The recent detections of gravitational waves from binary systems of black holes are in remarkable agreement with the predictions of General Relativity. In this pedagogical mini-review, I go through the physics of the different phases of the evolution of black hole binary systems, providing a qualitative physical interpretation of each one of them. I also briefly describe how these phases would be modified if gravitation were described by a theory extending or deforming General Relativity, or if the binary components turned out to be more exotic compact objects than black holes. Full article

In this paper, we review our current efforts to test General Relativity in the strong field regime by studying the reflection spectrum of supermassive black holes. So far we have analyzed 11 sources with observations of NuSTAR, Suzaku, Swift, and XMM-Newton. Our results are consistent with General Relativity, according to which the spacetime metric around astrophysical black holes should be well approximated by the Kerr solution. We discuss the systematic uncertainties in our model and we present a preliminary study on the impact of some of them on the measurement of the spacetime metric. Full article

Is there an unknown long-range force between dark matter (DM) and ordinary matters? When such a fifth force exists and in the case that it is ignored, the equivalence principle (EP) is violated apparently. The violation of EP was severely constrained by, for examples, the Eöt–Wash laboratory experiments, the lunar laser ranging, the MICROSCOPE satellite, and the long-term observation of binary pulsars. We discuss a recent bound that comes from PSR J1713+0747. When it is combined with the other bounds, a compelling limit on the hypothetical fifth force is derived. For the neutral hydrogen, the strength of such a fifth force should not exceed $1\%$ of the gravity. Full article

Gravitational waves (GWs) encode important information about the mass of the source. For binary black holes (BBHs), the templates that are used to retrieve the masses normally are developed under the assumption of a vacuum environment. However, theories suggest that some BBHs form in gas-rich environments. Here, we study the effect of hydrodynamic drag on the chirp signal of a stellar-mass BBH and the impact on the measurement of the mass. Based on theoretical arguments, we show that the waveform of a BBH in gas resembles that of a more massive BBH residing in vacuum. The effect is important for those GW sources in the band of space-borne detectors but negligible for those in ground-based ones. Furthermore, we carry out a matched-filtering search of the best fitting parameters. We find that the best-fit chirp mass could be significantly greater than the real mass when the gas effect is ignored. Our results have important implications for the future joint observation of BBHs using both ground- and space-based detectors. Full article

Gravitational wave observations offer unique opportunities to probe gravity in the strong and dynamical regime, which was difficult to access previously. We here review two theory-agnostic ways to carry out tests of general relativity with gravitational waves, namely (i) parameterized waveform tests and (ii) consistency tests between the inspiral and merger-ringdown portions. For each method, we explain the formalism, followed by results from existing events, and finally we discuss future prospects with upgraded detectors, including the possibility of using multi-band gravitational-wave observations with ground-based and space-borne interferometers. We show that such future observations have the potential to improve upon current bounds on theories beyond general relativity by many orders of magnitude. We conclude by listing several open questions that remain to be addressed. Full article

In this work, we obtain the deflection angle for a boosted Kerr black hole in the weak field approximation using the optics in a curved spacetime developed by J. L. Synge in 1960. We study the behavior of light in the presence of plasma by considering different distributions: uniform plasma, singular isothermal sphere, non-singular isothermal gas sphere, and plasma in a galaxy cluster. We found that the dragging of the inertial system along with the boosted parameter $\Lambda$ affect the value of the deflection angle. As an application, we studied the magnification for both uniform and singular isothermal distributions. Full article

In this paper, we briefly present RELXILL_NK, the first and currently only readily available model of the relativistic reflection spectrum of black hole accretion disks that includes non-Kerr solutions for the black hole spacetime, thus allowing for tests of the Kerr hypothesis and general relativity (GR). RELXILL_NK makes use of a general relativistic ray-tracing code to calculate the relativistic effects of any well-behaved, stationary, axisymmetric, and asymptotically flat black hole spacetime, while the disk physics is handled through the non-relativistic X-ray reflection code XILLVER. A number of different flavors are available within RELXILL_NK; we summarize and compare these flavors using the Johannsen metric for the black hole spacetime. Full article

X-ray time lags are complicated in nature. The exact reasons for complex lag spectra are as yet unknown. However, the hard lags, in general, are believed to be originated due to inverse Comptonization process. However, the origin of soft lags remained mischievous. Recent studies on “Disk–Jet Connections” revealed that the jets are also contributing in the X-ray spectral and timing properties in a magnitude which was more than what was predicted earlier. In this article, we first show an exact anticorrelation between X-ray time lag and radio flux for XTE J1550-546 during its 1998 outburst. We propose that the soft lags might be generated due to the change in the accretion disk structure along the line of sight during higher jet activity. Full article

The nearby M87 hosts an exceptional relativistic jet. It has been regularly monitored in radio to TeV bands, but little has been done in hard X-rays ≳10 keV. For the first time, we have successfully detected hard X-rays up to 40 keV from its X-ray core with joint Chandra and NuSTAR observations, providing important insights to the X-ray origins: from the unresolved jet or the accretion flow. We found that the hard X-ray emission is significantly lower than that predicted by synchrotron self-Compton models introduced to explain very-high-energy $\gamma$ -ray emission above a GeV. We discuss recent models to understand these high energy emission processes. Full article

