The Event Horizon Telescope (EHT) collaboration has produced the first resolved images of the supermassive black holes at the centre of our galaxy and at the centre of the elliptical galaxy M87. As both technology and analysis pipelines improve, it will soon become possible to produce spectral index maps of black hole accretion flows on event horizon scales. In this work, we predict spectral index maps of both M87* and Sgr A* by applying the general relativistic radiative transfer (GRRT) code IPOLE to a suite of general relativistic magnetohydrodynamic (GRMHD) simulations. We analytically show that the spectral index increases with increasing magnetic field strength, electron temperature, and optical depth. Consequently, spectral index maps grow more negative with increasing radius in almost all models, since all of these quantities tend to be maximised near the event horizon. Additionally, photon ring geodesics exhibit more positive spectral indices, since they sample the innermost regions of the accretion flow with the most extreme plasma conditions. Spectral index maps are sensitive to highly uncertain plasma heating prescriptions (the electron temperature and distribution function). However, if our understanding of these aspects of plasma physics can be tightened, even the spatially unresolved spectral index around 230 GHz can be used to discriminate between models. In particular, Standard and Normal Evolution (SANE) flows tend to exhibit more negative spectral indices than Magnetically Arrested Disk (MAD) flows due to differences in the characteristic magnetic field strength and temperature of emitting plasma.
The corona is a key component of most luminous accreting black holes, carrying 5 - 30 % of the power and in non-jetted Active Galactic Nuclei (AGN), creating all the X-ray emission above $\simeq 1-2$ keV. Its emission illuminates the inner accretion disc, creating the atomic line-rich reflection spectrum used to diagnose and map the accretion flow and measure black hole spin. The corona is likely powered magnetically by the strong differential rotation of the accretion disc and it may be intimately related to relativistic jets. Recent work shows that many black hole coronae may be dominated by…
Recent observational data on transiently-accreting neutron stars has unequivocally shown fast-cooling sources, such as in the case of neutron star MXB 1659-29. Previous calculations have estimated its total neutrino luminosity and heat capacity, as well as suggested that direct Urca reactions take place in $1 \%$ of the volume of the core. In this paper, we reproduce the inferred luminosity of this source with detailed models of equations of state (EOS) and nuclear pairing gaps. We show that three superfluidity gap models are inconsistent with data for all EOS and another three are disfavoured…
Observations of jets in X-ray binaries show a correlation between radio power and black hole spin. This correlation, if confirmed, points towards the idea that relativistic jets may be powered by the rotational energy of black holes. In order to examine this further, we perform general-relativistic radiative transport calculations on magnetically arrested accretion flows, which are known to produce powerful jets via the Blandford-Znajek (BZ) mechanism. We find that the X-ray and gamma-ray emission strongly depend on spin and inclination angle. Surprisingly, the high-energy power does not show …
The Event Horizon Telescope recently observed the first shadow of a black hole. Images like this can potentially be used to test or constrain theories of gravity and deepen the understanding in plasma physics at event horizon scales, which requires accurate parameter estimations. In this work, we present Deep Horizon, two convolutional deep neural networks that recover the physical parameters from images of black hole shadows. We investigate the effects of a limited telescope resolution and observations at higher frequencies. We trained two convolutional deep neural networks on a large image l…
We study the lamppost model, in which the X-ray source in accreting black-hole systems is located on the rotation axis close to the horizon. We point out a number of inconsistencies in the widely used lamppost model relxilllp, e.g., the neglect of the redshift of the photons emitted by the lamppost and directly observed. They appear to invalidate those model fitting results for which the source distances from the horizon are within several gravitational radii. Furthermore, if those results were correct, most of the photons produced in the lamppost would be trapped by the black hole, and the lu…
Upcoming Event Horizon Telescope (EHT) observations will provide a unique opportunity to reveal the innermost region of the radiative inefficient accretion flow (RIAF) around the Galactic black hole, Sgr A*. Depending on the flow dynamics and accumulated magnetic flux, the innermost region of an RIAF could have a quasi-spherical or disk-like geometry. Here we present a phenomenological model to investigate the characteristics of the black hole shadow images with different flow geometries, together with the effect of black hole spin and flow dynamics. The resulting image consists in general of …
Over the last decades X-ray spectroscopy has proven to be a powerful tool for the estimation of black hole spin and several other key parameters in dozens of AGN and black hole X-ray binaries. In this White Paper, we discuss the observational and theoretical challenges expected in the exploration, discovery, and study of astrophysical black holes in the next decade. We focus on the case of accreting black holes and their electromagnetic signatures, with particular emphasis on the measurement of the relativistic reflection component in their X-ray spectra.
