arXiv subjects

Indranil Chattopadhyay

Publications and source records attributed to Indranil Chattopadhyay.

At least 55 records · Page 3Linked to original sources

arXiv2014

Periodic massloss from viscous accretion flows around black holes

We investigate the behaviour of low angular momentum viscous accretion flows around black holes using Smooth Particle Hydrodynamics (SPH) method. Earlier, it has been observed that in a significant part of the energy and angular momentum parameter space, rotating transonic accretion flow undergoes shock transition before entering in to the black hole and a part of the post-shock matter is ejected as bipolar outflows, which are supposed to be the precursor of relativistic jets. In this work, we simulate accretion flows having injection parameters from the inviscid shock parameter space, and study the response of viscosity on them. With the increase of viscosity, shock becomes time dependent and starts to oscillate when the viscosity parameter crosses its critical value. As a result, the in falling matter inside the post-shock region exhibits quasi-periodic variations and causes periodic ejection of matter from the inner disc as outflows. In addition, the same hot and dense post-shock matter emits high energy radiation and the emanating photon flux also modulates quasi-periodically. Assuming a ten solar mass black hole, the corresponding power density spectrum peaks at the fundamental frequency of few Hz followed by multiple harmonics. This feature is very common in several outbursting black hole candidates. We discuss the implications of such periodic variations.

astro-ph.HE↗

arXiv2013

Interaction of galactic wind with halo gas and the origin of multiphase extraplanar material

We study the interaction of galactic wind with hot halo gas using hydrodynamical simulations. We find that the outcome of this interaction depends crucially on the wind injection density and velocity. Various phases of the extraplanar media such as high velocity clouds (HVCs), outflowing clouds, and OVI regions can originate in the interaction zones of wind with the halo gas, depending on the injection velocity and density. In our simulations the size of the HVCs is of the order of 100 pc. The total mass contained in the clouds is 10^5 -- 10^7 M_sun and they have a normal distribution of velocities in the galactic standard of rest frame, similar to HVCs. For high injection density and velocity, a significant number of clouds move outwards and resemble the case of cold neutral outflows. Furthermore a 10^5 -- 10^6 K phase is formed in our simulations which has a column density ~ 10^{18} cm^{-2}, and resembles the observed OVI regions. The injection velocity and density are linked with the mass loading factor of the outflow, efficiency of energy injection due to supernovae and the SFR. Comparison of the predicted morphology of extraplanar gas with observations can serve as a useful diagnostic for constraining feedback efficiency of outflows.

astro-ph.CO↗

arXiv2013

Accretion of multi-species plasma onto black holes

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.

astro-ph.HE↗

arXiv2013

Radiatively and thermally driven self-consistent bipolar outflows from accretion discs around compact objects

We investigate the role of radiative driving of shock ejected bipolar outflows from advective accretion discs in a self consistent manner. Radiations from the inner disc affects the subsonic part of the jet while those from the pre-shock disc affects the supersonic part, and there by constitutes a multi stage acceleration process. We show that the radiation from the inner disc not only accelerate but also increase the mass outflow rate, while the radiation from the pre-shock disc only increases the kinetic energy of the flow. With proper proportions of these two radiations, very high terminal speed is possible. We also estimated the post-shock luminosity from the pre-shock radiations, and showed that with the increase of viscosity parameter the disc becomes more luminous, and the resulting jet simultaneously becomes faster. This mimics the production of steady mildly relativistic but stronger jets as micro-quasars moves from low hard to intermediate hard spectral states.

astro-ph.HE↗

arXiv2013

How Plasma Composition affects the relativistic flows and the emergent spectra

It has been recently shown that transonic electron positron fluid is the least relativistic, compared to the fluid containing finite proportion of baryons. We compute spectra from these flows in general relativity (GR) including the effect of light bending. We consider the bremsstrahlung process to supply the seed photons. We choose accretion in the advective domain, and for simplicity the radial accretion or Bondi type accretion. We show that electron positron accreting flow produces the softest spectra and the lowest luminosity.

astro-ph.HE↗

arXiv2013

Effect of the flow composition on outflow rates from accretion discs around black holes

