arXiv subjects

Indranil Chattopadhyay

Publications and source records attributed to Indranil Chattopadhyay.

At least 37 records · Page 2Linked to original sources

arXiv2020

Simulations of radiation driven winds from Keplerian discs

We study the ejection of winds from thin accretion discs around stellar mass black holes and the time evolution of these winds in presence of radiation field generated by the accretion disc. Winds are produced by radiation, thermal pressure and the centrifugal force of the disc. The winds are found to be mildly relativistic, with speeds reaching up to terminal speeds $0.1$ for accretion rate $4$ in Eddington units. We show that the ejected matter gets its rotation by transporting angular momentum from the disc to the wind. We also show that the radiation drag affects the accretion disc winds in a very significant manner. Not only that the terminal speeds are reduced by an order of magnitude due to radiation drag, but we also show that the non-linear effect of radiation drag, can mitigate the formation of the winds from the matter ejected by the accretion disc. As radiation drag reduces the velocity of the wind, the mass outflow rate is reduced in its presence as well.

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arXiv2020

Two temperature solutions and emergent spectra from relativistic accretion discs around black holes

An accreting system in two-temperature regime, admits multiple solutions for the same set of constants of motion, producing widely different spectra. Comparing observed spectrum with that derived from a randomly chosen accretion solution, will give us a wrong estimation of the accretion parameters of the system. The form of entropy measure obtained by us, have helped in removing the degeneracy of the solutions, allowing us to understand the physics of the system, shorn of arbitrary assumptions. In this work, we have shown how the spectra and luminosities of an accreting system depends on the constants of motion, producing solutions ranging from radiatively inefficient flows to luminous flows. Increase in BH mass quantitatively changes the system, makes the system more luminous and the spectral bandwidth also increases. Higher BH mass system spans from radio to gamma-rays. However, increasing the accretion rate around a BH of certain mass, has little influence in the frequency range of the spectra.

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arXiv2019

Study of relativistic magnetized outflows with relativistic equation of state

We study relativistic magnetized outflows using relativistic equation of state having variable adiabatic index ($Γ$) and composition parameter $(ξ)$. We study the outflow in special relativistic magneto-hydrodynamic regime, from sub-Alfvénic to super-fast domain. We showed that, after the solution crosses the fast point, magnetic field collimates the flow and may form a collimation-shock due to magnetic field pinching/squeezing. Such fast, collimated outflows may be considered as astrophysical jets. Depending on parameters, the terminal Lorentz factors of an electron-proton outflow can comfortably exceed few tens. We showed that due to the transfer of angular momentum from the field to the matter, the azimuthal velocity of the outflow may flip sign. We also study the effect of composition $(ξ)$ on such magnetized outflows. We showed that relativistic outflows are affected by the location of the Alfvén point, the polar angle at the Alfvén point and also the angle subtended by the field lines with the equatorial plane, but also on the composition of the flow. The pair dominated flow experiences impressive acceleration and is hotter than electron proton flow.

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arXiv2019

Effect of plasma composition on magnetized outflows

In this paper, we study magnetized winds described by variable adiabatic index equation of state in Paczyński \& Wiita pseudo-Newtonian potential. We identify the flow solutions with the parameter space of the flow. We also confirm that the physical wind solution is the one which passes through the slow, Alfvén and fast critical points. We study the dependence of the wind solution on the Bernoulli parameter $E$ and the total angular momentum $L$. The adiabatic index, which is a function of temperature and composition, was found to be variable in all the outflow solutions. For the same values of the Bernoulli parameter and the total angular momentum, a wind in strong gravity is more accelerated, compared to a wind in Newtonian gravity. We show that flow variables like the radial and azimuthal velocity components, temperature all depend on the composition of the flow. Unlike the outflow solutions in hydrodynamic regime, the terminal speed of a magnetically driven wind also depends on the composition parameter.

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arXiv2018

General relativistic two-temperature accretion solutions for spherical flows around black holes

Matter falling onto black holes is hot, fully ionized and has to be necessarily transonic. Since the electrons are responsible for radiative cooling via processes like synchrotron, bremsstrahlung and inverse-Compton, therefore the electron gas and proton gas are supposed to settle into two separate temperature distribution. But the problem with two-temperature flow is that, there is one more variable than the number of equations. Accretion flow in its simplest form is radial, which has two constants of motion. While, the flow variables are, the radial bulk three-velocity, electron and proton temperatures. Therefore, unlike single temperature flow, in the two temperature regime, there are multiple transonic solutions, non-unique for any given set of constants of motion with a large variation in sonic points. We invoked the second law of thermodynamics to find a possible way to break the degeneracy, by showing only one of solutions among all possible, has maximum entropy and therefore is the correct solution. By considering these correct solutions, we showed that the accretion efficiency increase with the increase in the mass accretion rate. We showed that radial flow onto super-massive black hole can radiate with efficiency more than 10\%, if the accretion rate is more than 60\% of the Eddington accretion rate, but accretion onto stellar-mass black hole achieve the same efficiency, when it is close to the Eddington limit. We also showed that, dissipative heat quantitatively affects the two temperature solution. In presence of explicit heat processes the Coulomb coupling is weak.