Future gravitational wave detectors have been projected to be able to probe the nature of compact objects in great detail. In this work, we study the potential observability of the small length scale physics near black hole horizon with the tidal deformability of the compact objects in an inspiraling binary. We find that it is possible to probe them with extreme mass ratio inspirals. We discuss how the quantum effects can affect the gravitational wave observables. This as a consequence is bound to shape our understanding of the quantum scale near the horizon.
The strong version of the nonviolent nonlocality proposal of Giddings predicts "strong but soft" quantum metric fluctuations near black hole horizons in an attempt to resolve the information paradox. To study observable signatures of this proposal, we numerically solve Einstein's equations modified by these fluctuations and analyze the gravitational wave signal from the inspiral and merger of two black holes. In a model of evolution for such fluctuations, we show that they lead to significant deviations in the observed waveform, even when the black holes are still well separated, and could pot…
One of the most fundamental results of general relativity is that the event horizon of black hole is purely ingoing. On the other hand, semiclassical-gravity effects, such as particle creation and the quantization of black-hole area, suggest that black holes can emit energy. Since a black hole is characterized by the presence of the event horizon, the emitted energy must be extracted from the black hole through its horizon. These considerations provide a motivation to test the validity of the purely ingoing nature of black-hole horizon. In this paper, we propose a novel test of the purely ingo…
The effect of vacuum birefringence is one of the first predictions of quantum electrodynamics (QED): the presence of a charged Dirac field makes the vacuum birefringent when threaded by magnetic fields. This effect, extremely weak for terrestrial magnetic fields, becomes important for highly magnetized astrophysical objects, such as accreting black holes. In the X-ray regime, the polarization of photons traveling in the magnetosphere of a black hole is not frozen at emission but is changed by the local magnetic field. We show that, for photons traveling along the plane of the disk, where the f…
Astrophysical black holes are ideal laboratories for testing Einstein's theory of general relativity in the strong field regime. In this manuscript, we present an analysis of Suzaku and NuSTAR spectra of the black hole binary Cygnus X-1 using RELXILL_NK. Unlike our previous study on Cygnus X-1 with NuSTAR data, here we are able to constrain the Johannsen deformation parameter $α_{13}$. However, despite the high energy resolution near the iron line provided by Suzaku, our constraints on the Kerr metric from Cygnus X-1 are not very stringent in comparison with those that have been obtained from …
Observations of galactic nuclei help us to test General Relativity. Whereas the No-hair Theorem states that classical, isolated black holes eventually settle to a stationary state that can be characterized by a small number of parameters, cosmic black holes are neither isolated nor steady. Instead, they interact with the environment and evolve on vastly different time-scales. Therefore, the astrophysically realistic models require more parameters, and their values likely change in time. New techniques are needed in order to allow us to obtain independent constraints on these additional paramet…
The capability of the Event Horizon Telescope (EHT) to image the nearest supermassive black hole candidates at horizon-scale resolutions offers a novel means to study gravity in its strongest regimes and to test different models for these objects. Here, we study the observational appearance at 230 GHz of a surfaceless black hole mimicker, namely a non-rotating boson star, in a scenario consistent with the properties of the accretion flow onto Sgr A*. To this end, we perform general relativistic magnetohydrodynamic simulations followed by general relativistic radiative transfer calculations in …
Black holes are the most compact objects in the Universe. According to general relativity, black holes have a horizon that hides a singularity where Einstein's theory breaks down. Recently, gravitational waves opened the possibility to probe the existence of horizons and investigate the nature of compact objects. This is of particular interest given some quantum-gravity models which predict the presence of horizonless and singularity-free compact objects. Such exotic compact objects can emit a different gravitational-wave signal relative to the black hole case. In this thesis, we analyze the s…
