Lee S.-J.,Chungbuk National University |
Ryu D.,Chungnam National University |
Astrophysical Journal | Year: 2011
We investigated the instability of advective accretion flow as a consequence of angular momentum transfer in one-dimensional, quasi-spherical transonic accretion flow around a non-rotating black hole. The code is designed to include the effects of viscosity; the hydrodynamics component preserves angular momentum strictly with Lagrangian and remap method in the absence of viscosity, while the viscosity component updates viscous angular momentum transfer through the implicit method. We performed two tests to demonstrate the suitability of the code for accretion study. First, we simulated the inviscid, low angular momentum, transonic accretion flow with shocks around a black hole, and then the subsonic, self-similar ADAF solution around a Newtonian object. Both simulations fitted the corresponding analytical curves extremely well. We then simulated a rotating, viscous, transonic fluid with shocks. We showed that for low viscosity parameter, stable shocks at larger distance are possible. For higher viscosity parameter, more efficient angular momentum transfer in the post-shock disk makes the shock structure oscillatory. Moreover, as the shock drifts to larger distances, a secondary inner shock develops. We showed that the inner shock is the direct consequence of the expansion of the outer shock, as well as the creation of regions with ∂l/∂r < 0 due to more efficient angular momentum transfer near the inner sonic point. We showed that all disk parameters, including emissivity, oscillate with the same period as that of the shock oscillation. Our simulation may have implications for low frequency quasi-periodic oscillations, e.g., GRO J1655-40 and XTE J1550-564. © 2011. The American Astronomical Society. All rights reserved. Printed in the U.S.A.
12th Marcel Grossmann Meeting on Recent Dev. in Theoretical and Experimental General Relativity, Astrophysics and Relativistic Field Theories - Proc. of the MG 2009 Meeting on General Relativity | Year: 2012
Matter falling onto a black hole is trans-relativistic, transonic, and close to the horizon it is sub-Keplerian. Such a flow shows the existence of multiple critical point, shocks etc. Employing relativistic equation of state and realistic composition, it has been shown that the solution strongly depend on the composition of the fluid. Electron-positron fluid is the least relativistic, to the extent that multiple critical points, shocks etc do not form. The most relativistic fluid is the one whose ratio of proton to electron number density is ∼ 0.2. Since the solution strongly depend on composition, the emitted radiation should strongly depend on the composition too. Keywords: Accretion; accretion discs; black hole physics; hydrodynamics; shock; relativity. Copyright © 2012 by World Scientific Publishing Co. Pte. Ltd.
Pandey S.B.,ARIES |
Zheng W.,University of Michigan
EAS Publications Series | Year: 2013
Optical observations of afterglows at very times are very useful towards understanding the least known problem of GRB research field i.e. the transition from prompt emission to early afterglows. The comparison of a subset of well-monitored GRBs and their early time properties at optical are compared with that seen at XRT and BAT wavelengths. In most of the observed cases, the very early optical observations of GRBs do not trace the canonical decay nature seen at XRT wavelengths, suggesting different origins for the observed early emissions in the two bands. In some of the early optical light-curves, the decay followed by smooth rise features are consistent with the onset of the afterglow although such features are also expected if the emission is seen off-axis and/or the outflow is structured. © EAS, EDP Sciences 2013.
Joshi Y.C.,ARIES |
Narasimha D.,Tata Institute of Fundamental Research
Bulletin of the Astronomical Society of India | Year: 2012
Two classical novae were detected in the disk of M 31. The photometric observations were taken in Cousins R and I bands during the four year Naini Tal Microlensing Survey which was being carried out during 1998-2002 with the aim of detecting microlensing events in the direction of M 31. The first nova, NMS-1, which had an outburst in 2000 and was monitored before the outburst as well as after, showed an R-magnitude of 17.2 on 2000 October 20, and a fast decline of 0.11 mag/day. For the second nova NMS-2, outburst at peak was not detected; however, this nova was observed during the declining phase in 2001. The rate of decline of NMS-2 suggests that either it was a slow nova or we started monitoring it at late phase of its outburst. In the follow-up observations which were carried out from 2-m Hanle telescope in 2004-2005, we find that both the novae have reached their pre-outburst level flux, though the Rc-Ic color could be slightly different.
Das S.,Indian Institute of Technology Guwahati |
Chattopadhyay I.,ARIES |
Nandi A.,Space Astronomy Group |
Sarkar B.,Indian Institute of Technology Guwahati
Bulletin of the Astronomical Society of India | Year: 2014
We study the coupled disc-jet system around the black hole where the outflow solutions are obtained in terms of the inflow parameters. We observe that an advective accretion disc can eject outflows/jets for wide range of viscosity parameter. However, such possibility is reduced if the cooling is active as the energy dissipative process inside the disc. For mass outflow, we obtain the parameter space spanned by the inflow angular momentum and the viscosity in terms of cooling and quantify the limits of viscosity parameter. © 2014, Astronomical Society of India, Indian Institute of Astrophysics. All rights reserved.