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arXiv2018

Radiation driving and heating of general relativistic jets under Compton scattering regime

Interaction of intense radiation from the underlying accretion disc with steady, general-relativistic jet is studied. The radiation field imparts momentum as well as energy on to the outflowing jet under Compton scattering. As a result, the jet gains momentum and is simultaneously heated up. Jets can be classified as types A, B and C according to their base properties. We found that A type jets can undergo shock transition. It is also shown that, in the Compton scattering regime, radiation can drive jets starting with very small thermal energy at the base (B and C type jets). Such that, radiation can even} accelerate bound matter (generalized Bernoulli parameter $E<1$) {in the form of relativistic transonic jets. This is in stark contrast to radiatively driven jets in the Thomson scattering regime, where transonic jets were obtained only for $E>1$. We also show that for a given disc luminosity, jets in the Compton scattering regime exhibit a minimum terminal speed, unlike in the Thomson scattering domain. Further, the impact of accretion disc luminosity and jet plasma composition is studied. The ${{\rm e}^--{\rm p}^+}$ jets are accelerated up to Lorentz factors of about a few, while for lepton dominated jets the minimum Lorentz factor exceeds $10$ for moderate disc luminosities and can go up to few tens for highly luminous discs.

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arXiv2018

Study of Magnetized accretion flow with cooling processes

We have studied shock in magnetized accretion flow/funnel flow in case of neutron star with bremsstrahlung cooling and cyclotron cooling. All accretion solutions terminate with a shock close to the neutron star surface, but at some region of the parameter space, it also harbours a second shock away from the star surface. We have found that cyclotron cooling is necessary for correct accretion solutions which match the surface boundary conditions.

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arXiv2018

Study of magnetized accretion flow with variable $Γ$ equation of state

We present here the solutions of magnetized accretion flows on to a compact object with hard surface such as neutron stars. The magnetic field of the central star is assumed dipolar and the magnetic axis is assumed to be aligned with the rotation axis of the star. We have used an equation of state for the accreting fluid in which the adiabatic index is dependent on temperature and composition of the flow. We have also included cooling processes like bremsstrahlung and cyclotron processes in the accretion flow. We found all possible accretion solutions. All accretion solutions terminate with a shock very near to the star surface and the height of this primary shock do not vary much with either the spin period or the Bernoulli parameter of the flow, although the strength of the shock may vary with the period. For moderately rotating central star there are possible formation of multiple sonic points in the flow and therefore, a second shock far away from the star surface may also form. However, the second shock is much weaker than the primary one near the surface. We found that if rotation period is below a certain value $(\pmin)$, then multiple critical points or multiple shocks are not possible and $\pmin$ depends upon the composition of the flow. We also found that cooling effect dominates after the shock and that the cyclotron and the bremsstrahlung cooling processes should be considered to obtain a consistent accretion solution.

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arXiv2018

Radiatively driven general relativistic jets

We use moment formalism of relativistic radiation hydrodynamics to obtain equations of motion of radial jets and solve them using polytropic equation of state of the relativistic gas. We consider curved space-time around black holes and obtain jets with moderately relativistic terminal speeds. In addition, the radiation field from the accretion disc, is able to induce internal shocks in the jet close to the horizon. Under combined effect of thermal as well as radiative driving, terminal speeds up to 0.75 (units of light speed) are obtained.