Millimeter polarimetry of Sgr A* probes the linearly polarized emission region on a scale of $\sim 10$ Schwarzschild radii ($R_S$) as well as the dense, magnetized accretion flow on scales out to the Bondi radius ($\sim 10^5 R_S$) through Faraday rotation. We present here multi-epoch ALMA Band 6 (230 GHz) polarimetry of Sgr A*. The results confirm a mean rotation measure, ${\rm RM} \approx -5 \times 10^5 {\rm\ rad\ m^{-2}}$, consistent with measurements over the past 20 years and support an interpretation of the RM as originating from a radiatively inefficient accretion flow (RIAF) with $\dot{…
We consider a Compton cloud (CC) surrounding a black hole (BH) in an accreting black hole system, where electrons propagate with thermal and bulk velocities.In that cloud, soft (disk) photons may be upscattered off these energetic electrons and attain several MeV energies. They could then create pairs due to these photon-photon interactions. In this paper, we study the formation of the 511 keV annihilation line due to this photon-photon interaction, which results in the creation of electron-positron pairs, followed by the annihilation of the created positrons with the CC electrons. The appropr…
Supermassive binaries detectable by the future space gravitational-wave interferometer LISA might allow to distinguish black holes from ultracompact horizonless objects, even when the latter are motivated by quantum-gravity considerations. We show that a measurement of very small tidal Love numbers at the level of $10\%$ accuracy (as achievable with "golden binaries") may also allow to distinguish between different models of these exotic compact objects, even when taking into account an intrinsic uncertainty in the object radius putatively due to quantum mechanics. We argue that there is no co…
We explore a simple spherical model of optically thin accretion on a Schwarzschild black hole, and study the properties of the image as seen by a distant observer. We show that a dark circular region in the center --- a shadow --- is always present. The outer edge of the shadow is located at the photon ring radius $b_{\rm ph} \equiv \sqrt{27}r_g$, where $r_g=GM/c^2$ is the gravitational radius of the accreting mass $M$. The location of the shadow edge is independent of the inner radius at which the accreting gas stops radiating. The size of the observed shadow is thus a signature of the spacet…
We have examined a sample of 13 sub-Eddington supermassive black holes hosted by galaxies spanning a variety of morphological classifications to further understand the empirical fundamental plane of black hole activity. This plane describes black holes from stellar-mass to supermassive and relates the mass of an accreting black hole and its radio and X-ray luminosities. A key factor in studying the fundamental plane is the turnover frequency, the frequency at which the radio continuum emission becomes optically thin. We measured this turnover frequency using new VLA observations combined, when…
Several quantum-gravity models of compact objects predict microscopic or even Planckian corrections at the horizon scale. We explore the possibility of measuring two model-independent, smoking-gun effects of these corrections in the gravitational waveform of a compact binary, namely the absence of tidal heating and the presence of tidal deformability. For events detectable by the future space-based interferometer LISA, we show that the effect of tidal heating dominates and allows one to constrain putative corrections down to the Planck scale. The measurement of the tidal Love numbers with LISA…
Spin and mass of the nearest supermassive black hole
围绕吸积黑洞的热等离子体斑点或团块的准周期振荡（QPO）包含了关于黑洞质量和自旋的信息。吸积流中亮点的纬向振荡频率和黑洞事件视界旋转的频率是测量黑洞质量和自旋的有希望的观测特征。这两个频率都与吸积模型无关，完全由黑洞引力场的性质定义。借助吸积等离子体热点的信号调制对已知的QPO数据进行解释，揭示了银河系中心超大质量黑洞的克尔度规旋转参数$a=0.65\pm0.05$和质量$M=(4.2\pm0.2)10^6M_\odot$。同时，将观测到的11.5 min QPO周期分别与黑洞事件视界旋转周期和吸积流热点纬向振荡周期相对应.所描述的方法是适用于黑洞的吸积率低，吸积等离子体是透明的事件视界区域。
Magnetic fields are believed to drive accretion and relativistic jets in black hole accretion systems, but the magnetic-field structure that controls these phenomena remains uncertain. We perform general relativistic (GR) polarized radiative transfer of time-dependent three-dimensional GR magnetohydrodynamical (MHD) simulations to model thermal synchrotron emission from the Galactic Center source Sagittarius A$^\ast$ (Sgr A*). We compare our results to new polarimetry measurements by the Event Horizon Telescope (EHT) and show how polarization in the visibility (Fourier) domain distinguishes an…
Indian Institute of Astrophysics（印度天体物理研究所）；University of Oxford（牛津大学）；University of Cape Town（开普敦大学）；Curtin University（科廷大学）；Institute of Space Science（空间科学研究所）；Anton Pannekoek Institute, University of Amsterdam（阿姆斯特丹大学安东·范·纽文克研究所）
We examine radiative standing shocks in advective accretion flows around stellar-mass black holes by 2D radiation hydrodynamic simulations, focusing on the super-Eddington accreting flow. Under a set of input flow parameters responsible for the standing shock, the shock location on the equator decreases toward the event horizon with an increasing accretion rate. The optically thin and hot gas in the narrow funnel region along the rotational axis changes gradually into a dense and optically thick state with the increasingly dense gas transported from the base of the radiative shock near the equ…