TV Cassiopeia (TV Cas) is a well-known eclipsing Algol-type binary comprising a B9V primary and F7 IV secondary in an orbit with a period of 1.81 days, together with a purported low-mass companion in a wide orbit. Despite the considerable attention TV Cas has received in optical and UV bands, no X-ray analysis has been reported. Chandra has detected TV Cas six times and XMM–Newton observed it twice, all serendipitously during observations of the starburst galaxy IC 10. We have put all the X-ray data together to investigate its coronal morphology and spectral properties. We use the X-ray light curves and eclipses to probe the emitting geometry and the X-ray spectra at different epochs to investigate the activity mechanisms and test speculation that TV Cas undergoes significant mass transfer episodes. Full article

A compact object captured by a supermassive black hole, named as extreme-mass-ratio inspiral (EMRI), is one of the most important gravitational wave sources for low-frequency interferometers such as LISA, Taiji, and TianQin. EMRIs can be used to accurately map the space-time of the central massive body. In the present paper, we introduce our recent progress on testing gravity theory with EMRIs. We demonstrate how to constrain gravitational wave dispersion and measure the deviation of the central body from the Kerr black hole. By using binary-EMRIs, the gravitational recoil and mass loss due to merger will be measured in a higher accuracy compared with the current LIGO observations. All these potential constrains and measurements will be useful for test of the gravity theory. Full article

We present results regarding the disc response functions and the corresponding time lags assuming a standard Novikov–Thorne accretion disc illuminated by a point-like X-ray source. We took into account all relativistic effects in the light propagation from the X-ray source to the disc and then to the observer, and we computed the disc reflection, accounting for its radial ionization profile. Our results suggest that the thermal reverberation effects should be stronger in sources with large X-ray source height and low accretion rate. We found that time lags increase with height and accretion rate. The amplitude of the observed time lags as a function of wavelength (in NGC 5548) is consistent with the model predictions. It is not necessary for the disc to be too hot, it may be that the X-ray source is located further from the disc. Full article

Properties of charged particle motion in the field of magnetized black holes (BHs) imply four possible regimes of behavior of ionized Keplerian disks: survival in regular epicyclic motion, transformation into chaotic toroidal state, destruction due to fall into the BHs, destruction due to escape along magnetic field lines (escape to infinity for disks orbiting Kerr BHs). The regime of the epicyclic motion influenced by very weak magnetic fields can be related to the observed high-frequency quasiperiodic oscillations. In the case of very strong magnetic fields particles escaping to infinity could form UHECR due to extremely efficient magnetic Penrose process – protons with energy $E>{10}^{21}$ eV can be accelerated by supermassive black holes with $M\sim {10}^{10}{\mathrm{M}}_{\odot }$ immersed in magnetic field with $B\sim {10}^4$ Gs. Full article

The continuous X-ray flux of stellar-mass black holes provides an excellent source of data to learn about the astrophysics of accretion disks and about the spacetime itself. The extraction of information, however, depends heavily on our ability to correctly model the astrophysics and the theory of gravity, and the quality of the data. By combining a relativistic ray-tracing and Markov-Chain Monte-Carlo sampling technique, I show that the incorporation of the spin parameter through a slowly-rotating approximation, is not able to break the complex degeneracies of the model and therefore, when introducing modifications beyond general relativity it is very challenging to perform tests of general relativity with this type of observations. As a particular case, I show that it not possible to distinguish the small-coupling, slow-rotation black hole solution of dynamical Chern–Simons gravity from the Kerr solution with current instruments. Full article

The search for periodic signals from blazars has become an actively pursued field of research in recent years. This is because periodic signals bring us information about the processes occurring near the innermost regions of blazars, which are mostly inaccessible to our direct view. Such signals provide insights into some of the extreme conditions that take place in the vicinity of supermassive black holes that lead to the launch of the relativistic jets. In addition, studies of characteristic timescales in blazar light curves shed light on some of the challenging issues in blazar physics that include disk-jet connection, strong gravity near fast-rotating supermassive black holes and release of gravitational waves from binary supermassive black hole systems. However, a number of issues associated with the search for quasi-periodic oscillations (QPOs) in blazars e.g., red-noise dominance, modest significance of the detection, periodic modulation lasting for only a couple of cycles and their transient nature, make it difficult to estimate the true significance of the detection. Consequently, it also becomes difficult to make meaningful inferences about the nature of the on-going processes. In this proceedings, results of study focused on searching for QPOs in a number of blazar multi-frequency light curves are summarized. The time series analyses of long term observations of the blazars revealed the presence of year-timescale QPOs in the sources including OJ 287 (optical), Mrk 501 (gamma-ray), J1043+2408 (radio) and PKS 0219-164 (radio). A likely explanations, we discuss a number of scenarios including binary supermassive black hole systems, lense-thirring precession, and jet precession. Full article