In 2012 November MAGIC detected a bright flare from IC 310. The flare consisted of two sharp peaks with a typical duration of ~ 5 min. The energy released during that event has been estimated to be at the level of 2 × 1044 erg s−1.

In this work we derive an upper limit on the possible luminosity of flares generated in black hole (BH) magnetosphere, which depends very weakly on the mass of BH and is determined by disk magnetisation, viewing angle, and pair multiplicity. Since all these parameters are smaller than a unit, the luminosity 2 × 1043 erg s−1 can be taken as a strict upper limit for flare luminosity for several minutes variability time. This upper limit appears to be approximately an order of magnitude below the value measured with MAGIC. Thus, we conclude that it seems very unfeasible that the magnetospheric processes can be indeed behind the bright flaring activity recorded from IC 310.

Tidal disruption events are a powerful tool to study quiescent massive black holes residing in the centre of galaxies. Occasionally, astrophysical objects such as stars, planets and smaller bodies are captured and tidally disrupted by the massive black hole, giving rise to a luminous flare. A detailed study of disruption parameters and the emitted radiation can give important insights on the black hole and its surroundings.

Generalizations of the Schwarzschild and Kerr black holes are discussed in an astrophysically viable generalized theory of gravity, which includes higher curvature corrections in the form of the Gauss-Bonnet term, coupled to a dilaton. The angular momentum of these black holes can slightly exceed the Kerr bound. The location and the orbital frequency of particles in their innermost stable circular orbits can deviate significantly from the respective Kerr values. Study of the quasinormal modes of the static black holes gives strong evidence that they are mode stable against polar and axial perturbations. Future gravitational wave observations should improve the current bound on the Gauss-Bonnet coupling constant, based on observations of the low-mass x-ray binary A 0620-00.

We investigate the growth of the Rayleigh–Taylor and Richtmyer–Meshkov instabilities at the interface of the relativistic jet using three-dimensional hydrodynamic simulations. The propagation of the relativistic jet that is continuously injected from the boundary of the calculation domain into a uniform ambient medium is solved. We find that the interface of the jet is deformed by a synergetic growth of the Rayleigh–Taylor and Richtmyer–Meshkov instabilities regardless of the launching condition, such as the specific enthalpy of the jet or the effective inertia ratio between the jet and ambient medium. The material mixing between the jet and external medium due to these instabilities causes the deceleration of the jet.

We have investigated particle motion around Schwarzschild black holes in the presence of a plasma with radial power-law density profile has been shown that the photon sphere around a spherically symmetric black hole is unchanged under the influence of the plasma; however, the Schwarzschild black hole shadow size is reduced due to the refraction of electromagnetic radiation in the plasma environment of the black hole.

In this paper, we focus on studying the high energy emission of GRB 160625B. The lightcurve of prompt emission is composed of three episodes: short-soft precursor, hard main burst, and possible long extended emission. The spectra of first and third episode can be fitted by a multi-color blackbody and cutoff power-law model, respectively. However, the spectrum of second episode was contributed by both multi-color blackbody and cutoff power-law. One can estimate the Lorenz factor of jet of first two episodes by invoking photosphere model as Γ0 ~ 175 and 1694, respectively. It suggests that the ejecta of this case evolved from photosphere dominated initially to internal shock later. On the other hand, the optical emission is very bright during the second episode, which is likely a prompt optical emission. Finally, a more shallower normal decay segment appeared, which is consistent with standard external shock model.

The Fermi-LAT gamma-ray data in the inner Galaxy region show several prominent features possibly related to the past activity of the Milky Way’s super massive black hole. At a large, 50 deg scale, the Fermi LAT revealed symmetric hour glass structures with hard energy spectra extending up to 100 GeV (and dubbed ‘the Fermi bubbles’). More recently and closer to the Galactic centre, at the 10 deg scale, several groups have claimed evidence for excess gamma-ray emission that appears symmetric around the Galactic center and has an energy spectrum peaking at few GeVs. We explore here the possibility that this emission originates in inverse Compton emission from high-energy electrons produced in a short duration, burst-like event injecting 1052 − 1053 erg, roughly 106 yrs ago. Several lines of evidence suggest that a series of ‘burst like’ events happened in the vicinity of our black hole in the past and gamma-ray observations may offer a new view of that scenario.