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arXiv2018

Radiatively driven relativistic jets in Schwarzschild space-time

We carry out a general relativistic study of radiatively driven, conical fluid jets around non-rotating black holes and investigate the effects and significance of radiative acceleration, as well as radiation drag. We apply relativistic equations of motion in curved space-time around a Schwarzschild black hole for axis-symmetric 1-D jet in steady state, plying through the radiation field of the accretion disc. Radiative moments are computed using information of curved space-time. Slopes of physical variables at the sonic points are found using L$^{\prime}$Hôpital's rule and employed Runge-Kutta's $4^{th}$ order method to solve equations of motion. The analysis is carried out, using the relativistic equation of state of the jet fluid. The terminal speed of the jet depends on how much thermal energy is converted into jet momentum and how much radiation momentum is deposited on to the jet. Many classes of jet solutions with single sonic points, multiple sonic points as well as, those having radiation driven internal shocks are obtained. Variation of all flow variables along the jet-axis has been studied. Highly energetic electron-proton jets can be accelerated by intense radiation to terminal Lorentz factors $γ_{\rm \small T} \sim 3$. Moderate terminal speed $v_{\rm \small T} \sim 0.5$ is obtained for moderately luminous discs. Lepton dominated jets may achieve $γ_{\rm \small T} \sim 10$. Thermal driving of the jet itself and radiation driving by accretion disc photons produce a wide-ranging jet solutions staring from moderately strong jets to the relativistic ones. Interplay of intensity and nature of radiation field and the energetics of the jet result in such a variety in jet solutions. We show that radiation field is able to induce steady shocks in jets, one of the criteria to explain high energy power law emission observed in spectra of some of the astrophysical objects.

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arXiv2017

Estimation of bipolar jets from accretion discs around Kerr black holes

We analyse flows around a rotating black hole and obtain self-consistent accretion-ejection solutions in full general relativistic prescription. Entire energy-angular momentum parameter space is investigated in the advective regime to obtain shocked and shock-free accretion solutions. Jet equations of motion are solved along the von-Zeipel surfaces computed from the post-shock disc, simultaneously with the equations of accretion disc along the equatorial plane. For a given spin parameter, the mass outflow rate increases as the shock moves closer to the black hole, but eventually decreases, maximizing at some intermediate value of shock location. Interestingly, we obtain all types of possible jet solutions, for example, steady shock solution with multiple critical points, bound solution with two critical points and smooth solution with single critical point. Multiple critical points may exist in jet solution for spin parameter $a_s\ge 0.5$. The jet terminal speed generally increases if the accretion shock forms closer to the horizon and is higher for corotating black hole than the counter-rotating and the non-rotating one. Quantitatively speaking, shocks in jet may form for spin parameter $a_s>0.6$ and jet shocks range between $6r_g$ and $130r_g$ above the equatorial plane, while the jet terminal speed $v_{{\rm j}\infty} > 0.35{\cm}$ if Bernoulli parameter ${\cal E}\geq1.01$ for $a_s>0.99$.

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arXiv2017

General relativistic study of astrophysical jets with internal shocks

We explore the possibility of formation of steady internal shocks in jets around black holes. We consider a fluid described by a relativistic equation of state, flowing about the axis of symmetry ($θ=0$) in a Schwarzschild metric. We use two models for the jet geometry, (i) a conical geometry and (ii) a geometry with non-conical cross-section. Jet with conical geometry is smooth flow. While the jet with non-conical cross section undergoes multiple sonic point and even standing shock. The jet shock becomes stronger, as the shock location is situated further from the central black hole. Jets with very high energy and very low energy do not harbour shocks, but jets with intermediate energies do harbour shocks. One advantage of these shocks, as opposed to shocks mediated by external medium is that, these shocks have no effect on the jet terminal speed, but may act as possible sites for particle acceleration. Typically, a jet with energy $1.8~c^2$, will achieve a terminal speed of $v_\infty=0.813c$ for jet with any geometry. But for a jet of non-conical cross-section for which the length scale of the inner torus of the accretion disc is $40\rg$, then in addition, a steady shock will form at $\rsh \sim 7.5\rg$ and compression ratio of $R\sim 2.7$. Moreover, electron-proton jet seems to harbour the strongest shock. We discuss possible consequences of such a scenario.

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arXiv2016

Simulations of Viscous Accretion Flow around Black Holes in Two-Dimensional Cylindrical Geometry

We simulate shock-free and shocked viscous accretion flow onto a black hole in a two dimensional cylindrical geometry, where initial conditions were chosen from analytical solutions. The simulation code used the Lagrangian Total Variation Diminishing (LTVD) and remap routine, which enabled us to attain high accuracy in capturing shocks and to handle the angular momentum distribution correctly. Inviscid shock-free accretion disk solution produced a thick disk structure, while the viscous shock-free solution attained a Bondi-like structure, but in either case, no jet activity nor any QPO-like activity developed. The steady state shocked solution in the inviscid, as well as, in the viscous regime, matched theoretical predictions well. However, increasing viscosity renders the accretion shock unstable. Large amplitude shock oscillation is accompanied by intermittent, transient inner multiple shocks. Such oscillation of the inner part of disk is interpreted as the source of QPO in hard X-rays observed in micro-quasars. Strong shock oscillation induces strong episodic jet emission. The jets also showed existence of shocks, which are produced as one shell hits the preceding one. The periodicity of jets and shock oscillation were similar. The jets for higher viscosity parameter are evidently stronger and faster.