We review the physical origins for possible visible images of the supermassive black hole M87* in the galaxy M87 and SgrA* in the Milky Way Galaxy. The classical dark black hole shadow of the maximal size is visible in the case of luminous background behind the black hole at the distance exceeding the so-called photon spheres. The notably smaller dark shadow (dark silhouette) of the black hole event horizon is visible if the black hole is highlighted by the inner parts of the luminous accreting matter inside the photon spheres. The first image of the supermassive black hole M87 *, obtained by …
For general relativistic, inviscid, axisymmetric flow around Kerr black hole one may choose different flow thickness. The stationary flow equations can be solved using methods of dynamical system to get transonic accretion flows , i.e, flow infalling in the blackhole that turns supersonic from subsonic with decreasing radial distance, or vice versa. This transonic flows are obtained by choosing the particular flow passing through critical points of phase portrait. For certain flow thickness like the one maintaining conical shape, the sonic point coincide with the critical point. But there are …
We report the broadband spectro-temporal study of the poorly studied accreting neutron star (NS) low mass X-ray binary (LMXB) 4U 1724-30 using data from Soft X-ray Telescope (SXT) and Large Area X-ray Proportional Counters (LAXPC) instruments on board AstroSat. The dim persistent LMXB source was observed with AstroSat over 4 epochs in 2017, all of which corresponded to a low-luminosity non-thermal emission dominated (hard/island) emission state with modest spectral evolution. All the X-ray broadband spectra can be modelled by a combination of thermal emission from the NS boundary layer (BL) or…
Quantum nature of black hole horizons has been a subject of recent interest and scrutiny. In particular, a near-horizon quantum violation of the equivalence principle has been proposed as a resolution of the black hole information paradox. Such a violation may lead to a modified dispersion relation at high energies, which could become relevant due to the intense gravitational blueshift experienced by ingoing gravitational waves. We investigate the ringdown for a perturbed black hole with such a modified dispersion relation and find that infalling gravitational waves are partially reflected nea…
For axially symmetric accretion maintained in hydrostatic equilibrium along the vertical direction, we investigate how the characteristic features of the embedded acoustic geometry depends on the background Kerr metric, and how such dependence is governed by three different expressions of the thickness of the matter flow. We first obtain the location of the sonic points and stationary shock between the sonic points. We then linearly perturb the flow to obtain the corresponding metric elements of the acoustic space-time. We thus construct the causal structure to establish that the sonic points …
X-ray reverberation has proven to be a powerful tool capable of probing the innermost region of accretion disks around compact objects. Current theoretical effort generally assumes that the disk is geometrically thin, optically thick and orbiting with Keplerian speed. Thus, these models cannot be applied to systems where super-Eddington accretion happens because the thin disk approximation fails in this accretion regime. Furthermore, state-of-the-art numerical simulations show that optically thick winds are launched from the super-Eddington accretion disks, and thereby changing the reflection …
A fundamental difference between a neutron star (NS) and a black hole (BH) is the absence of a physical surface in the latter. For this reason, any remaining kinetic energy of the matter accreting onto a BH is advected inside its event horizon. In the case of an NS, on the contrary, accreting material is decelerated on the NS surface, and its kinetic energy is eventually radiated away. Copious soft photons produced by the NS surface will affect the properties of the Comptonised component dominating spectra of X-ray binaries in the hard state. Thus, parameters of the Comptonised spectra -- the …
We discuss the importance of large scale strong magnetic field in the removal of angular momentum outward, as well as the possible origin of different kinds of magnetic barrier in advective, geometrically thick, sub-Keplerian accretion flows around black holes. The origin of this large scale strong magnetic field near the event horizon is due to the advection of the magnetic flux by the accreting gas from the environment, say, the interstellar medium or a companion star, because of flux freezing. In this simplest vertically averaged, 1.5-dimensional disc model, we choose the maximum upper limi…