Feedback released during the growth of supermassive black holes is expected to play a key role in shaping black hole-host galaxy co-evolution. Powerful, accretion disc driven winds have been invoked to explain both observed scaling relations (e.g., M − σ) and large-scale outflows with mass outflow rates of ~ 100 − 1000 M⊙ yr−1 and momentum rates of up to ~ 30 LAGN/c. Critically, how these winds couple to the host galaxy depends on if they are momentum or energy conserving. I outline observational signatures that could distinguish between these regimes and discuss their roles in establishing galaxy properties. Furthermore, I discuss high-resolution simulations exploring feedback in a multi-phase medium, highlighting how structural properties of galaxies can impact feedback efficiency. Finally, feedback, in the form of collimated jets, is expected to regulate cooling in galaxy clusters. I discuss new simulations of jet feedback using the moving-mesh code AREPO and outline the scope of our new study.

We present a study performed to understand the role of magnetic field during a historical outburst of IBL S5 0716+714 witnessed in early 2015. The two month long profile of the outburst reveals several episodes of sub-flares superimposed over the diminishing trend of the peak-flux. The broadband X-ray spectrum from Swift + Nustar exhibits a break at 4.9+0.4 −0.5 keV which is consistent with the valley in the SED predicted by model. The spectral index variations, closely correlated to the flux variations during the first prominent sub-flare, indicate the alterations in the particle energy distribution coinciding the onset of the flare. The PA rotations approximately at the same epoch are used to constrain the field and its geometry.

We present the results of numerical particle-in-cell (PIC) simulations of the magnetic field generation and decay in the upstream of collisionless shocks. We use the model, where the magnetic field in the incoming flow is generated by continuous injection of anisotropic electron-positron pairs. We found that the continuous injection of anisotropic plasma in the upstream of the shock-wave generates the large-scale, slowly decaying magnetic field that is later amplified during the passage of the shock front. In our simulations the magnetic field energy reached ~0.01 of the equipartition value, after that it slowly decays on the time scale proportional to the duration of the injection in the upstream. Thus, the magnetic field survives for a sufficiently long time, and supports efficient synchrotron radiation from relativistic shocks, e.g., in GRBs.

The Large Synoptic Survey Telescope (LSST), the next-generation optical imaging survey sited at Cerro Pachon in Chile, will provide hundreds of detections for a sample of more than ten million quasars with redshifts up to about seven. The LSST design, with an 8.4m (6.7m effective) primary mirror, a 9.6 sq. deg. field of view, and a 3.2 Gigapixel camera, will allow about 10,000 sq. deg. of sky to be covered twice per night, every three to four nights on average, with typical 5-sigma depth for point sources of r=24.5 (AB). With about 1000 observations in ugrizy bands over a 10-year period, these data will enable a deep stack reaching r=27.5 (about 5 magnitudes deeper than SDSS) and faint time-domain astronomy. The measured properties of newly discovered and known astrometric and photometric transients will be publicly reported within 60 sec after closing the shutter. In addition to a brief introduction to LSST, I review optical quasar selection techniques, with emphasis on methods based on colors, variability properties, and astrometric behavior.

Narrow-line Seyfert 1 (NLS1) galaxies provide us with unique insights into the drivers of AGN activity under extreme conditions. Given their low black hole (BH) masses and near-Eddington accretion rates, they represent a class of galaxies with rapidly growing supermassive BHs in the local universe. Here, we present the results from our multi-frequency radio monitoring of a sample of γ-ray loud NLS1 galaxies (γNLS1s), including systems discovered only recently, and featuring both the nearest and the most distant γNLS1s known to date. We also present high-resolution radio imaging of 1H 0323+342, which is remarkable for its spiral or ring-like host. Finally, we present new radio data of the candidate γ-emitting NLS1 galaxy RX J2314.9+2243, characterized by a very steep radio spectrum, unlike other γNLS1s.

