Samir Mandal

ON THE EVOLUTION OF ACCRETION RATES IN COMPACT OUTBURST SOURCES

Spectra in outburst sources are found to become soft in viscous timescales before going back to hard as in the pre-outburst phase. By using two component accretion disks we show that major characteristics of the spectral evolution can be reproduced. We find that the outburst is possibly caused by a sudden rise in viscosity which gradually converts the sub-Keplerian flow into a Keplerian flow. The decline of the viscosity reduces the Keplerian accretion rate, and the system goes back to hard states. We discuss the genesis of the characteristic shape of the hardness-intensity diagram and reproduce this for the well-known outburst source GRO J1655-40.

Spectrum of Two-Component Flows around a Supermassive Black Hole: An Application to M87

We calculate the spectra of two-component accretion flows around black holes of various masses, from quasars to nanoquasars. Specifically, we fit the observational data of M87 very satisfactorily using our model and find that the spectrum may be well fitted by a sub-Keplerian component alone, and there is little need of any Keplerian component. The nonthermal distribution of electrons produced by their acceleration across the standing shock in the sub-Keplerian component is enough to produce the observed flat spectrum through the synchrotron radiation.

Instruments of RT-2 experiment onboard CORONAS-PHOTON and their test and evaluation IV: background simulations using GEANT-4 toolkit

Hard X-ray detectors in space are prone to background signals due to the ubiquitous cosmic rays and cosmic diffuse background radiation that continuously bombards the satellites which carry the detectors. In general, the background intensity depends on the space environment as well as the material surrounding the detectors. Understanding the behavior of the background noise in the detector is very important to extract the precise source information from the detector data. In this paper, we carry out Monte Carlo simulations using the GEANT-4 toolkit to estimate the prompt background noise measured with the detectors of the RT-2 Experiment onboard the CORONAS-PHOTON satellite.

SPECTRAL SIGNATURE OF MASS LOSS FROM (AND MASS GAIN BY) AN ACCRETION DISK AROUND A BLACK HOLE

Accretion and outflows are common in systems that include black holes. Assuming that outflows are primarily produced at the centrifugal pressure-supported boundary layer at the inner disk, the hard X-ray spectrum is softened in the presence of winds. Conversely, the spectrum would be hardened when mass gain takes place at the inner disk. We first demonstrate these changes in the spectrum from theoretical consideration. We then give examples of the spectra of a few black hole candidates that indicate that a significant mass loss or mass gain may be taking place in short timescales, causing changes is spectral slopes.

Properties of magnetically supported dissipative accretion flow around black holes with cooling effects

We investigate the global structure of the advection dominated accretion flow around a Schwarzschild black hole where the accretion disc is threaded by toroidal magnetic fields. We consider synchrotron radiative process as an effective cooling mechanism active in the flow. With this, we obtain the global transonic accretion solutions by exploring the variety of boundary conditions and dissipation parameters, namely accretion rate (

SS 433: results of a recent multiwavelength campaign

{\dot m}

Properties of two-temperature dissipative accretion flow around black holes

) and viscosity (

HOW UNUSUAL IS XRF 060218

\alpha_B

SIGNATURES OF ACCRETION SHOCKS IN BROADBAND SPECTRUM OF ADVECTIVE FLOWS AROUND BLACK HOLES

). The fact that depending on the initial parameters, steady state accretion flows can possess centrifugally supported shock waves. These global shock solutions exist even when the level of dissipation is relatively high. We study the properties of shock waves and observe that the dynamics of the post-shock corona (hereafter, PSC) is regulated by the flow parameters. Interestingly, we find that shock solution disappears completely when the dissipation parameters exceed their critical values. We calculate the critical values of viscosity parameter (

Spectral Properties of M87 Using Two‐Component Flow

\alpha^{\rm cri}_B