We studied the outflow behaviour from accretion discs around black holes taking into account the vertical equilibrium accretion flow model. The outflow rate is found to depend crucially on flow composition. Our approach is to study the outflow behaviour as function of inflow around black holes with an equation of state which allows flow to be thermally relativistic close to black holes and non relativistic far away from black holes. We studied shock ejection model. A pure electron positron pair flow never undergoes shock transition while presence of some baryons (common in outflows and jets) makes it possible to have standing shock waves in the flow. It can be concluded that the presence of protons is necessary for the flow to show the outflow behaviour. The outflow rate is maximum when the flow contains the proton number density which is 27% of the electron number density. We conclude that a pure electron-positron jet is unlikely to form.

astro-ph.HE↗

arXiv2013

Effect of plasma composition on accretion on to black holes

Matter makes a transition from non-relativistic to relativistic regime, as it falls onto a black hole. We employ a relativistic equation of state, abbreviated as RC, to study multi-species fluid flow around black holes. We show that pair-plasma fluid around a black hole is thermally not relativistic. In order to make it relativistic, a finite baryon loading is necessary. As a consequence of this, pair-plasma flow do not suffer centrifugal pressure driven shock in accretion. However, fluid with finite baryon content may undergo shock transition.

astro-ph.HE↗

arXiv2013

Effects of the composition on transonic properties of accretion flows around black holes

We study the properties of a steady, multi-species, low angular momentum accretion flow around a Schwarzschild black hole. Each species is described by a relativistic equation of state. We find that the transonic properties depend strongly on the composition of the flow. We find that an electron-positron pair plasma is the least relativistic one. This flow produces only one sonic point very close to the event horizon and does not show multiple critical points for any angular momentum or energy. When the baryons are present, the number of critical points depend on the specific energy content. Since the number of critical points decide whether the flow will have non-linearities or shock waves, our results implies that whether or not standing shocks forms will depend on the flow composition. Thus, for instance, a pure electron-positron pair plasma will never undergo a shock transition, while mixing it with some baryons (common in outflows and jets, for example) as in a completely ionized gas, will have shocks. We study in detail how the baryon loading affects the shock properties and discuss implication in astrophysical observations.

astro-ph.HE↗

arXiv2012

Estimation of the mass outflow rates from viscous accretion discs

We study viscous accretion disc around black holes, and all possible accretion solutions, including shocked as well as shock free accretion branches. Shock driven bipolar outflows from a viscous accretion disc around a black hole has been investigated. One can identify two critical viscosity parameters $α_{cl}$ and $α_{cu}$, within which the stationary shocks may occur, for each set of boundary conditions. Adiabatic shock has been found for upto viscosity parameter $α=0.3$, while in presence of dissipation and massloss we have found stationary shock upto $α=0.15$. The mass outflow rate may increase or decrease with the change in disc parameters, and is usually around few to 10% of the mass inflow rate. We show that for the same outer boundary condition, the shock front decreases to a smaller distance with the increase of $α$. We also show that the increase in dissipation reduces the thermal driving in the post-shock disc, and hence the mass outflow rate decreases upto a few %.

astro-ph.HE↗

arXiv2012

Simulation of radiation driven wind from disc galaxies

We present 2-D hydrodynamic simulation of rotating galactic winds driven by radiation. We study the structure and dynamics of the cool and/or warm component($T \simeq 10^4$ K) which is mixed with dust. We have taken into account the total gravity of a galactic system that consists of a disc, a bulge and a dark matter halo. We find that the combined effect of gravity and radiation pressure from a realistic disc drives the gas away to a distance of $\sim 5$ kpc in $\sim 37$ Myr for typical galactic parameters. The outflow speed increases rapidly with the disc Eddington parameter $Γ_0(=κI/(2 c G Σ)$) for $Γ_0 \ge 1.5$. We find that the rotation speed of the outflowing gas is $\lesssim 100$ km s$^{-1}$. The wind is confined in a cone which mostly consist of low angular momentum gas lifted from the central region.

astro-ph.GA↗

arXiv2011

Relativistic Hydrodynamic Codes for Adiabatic and Isothermal Flows

The equation of state (EOS) is an important issue in numerical simulation codes for relativistic hydrodynamics. We describe a code for adiabatic flows, employing an EOS which is simple and yet approximates very closely the EOS of perfect gas in relativistic regime. We also describe a new code for isothermal flows, where the EoS is trivially given.

