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DYNAMICAL STRUCTURE OF VISCOUS ACCRETION DISKS WITH SHOCKS
Das, Santabrata,Becker, Peter A.,Le, Truong IOP Publishing 2009 The Astrophysical journal Vol.702 No.1
<P>We develop and discuss global accretion solutions for viscous advection-dominated accretion flow (ADAF) disks containing centrifugally supported isothermal shock waves. The fact that such shocks can exist at all in ADAF disks is a new result. Interestingly, we find that isothermal shocks can form even when the level of viscous dissipation is relatively high. In order to better understand this phenomenon, we explore all possible combinations of the fundamental flow parameters, such as specific energy, specific angular momentum, and viscosity, to obtain the complete family of global solutions. This procedure allows us to identify the region of the parameter space where isothermal shocks can exist in viscous ADAF disks. The allowed region is maximized in the inviscid case, and it shrinks as the level of viscous dissipation increases. Adopting the canonical value gamma = 1.5 for the ratio of specific heats, we find that the shock region disappears completely when the Shakura-Sunyaev viscosity parameter alpha exceeds the critical value similar to 0.27. This establishes for the first time that steady ADAF disks containing shocks can exist even for relatively high levels of viscous dissipation. If an isothermal shock is present in the disk, it would have important implications for the acceleration of energetic particles that can escape to power the relativistic jets commonly observed around underfed, radio-loud black holes. In two specific applications, we confirm that the kinetic luminosity lost from the disk at the isothermal shock location is sufficient to power the observed relativistic outflows in M87 and Sgr A*.</P>
Studies of dissipative standing shock waves around black holes
Das, Santabrata,Chakrabarti, Sandip K.,Mondal, Soumen Blackwell Publishing Ltd 2010 MONTHLY NOTICES- ROYAL ASTRONOMICAL SOCIETY Vol.401 No.3
<P>ABSTRACT</P><P>We investigate the dynamical structure of advective accretion flow around stationary as well as rotating black holes. For a suitable choice of input parameters, such as accretion rate <IMG SRC='http://onlinelibrary.wiley.com/store/10.1111/j.1365-2966.2009.15793.x/asset/equation/MNR_15793_mu1.gif?v=1&s=1f863aabc4afe4e815441d3668ba8bef331dbb46'/> and angular momentum (λ), a global accretion solution may include a shock wave. The post-shock flow is located at a few tens times the Schwarzchild radius and is generally very hot and dense. This successfully mimics the so-called Compton cloud, which is believed to be responsible for emitting hard radiation. Owing to the radiative loss, significant energy is removed from the accreting matter and the shock moves forward towards the black hole in order to maintain the pressure balance across it. We identify the effective area of parameter space <IMG SRC='http://onlinelibrary.wiley.com/store/10.1111/j.1365-2966.2009.15793.x/asset/equation/MNR_15793_mu2.gif?v=1&s=390e74728c22c2f001e31788bfde14ab9deeb019'/> that allows accretion flows to have some energy dissipation at the shock (Δε). As the dissipation is increased, the parameter space is reduced and finally disappears when the dissipation reaches a critical value. The dissipation has a profound effect on the dynamics of post-shock flow. By moving forward, an unstable shock, the oscillation of which causes quasi-periodic oscillations (QPOs) in the emitted radiation, will produce oscillations of high frequency. Such an evolution of QPOs has been observed in several black hole candidates during their outbursts.</P>
Behaviour of dissipative accretion flows around black holes
Blackwell Publishing Ltd 2007 MONTHLY NOTICES- ROYAL ASTRONOMICAL SOCIETY Vol.376 No.4
<P>ABSTRACT</P><P>We investigate the behaviour of dissipative accreting matter close to a black hole, as this provides important observational features of galactic and extragalactic black hole candidates. We find a complete set of global solutions in the presence of viscosity and synchrotron cooling. We show that advective accretion flow can have a standing shock wave and the dynamics of the shock is controlled by the dissipation parameters (both viscosity and cooling). We study the effective region of the parameter space for standing as well as oscillating shock. We find that the shock front always moves towards the black hole as the dissipation parameters are increased. However, viscosity and cooling have opposite effects in deciding the solution topologies. We obtain two critical cooling parameters that separate the nature of the accretion solution.</P>
INTERGALACTIC MAGNETIC FIELD AND ARRIVAL DIRECTION OF ULTRA-HIGH-ENERGY PROTONS
Ryu, Dongsu,Das, Santabrata,Kang, Hyesung IOP Publishing 2010 The Astrophysical journal Vol.710 No.2
<P>We studied how the intergalactic magnetic field (IGMF) affects the propagation of super-Greisen-Zatsepin-Kuz'min (GZK) protons that originate from extragalactic sources within the local GZK sphere. To this end, we set up hypothetical sources of ultra-high-energy cosmic rays (UHECRs), virtual observers, and the magnetized cosmic web in a model universe constructed from cosmological structure formation simulations. We then arranged a set of reference objects mimicking active galactic nuclei (AGNs) in the local universe, with which correlations of simulated UHECR events are analyzed. With our model IGMF, the deflection angle between the arrival direction of super-GZK protons and the sky position of their actual sources is quite large with a mean value of <theta > similar to 15 degrees and a median value of (0) over tilde similar to 7 degrees-10 degrees. On the other hand, the separation angle between the arrival direction and the sky position of nearest reference objects is substantially smaller with < S > similar to 3 degrees.5-4 degrees, which is similar to the mean angular distance in the sky to nearest neighbors among the reference objects. This is a direct consequence of our model that the sources, observers, reference objects, and the IGMF all trace the matter distribution of the universe. The result implies that extragalactic objects lying closest to the arrival direction of UHECRs are not necessarily their actual sources. With our model for the distribution of reference objects, the fraction of super-GZK proton events, whose closest AGNs are true sources, is less than 1/3. We discussed implications of our findings for correlation studies of real UHECR events.</P>