Highly accreting quasars are possible cosmological probes, as their Eddington ratio is expected to saturate toward values of order unity. We present preliminary estimates of redshift- independent source luminosities and the Hubble diagram for quasars in the redshift range 0.1 ≲ z ≲ 2.6.

We studied the imprints that feedback from Active Galactic Nuclei (AGN) leaves on the intracluster plasma during the assembly history of galaxy clusters. To this purpose we used state-of-the-art cosmological hydrodynamical simulations based on an updated version of the Tree-PM SPH GADGET-3 code, comparing three sets of simulations with different prescriptions for the physics of baryons (including AGN and/or stellar feedback). We explore the effect of these different physics, in particular AGN feedback, on IntraCluster medium (ICM) properties observed via Sunyaev-Zel’dovich (SZ) effect using an extended set of galaxy clusters (~100 clusters with M 500 masses above 5 × 1013 M ⊙/h). Some of the main findings are that the scaling relation between the integrated SZ flux and the galaxy cluster total mass is in good accordance with several observed samples, especially for massive clusters, and does not show any clear redshift evolution, with the slope of the relation close to the theoretical one in the AGN feedback case. As for the scatter of this relation, we obtain a mild dependence on the cluster dynamical state.

With relatively small black hole masses and high accretion rates, narrow-line Seyfert 1 galaxies are thought to be young AGNs. About 7% of them are radio-loud narrow-line Seyfert 1 galaxies (RLNLS1s). RLNLS1s allow us to re-address some of the key questions regarding the physics of jet formation. As the first step of the systematic study on the jet properties of RLNLS1s, we present the radio structure of fourteen RLNLS1s from VLBA observations at 5 GHz in 2013. Although all these sources are very radio-loud with R > 100, their jet properties are diverse, in terms of their pc-scale morphology and overall radio spectral shape. The core brightness temperatures of our sources are significantly lower than those of blazars, therefore, the beaming effect is generally not significant, compared to blazars. This implies that the bulk jet speed may likely be low in our sources.

We have constructed self similar models of time dependent and non relativistic accretion disks in both sub and super-Eddington phase of TDEs with wind outflows for a general viscosity prescription which is a function of surface density of the disk Σd and radius r. The physical parameters are black hole (BH) mass M •, specific orbital energy E and angular momentum J, star mass M ⋆ and radius R ⋆. We have considered an accretion disk where matter is lost due to accretion by black hole and out flowing wind (in case of super-Eddington) and added through fallback of the disrupted debris. We have simulated the light curve profiles in various spectral bands and fit them to the observations to determine the above mentioned physical parameters.

Here we present the evidence for periodicity of an optical emission detected in several AGN. Significant periodicity is found in light curves and radial velocity curves. We discuss possible mechanisms that could produce such periodic variability and their implications. The results are consistent with possible detection of the orbital motion in proximity of the AGN central supermassive black holes.

I present an overview of observational efforts across the electromagnetic spectrum to identify and study tidal disruption flares (TDFs), when a star wanders too close to a super-massive black hole and is torn apart by tidal forces. In particular I will focus on four unexpected surprises that challenge the most basic analytic picture of these events: 1) large inferred radii for the optical/UV-emitting material; 2) the ubiquity of outflows, detected at radio, X-ray, and UV wavelengths, ranging from speeds of 100 km/s to near the speed of light; 3) the peculiar atomic abundances observed in the UV and optical spectra of these objects; and, 4) the preference for these events to occur in post-starburst galaxies.

Active galactic nuclei, hosting supermassive black holes and launching relativistic jets, are the most numerous objects on the gamma-ray sky. At the other end of the mass scale, phenomena related to stellar mass black holes, in particular gamma-ray bursts and microquasars, are also seen on the gamma-ray sky. While all of them are thought to launch relativistic jets, the diversity even within each of these classes is enormous. In this review, I will discuss recent very high energy gamma-ray results that underline both the similarity of the black hole systems, as well as their diversity.