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Low Frequency Quasi-Periodic Oscillations or LF QPOs are ubiquitous in BH X-ray binaries and provide strong constraints on the accretion-ejection processes. Although several models have been proposed so far, none has been proven to…

Long-term gamma-ray variability of a non-blazar Active Galactic Nucleus (AGN) PKS 0521-36 is investigated by using Fermi-LAT pass 8 data covering from 2008 August to 2021 March. The results show that the histogram of the gamma-ray fluxes…

Black hole X-ray binaries routinely exhibit Quasi Periodic Oscillations (QPOs) in their Power density spectrum. Studies of QPOs have demonstrated immense ability to understand these dynamical systems although their unambiguous origin still…

Many accreting black holes and neutron stars exhibit rapid variability in their X-ray light curves, termed quasi-periodic oscillations (QPOs). The most commonly observed type is the low-frequency ($\lesssim 10$ Hz), type-C QPO, while only a…

X-ray quasi-periodic oscillations (QPOs) are a characteristic feature of low-mass X-ray binaries (LMXBs). These oscillations have been studied for decades and revealed a rich and complex phenomenology that is still not fully understood.…

In previous papers we proposed a model that high-frequency quasi-periodic oscillations (QPOs) observed in black-hole and neutron-star X-ray binaries are disk oscillations (inertial-acoustic and/or g-mode oscillations) resonantly excited on…

High-frequency twin peak quasiperiodic oscillations (QPOs) are observed in four microquasars, i.e., Galactic black hole binary systems, with frequency ratio very close to 3:2. In the microquasar GRS 1915+105, the structure of QPOs exhibits…

The evolution of different types of quasi-periodic oscillations (QPOs) and the coupled radiative/physical changes in the accretion disk are still poorly understood. In a few black hole binaries it was found that fast evolution of QPOs is…

The shape of the power spectrum of the black hole low-mass X-ray binary evolves systematically over different spectral states during an outburst. Therefore, the power colours (ratio of the variability amplitude at different frequency…

We investigate the dynamical properties of low frequency quasi-periodic oscillations (QPOs) observed from the black hole X-ray binary MAXI J1820+070 during the early part of its 2018 outburst, when the system was in a bright hard state. To…

Reverberation lags have recently been discovered in a handful of nearby, variable AGN. Here, we analyze a ~100 ksec archival XMM-Newton observation of the highly variable AGN, ESO 113-G010 in order to search for lags between hard, 1.5 - 4.5…

The shocked wave created on the accretion disk after different physical phenomena (accretion flows with pressure gradients, star-disk interaction etc.) may be responsible observed Quasi Periodic Oscillations (QPOs) in $X-$ray binaries. We…

We report the discovery with the proportional counter array (PCA) onboard the Rossi X-ray Timing Explorer (RXTE) of a 450 Hz quasiperiodic oscillation (QPO) in the hard X-ray flux from the galactic microquasar GRO J1655-40. This is the…

X-ray inter-band time lags are observed during the outbursts of black hole X-ray binaries (BHXRBs). Timing analysis of fast variability in low Fourier frequency bands shows that high-energy photons lag behind low-energy photons, a…

We report a 325(-7, +8) day quasi-periodic oscillation (QPO) in the X-ray emission of the blazar Mkn 421, based on data obtained with the Rossi X-ray Timing Explorer (RXTE). The QPO is seen prominently in the ASM data (at least 15 cycles),…

In the propagating oscillatory shock model, the oscillation of the post-shock region, i.e., the Compton cloud, causes the observed low-frequency quasi-periodic oscillations (QPOs). The evolution of QPO frequency is explained by the…

This study aims to detect and characterize quasi-periodic oscillations (QPOs) signals in X-ray observations of NGC 4151. We employed the Weighted Wavelet Z-transform (WWZ) and Lomb-Scargle periodogram (LSP) methods for our analysis. QPO…

With a systematic timing investigation of the persistent X-ray binary pulsar Her X-1 based on a large number of Insight-HXMT observations between 2017 to 2019, we confirm the presence of X-ray millihertz quasi-periodic oscillations (mHz…

We report here a study of the long term properties of Quasi Periodic Oscillations (QPO) in an unusual accreting X-ray pulsar, 4U 1626--67. This is a unique accretion powered X-ray pulsar in which we have found the QPOs to be present during…

We present the results of an extensive timing analysis of the 1999 outburst of the soft X-ray transient and black hole candidate XTE J1859+226 as observed with the Rossi X-Ray Timing Explorer. Three main different types of low frequency…