propose a test of the weak-equivalence principle in the strong gravitational field of black holes. We construct a relativistic reflection model in which either the massive particles of the gas of the accretion disk or the photons emitted by the disk may not follow the geodesics of the spacetime. We employ our model to analyze the reflection features of a NuSTAR spectrum of the black hole binary EXO 1846-031 and we constrain two parameters that quantify a possible violation of the weak-equivalence principle by massive particles and x-ray photons, respectively. [ABSTRACT FROM AUTHOR]

Like two ballet dancers spinning on a grand stage, these supermassive black holes gracefully sweep toward one another in a visualization created by NASA's Goddard Space Flight Center. Light from the accretion disks, colored blue and red, shows how the extreme gravitational forces of the objects distort their appearance when one black hole crosses in front of the other. Surrounding each black hole is an accretion disk, a superheated soup of material that swirls around before flowing inward like water disappearing down a drain. [Extracted from the article]

We study the quasi-periodic oscillations from the accretion disk around rotating traversable wormholes by means of the resonance models. We investigate the linear stability of the circular geodesic orbits in the equatorial plane for a general class of wormhole geometries deriving analytical expressions for the epicyclic frequencies. Since wormholes can often mimic black holes in the astrophysical observations, we analyze the properties of the quasi-circular oscillatory motion in comparison with the Kerr black hole. We demonstrate that wormholes possess distinctive features, which can be observationally significant. It is characteristic for the Kerr black hole that the orbital and the epicyclic frequencies obey a constant ordering in the whole range of the spin parameter. In contrast, for wormhole spacetimes we can have various types of orderings between the frequencies in the different regions of the parametric space. This enables the excitation of much more diverse types of resonances including parametric and forced resonances of lower order, which could lead to stronger observable signals. In addition, for corotating orbits the resonances can be excited in a very close neighborhood of the wormhole throat for a wide range of values of the angular momentum, making wormholes a valuable laboratory for testing strong gravity. [ABSTRACT FROM AUTHOR]

The article discusses a joint paper published in the journal "Classical and Quantum Gravity" from the film "Interstellar"'s special effects team and scientific consultant Kip Thorne, on the team's decisions to modify real-life models of a black hole deemed to complex for audiences to understand. Topics include the visual effects studio Double Negative and director Christopher Nolan; and elements such as the black hole's accretion disk, spin, color scheme, time dilation, and Doppler shift.

We study a viable connection between the circular-equatorial orbits and reflection symmetry across the equatorial plane of a vacuum stationary axis-symmetric spacetime in general relativity. The behavior of the circular equatorial orbits in the direction perpendicular to the equatorial plane is studied, and different outcomes in the presence and in the absence of the reflection symmetry are discussed. We conclude that in the absence of the equatorial reflection symmetry neither stable nor unstable circular orbit can exist on the equatorial plane. Moreover, to address the observational aspects, we provide two possible examples relating gravitational wave astronomy and the thin accretion disk which can put constraints on the symmetry breaking parameters. [ABSTRACT FROM AUTHOR]

Very steep reflection emissivity profiles in the inner part of accretion disks are commonly found in the analysis of x-ray observations of black hole binaries and active galactic nuclei, but there is some debate about their exact origin. While steep reflection emissivity profiles can be naturally produced by compact coronae close to black holes, the measured radial emissivity parameter can be further increased by the radial disk ionization profile when the theoretical model assumes a disk with constant ionization. In this paper, we implement the possibility of a radial disk ionization profile in the reflection model relxill_nk, which is a package designed to calculate reflection spectra of "deformed" Kerr black holes. We analyze a NuSTAR observation of the black hole binary EXO 1846-031, which was previously found to have a very high inner emissivity index. We find that the model with a radial disk ionization profile improves the fit, but the impact on the estimate of the black hole spin parameter and on the constraint of the deformation parameter is modest. However, we show that the analysis of future observations of Athena and eXTP will necessarily require models with a radial disk ionization profile to have accurate constraints of the deformation parameters. [ABSTRACT FROM AUTHOR]

Extreme mass ratio inspirals (EMRIs) are important sources for space-borne gravitational wave detectors, such as the laser interferometer space antenna and TianQin. Previous EMRI rate studies have focused on the "loss cone" scenario, where stellar-mass black holes (sBHs) are scattered into highly eccentric orbits near the central massive black hole (MBH) via multibody interaction. In this work, we calculate the rate of EMRIs of an alternative formation channel: EMRI formation assisted by the accretion flow around accreting massive black holes. In this scenario, sBHs and stars on inclined orbits are captured by the accretion disk, and then subsequently migrate towards the MBH, under the influence of density wave generation and head wind. By solving the Fokker-Planck equation incorporating both sBH-sBH-sBH-star scatterings and sBH-star-disk interactions, we find that an accretion disk usually boosts the EMRI formation rate per individual MBH by Ϭ(10¹-10³) compared with the canonical loss cone formation channel. Taking into account that the fraction of active Galactic nuclei (AGNs) is Ϭ(10-2-10-1), where the MBHs are expected to be rapidly accreting, we expect EMRI formation assisted by AGN disks to be an important channel for all EMRIs observed by space-borne gravitational wave detectors. These two channels also predict distinct distributions of EMRI eccentricities and orbit inclinations with respect to the MBH spin equatorial plane, which can be tested by future gravitational wave observations. [ABSTRACT FROM AUTHOR]

Neutrinos drive core-collapse supernovae, launch outflows from neutron star merger accretion disks, and set the ratio of protons to neutrons in ejecta from both systems that generate heavy elements in the Universe. Neutrinos of different flavors interact with matter differently, and much recent work has suggested that fast flavor instabilities are likely ubiquitous in both systems, but the final flavor content after the instability saturates has not been well understood. In this work we present particle-in-cell calculations which follow the evolution of all flavors of neutrinos and antineutrinos through saturation and kinematic decoherence. We conduct one-dimensional three-flavor simulations of neutrino quantum kinetics to demonstrate the outcome of this instability in a few example cases. We demonstrate the growth of both axially symmetric and asymmetric modes whose wavelength and growth rate match predictions from linear stability analysis. Finally, we vary the number density, flux magnitude, and flux direction of the neutrinos and antineutrinos and demonstrate that these factors modify both the growth rate and postsaturation neutrino flavor abundances. Weak electron lepton number crossings in these simulations produce both slow growth of the instability and little difference between the flavor abundances in the initial and final states. In all of these calculations the same number of neutrinos and antineutrinos change flavor, making the least abundant between them the limiting factor for postsaturation flavor change. Many more simulations and multidimensional simulations are needed to fully probe the parameter space of the initial conditions. [ABSTRACT FROM AUTHOR]