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arXiv2016

Estimation of mass outflow rates from viscous relativistic accretion discs around black holes

We investigated flow in Schwarzschild metric, around a non-rotating black hole and obtained self-consistent accretion - ejection solution in full general relativity. We covered the whole of parameter space in the advective regime to obtain shocked, as well as, shock-free accretion solution. We computed the jet streamline using von - Zeipel surfaces and projected the jet equations of motion on to the streamline and solved them simultaneously with the accretion disc equations of motion. We found that steady shock cannot exist {for $α\gsim0.06$} in the general relativistic prescription, but is lower if mass - loss is considered too. We showed that for fixed outer boundary, the shock moves closer to the horizon with increasing viscosity parameter. The mass outflow rate increases as the shock moves closer to the black hole, but eventually decreases, maximizing at some intermediate value of shock {location}. The jet terminal speed increases with stronger shocks, quantitatively speaking, the terminal speed of jets $v_{{\rm j}\infty} > 0.1$ if $\rsh < 20 \rg$. The maximum of the outflow rate obtained in the general relativistic regime is less than $6\%$ of the mass accretion rate.

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arXiv2015

Radiatively driven relativistic jets with variable adiabatic index equation of state

We investigate a relativistic fluid jet driven by radiation from a shocked accretion disc around a non-rotating black hole approximated by Paczyński-Wiita potential. The sub-Keplerian and Keplerian accretion rates control the shock location and therefore, the radiation field around the accretion disc. We compute the radiative moments with full special relativistic transformation. The effect of a fraction of radiation absorbed by the black hole has been approximated, over and above the special relativistic transformations. We show that the radiative moments around a super massive black hole are different compared to that around a stellar mass black hole. We show that the terminal speed of jets increases with the mass accretion rates,synchrotron emission of the accretion disc and reduction of proton fraction of the flow composition. To obtain relativistic terminal velocities of jets, both thermal and radiative driving are important. We show for very high accretion rates and pair dominated flow, jets around super massive black holes are truly ultra-relativistic, while for jets around stellar mass black holes, terminal Lorentz factor of about $10$ is achievable.

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arXiv2014

Dissipative advective accretion disc solutions with variable adiabatic index around black holes

We investigated accretion on to black holes in presence of viscosity and cooling, by employing an equation of state with variable adiabatic index and multi-species fluid. We obtained the expression of generalized Bernoulli parameter which is a constant of motion for an accretion flow in presence of viscosity and cooling. We obtained all possible transonic solutions for a variety of boundary conditions, viscosity parameters and accretion rates. We identified the solutions with their positions in the parameter space of generalized Bernoulli parameter and the angular momentum on the horizon. We showed that a shocked solution is more luminous than a shock-free one. For particular energies and viscosity parameters, we obtained accretion disc luminosities in the range of $10^{-4}-1.2$ times Eddington limit, and the radiative efficiency seemed to increase with the mass accretion rate too. We found steady state shock solutions even for high-viscosity parameters, high accretion rates, and for wide range of composition of the flow, starting from purely electron-proton to lepton-dominated accretion flow. However, similar to earlier studies of inviscid flow, accretion shock was not obtained for electron-positron pair plasma.

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arXiv2014

On the possibilities of mass loss from an advective accretion disc around stationary black holes

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.

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arXiv2014

Formation of non-steady outflows and QPOs around black hole

We study the time dependent properties of sub-Keplerian viscous accretion flow around the black holes. We find that rotating matter feels centrifugal barrier on the way towards the black holes and eventually, shock transition is triggered allowing a part of the post-shock matter to eject out as bipolar outflow. This outflowing matters are supposed to be the precursor of relativistic jets. As viscosity is increased, shock becomes unsteady and start to oscillate when viscosity reached its critical value. This causes the inner part of the disk unsteady resulting periodic ejection of matter from the post-shock region. Also, the same hot and dense post-shock matter emits high energy radiation which modulates quasi-periodically. The power density spectra confirms this features as most of the power is concentrated at a narrow frequency range --- a characteristics (i. e. Quasi-Periodic Oscillation) commonly seen in several outbursting black hole candidates.

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