astro-ph.HE↗

arXiv2010

Quasi-Spherical, Time-Dependent Viscous Accretion Flow: One-Dimensional Results

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 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 expansion of the outer shock, as well as creation of regions with $\partial l / \partial r < 0$ 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 implication for low frequency QPOs, e.g., GRO J1655-40 and XTE J1550-564.

astro-ph.CO↗

arXiv2008

Effects of Fluid Composition on Spherical Flows around Black Holes

Steady, spherically symmetric, adiabatic accretion and wind flows around non-rotating black holes were studied for fully ionized, multi-component fluids, which are described by a relativistic equation of state (EoS). We showed that the polytropic index depends on the temperature as well as on the composition of fluids, so the composition is important to the solutions of the flows. We demonstrated that fluids with different composition can produce dramatically different solutions, even if they have the same sonic point, or they start with the same specific energy or the same temperature. Then, we pointed that the Coulomb relaxation times can be longer than the dynamical time in the problem considered here, and discussed the implication.

astro-ph↗

arXiv2008

Computation of mass loss from viscous accretion disc in presence of cooling

Rotating accretion flow may undergo centrifugal pressure mediated shock transition even in presence of various dissipative processes, such as viscosity and cooling mechanism. The extra thermal gradient force along the vertical direction in the post shock flow drives a part of the accreting matter as bipolar outflows which are believed to be the precursor of relativistic jets. We compute mass loss rates from a viscous accretion disc in presence of synchrotron cooling in terms of the inflow parameters. We show cooling significantly affects the mass outflow rate, to the extent that, jets may be generated from flows with higher viscosity. We discuss that our formalism may be employed to explain observed jet power for a couple of black hole candidates. We also indicate that using our formalism, it is possible to connect the spectral properties of the disc with the rate of mass loss.

astro-ph↗

arXiv2007

Massloss from viscous advective disc

Rotating transonic flows are long known to admit standing or oscillating shocks and that the excess thermal energy in the post shock flow drives a part of the infalling matter as bipolar outflows. We compute massloss from a viscous advective disc. We show that the mass outflow rate decreases with increasing viscosity of the accretion disc, since viscosity weakens the centrifugal barrier that generates the shock. We also show that the optical depth of the post-shock matter decreases due to massloss which may soften the spectrum from such a mass losing disc.

astro-ph↗

arXiv2006

Equation of State in Numerical Relativistic Hydrodynamics

Relativistic temperature of gas raises the issue of the equation of state (EoS) in relativistic hydrodynamics. We study the EoS for numerical relativistic hydrodynamics, and propose a new EoS that is simple and yet approximates very closely the EoS of the single-component perfect gas in relativistic regime. We also discuss the calculation of primitive variables from conservative ones for the EoS's considered in the paper, and present the eigenstructure of relativistic hydrodynamics for a general EoS, in a way that they can be used to build numerical codes. Tests with a code based on the Total Variation Diminishing (TVD) scheme are presented to highlight the differences induced by different EoS's.

astro-ph↗

arXiv2004

Radiatively Driven Jets around Black Holes

The hot, puffed up, post-shock region of an advective disc is the source of high energy photons and also the jets and outflows. We study the relativistic equations of motion of jets as these high energy photons interact with them. We show that the much discussed terminal velocity of jets depends on the comparative value of radiative energy density, flux and the radiative pressure. We show that electron-positron pair plasma jets achieves highly relativistic terminal speeds for higher disc luminosities.

astro-ph↗

arXiv2004

Radiatively driven rotating pair-plasma jets from two component accretion flows

Centrifugal pressure of matter spiralling onto black holes, have long been known to produce standing or oscillating shocks. The post-shock disc puffs up in the form of a torus, which intercepts soft photons from the outer Keplerian disc and inverse Comptonizes them to produce hard photons. The post-shock region also produces jets. We study the interaction of both hard photons and soft photons, with rotating electron-positron jets. We show that hard photons from the post-shock torus are instrumental in acceleration of jets, while soft photons from the Keplerian disc is a better collimating agent. We also show that if the jets are launched closer to the black hole, relativistic and collimated jets are produced; if they are launched at larger distances both collimation and acceleration are less. We also show that if the shock location is relatively at larger distances from the black hole, collimation is better.

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