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We study the Doppler factors for a group blazars at soft X-ray band. In our estimates, we have made the assumptions that (i) blazars can be divided into high-energy-peaked (HEP) objects whose synchrotron peak frequencies ν_p > 10^14.7Hz, and the low-energy-peaked (LEP) objects whose synchrotron peak frequencies ν_p≤10^14.7Hz, and (ii) the intrinsic radiation from a blazar in the energy range from radio to soft X-ray bands is the synchrotron radiation for HEP objects and the soft X-ray emission comes from inverse Compton scattering for LEP objects. Under the above assumptions, we estimate Doppler factors at optical (δ_O) and X-rays (δ_x) for 54 blazars by using the known radio Doppler factors and the observed flux densities in radio, optical and X-ray bands, and Doppler factors at X-ray band in which X-rays are assumed to be produced only by the synchrotron radiation. We get . The Doppler factors are different in various wavebands, and on average, the Doppler factor decreases with frequency from radio to X-ray bands.

The evidence is reviewed that the primary form of energy that escapes to infinity from gamma-ray bursts (GRBs) is gamma-rays, and/or Poynting flux, and that the kinetic energy in ultrarelativistic baryons is a secondary component resulting from acceleration of baryons by radiation pressure near or beyond the photosphere. This could account for several observed characteristics of observed GRB spectra and light curves, such as the typical peak photon energy, the correlation of this peak with apparent GRB energy, and the profiles and spectral lagging of GRB subpulses.

The environs of supermassive black holes are among the universe's most extreme phenomena. Understanding the physical processes occurring in the vicinity of black holes may provide the key to answer a number of fundamental astrophysical questions including the detectability of strong gravity effects, the formation and propagation of relativistic jets, the origin of the highest energy gamma-rays and cosmic rays, and the nature and evolution of the central engine in active galactic nuclei (AGN). As a step towards this direction, this paper reviews some of the progress achieved in the field based on observations in the very high energy domain. It particularly focuses on nonthermal particle acceleration and emission processes that may occur in the rotating magnetospheres originating from accreting, supermassive black hole systems. Topics covered include direct electric field acceleration in the black hole's magnetosphere, ultra-high energy cosmic ray production, Blandford–Znajek mechanism, centrifugal acceleration and magnetic reconnection, along with the relevant efficiency constraints imposed by interactions with matter, radiation and fields. By way of application, a detailed discussion of well-known sources (Sgr A*; Cen A; M87; NGC1399) is presented.

We study time variability properties of black hole transients densely monitored by the RXTE instruments. We systematically study the time/phase lag at QPO frequency. We find hard lag, intregated over Quasi Periodic Oscillation (QPO) frequency for the low inclination source (such as GX 339-4). The hard lag monotonically decrease and become negative (i.e., soft lag happens) close to 3.0 Hz for the high inclination sources (e.g., XTE J1550-564). Thus we find two different behaviours for the high inclination and the low inclination systems. We also find that the evolution properties of low-frequency quasi-periodic oscillations (QPOs) do not depend on the orbital inclination though the amplitude of low-frequency quasi-periodic oscillations (QPOs) depends on the orbital inclination. We conclude these evolutions could be due to the systematic movement of the Comptonizing region itself confirming the propagatory shock oscillation model.

Variability classes in the enigmatic black hole candidate GRS 1915+105 are known to be correlated with the variation of the Comptonizing Efficiency (CE) which is defined to be the ratio between the number of power-law (hard) photons and seed (soft) photons injected into the Compton cloud. Similarities of light curves of several variability classes of GRS 1915+105 and IGR J17091-3624, some of which are already reported in the literature, motivated us to compute CE for IGR J17091-3624 as well. We find that they are similar to what were reported earlier for GRS 1915+105, even though masses of these objects could be different.