Related papers: Fourier-phase analysis of the orbiting bright-spot…
Black hole binaries exhibit a wide range of variability phenomena, from large-scale state changes to broadband noise and quasi-periodic oscillations, but the physical nature of much of this variability is poorly understood. We examine the…
A phenomenological model of X-ray variability of accreting black holes is considered, where the variable emission is attributed to multiple active regions/perturbations moving radially towards the central black hole. The hard X-rays are…
Black hole dynamical instabilities have been mostly studied in specific models. We here study the general properties of the complex-frequency modes responsible for such instabilities, guided by the example of a charged scalar field in an…
In this review I briefly describe the latest advances in studies of aperiodic variability of accreting X-ray binaries and outline the model which currently describe the majority of observational appearances of variability of accreting…
Context: In addition to coherent pulsation, many accreting neutron stars exhibit flaring activity and strong aperiodic variability on time scales comparable to or shorter than their pulsation period. Such a behavior shows that the accretion…
We study energy dependencies of quasi-periodic oscillations from a number of black hole X-ray binaries. The selected sources were observed by RXTE at time periods close to state transitions and showed QPO in the 1-10 Hz range. We have…
Axion-like particles (ALPs) are compelling candidates for dark matter and potential portals to new physics beyond the Standard Model. Photons traversing magnetized regions can convert into ALPs, producing characteristic, energy-dependent…
Context. Variability is a ubiquitous feature of active galactic nuclei (AGNs), and the characterisation of the variability is crucial to constrain its physical mechanism and proper applications in AGN studies. The advent of all-sky and…
We review the recent multifrequency studies of galactic black hole binaries, aiming at revealing the underlying emission processes and physical properties in these systems. The optical and infrared observations are important for determining…
We investigate the properties of a variability-selected complete sample of AGN in order to identify the mechanisms which cause large amplitude X-ray variability on time scales of years. A complete sample of 24 sources was constructed, from…
This paper presents the results of a study of X-ray spectral and flux variability on time scales from months to years, of the 123 brightest objects (including 46 type-1 AGN and 28 type-2 AGN) detected with XMM-Newton in the Lockman Hole…
We conducted a $369\;{\rm ks}$ NuSTAR observation on the X-ray pulsar Centaurus X-3, which covered two consecutive orbital cycles of the source, including two eclipse durations. We investigated the orbital-phase spectral variability over…
Two component X-ray spectra (soft multicolor black body plus harder power law) are frequently observed from accreting black holes. These components are presumably associated with the different parts of the accretion flow (optically thick…
We have obtained discrete Fourier power spectra of a sample of persistent low-mass neutron-star X-ray binaries using long-term light curves from the All Sky Monitor on board the Rossi X-ray Timing Explorer. Our aim is to investigate their…
There have been a number of recent spectral models that have been successful in reproducing the observed X-ray spectra of galactic black hole candidates (GBHC). However, there still exists controversy over such issues as: what are the…
Recent infrared (IR) observations of the center of our Galaxy indicate that the supermassive black hole source Sgr A* is strongly variable in the IR. The timescale for the variability, $\sim 30$ min, is comparable to that of the X-ray…
Knowing the site of gamma-ray emission in AGN jets will do much for our understanding of the physics of the source. In particular, if the emission region is close to the black hole then absorption of gamma-rays with photons from the…
We report on our attempt to understand the outbursting profile of Galactic Black Hole (GBH) sources, keeping in mind the evolution of temporal and spectral features during the outburst. We present results of evolution of Quasi-periodic…
Blazars are a subclass of radio-loud active galactic nuclei (AGNs), where the jet is aligned close to the line of sight. Blazars emission is dominated by non-thermal processes, where Doppler boosted radiation originates from a relativistic…
Context. Considerable effort has been put into using light curves observed by space telescopes such as CoRoT, Kepler and TESS for determining stellar rotation periods. While rotation periods of active stars can be reliably determined, the…