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We investigate two-temperature accretion flows onto strongly magnetized compact stars. Matter is accreted in the form of an accretion disc upto the disc radius ($r_{\rm d}$), where, the magnetic pressure exceeds both the gas and ram pressure and thereafter the matter is channelled along the field lines onto the poles. We solve the equations of motion self-consistently along the field lines, incorporating radiative processes like bremsstrahlung, synchrotron and inverse-Comptonization. For a given set of constants of motion, the equations of motion do not produce unique transonic solution. Following the second law of thermodynamics the solution with the highest entropy is selected and thereby eliminating the degeneracy in solution. We study the properties of these solutions and obtain corresponding spectra as a function of the magnetic field ($B_*$), spin period ($P$) and accretion rate of the star ($\dot{M}$). A primary shock is always formed just near the surface. The enhanced radiative processes in this post-shock region slows down the matter and it finally settles on the surface of the star. This post-shock region contributes to $\gtrsim 99.99\%$ of the total luminosity obtained from the accretion flow. It is still important to study the full accretion flow because secondary shocks may be present for some combination of $B_*$, $P$ and ${\dot{M}}$ in addition to primary shocks. We find that secondary shocks, if present, produce an extended emission at higher energies in the spectra., 18 pages, 17 figures and accepted for publication in MNRAS

We study time-dependent relativistic jets under the influence of radiation field of the accretion disk. The accretion disk consists of an inner compact corona and an outer sub-Keplerian disk. The thermodynamics of the fluid is governed by a relativistic equation of state (EoS) for multispecies fluid which enables to study the effect of composition on jet-dynamics. Jets originate from the vicinity of the central black hole where the effect of gravity is significant and traverses large distances where only special relativistic treatment is sufficient. So we have modified the flat metric to include the effect of gravity. In this modified relativistic framework we have developed a new total variation diminishing (TVD) routine along with multispecies EoS for the purpose. We show that the acceleration of jets crucially depends on flow composition. All the results presented are transonic in nature, starting from very low injection velocities, the jets can achieve high Lorentz factors. For sub-Eddington luminosities, lepton dominated jets can be accelerated to Lorentz factors > 50. The change in radiation field due to variation in the accretion disk dynamics will be propagated to the jet in a finite amount of time. Hence any change in radiation field due to a change in disk configuration will affect the lower part of the jet before it affects the outer part. This can drive shock transition in the jet flow. Depending upon the disk oscillation frequency, amplitude and jet parameters these shocks can collide with each other and may trigger shock cascades., 21 pages, 11 figures. Accepted for publication in ApJ

We study the effect of plasma composition on the dynamics and morphology of the relativistic astrophysical jets. Our work is based on a relativistic total variation diminishing (TVD) simulation code. We use a relativistic equation of state in the simulation code which accounts for the thermodynamics of a multispecies plasma which is a mixture of electrons, positrons, and protons. To study the effect of plasma composition we consider various jet models. These models are characterized by the same injection parameters, same jet kinetic luminosity, and the same Mach numbers. The evolution of these models shows that the plasma composition affects the jet head propagation speed, the structure of the jet head, and the morphology despite fixing the initial parameters. We conclude that the electron-positron jets are the slowest and show more pronounced turbulent structures in comparison to other plasma compositions. The area and locations of the hot-spots also depend on the composition of jet plasma. Our results also show that boosting mechanisms are also an important aspect of multi-dimensional simulations which are also influenced by the change in composition., Accepted for publication in ApJ, 13 pages and 9 figures

In an attempt to investigate the structures of ultra-relativistic jets injected into the intracluster medium (ICM) and the associated flow dynamics, such as shocks, velocity shear, and turbulence, we have developed a new special relativistic hydrodynamic (RHD) code in the Cartesian coordinates, based on the weighted essentially non-oscillatory (WENO) scheme. It is a finite difference scheme of high spatial accuracy, which has been widely employed for solving hyperbolic systems of conservation equations. The code is equipped with different WENO versions, such as the 5th-order accurate WENO-JS (Jiang & Shu 1996), WENO-Z, and WENO-ZA, and different time integration methods, such as the 4th-order accurate Runge-Kutta (RK4) and strong stability preserving RK (SSPRK), as well as the implementation of the equations of state (EOSs) that closely approximate the EOS of the single-component perfect gas in relativistic regime. In addition, it incorporates a high-order accurate averaging of fluxes along the transverse directions to enhance the accuracy of multi-dimensional problems, and a modification of eigenvalues for the acoustic modes to effectively control the carbuncle instability. Through extensive numerical tests, we assess the accuracy and robustness of the code, and choose WENO-Z, SSPRK, and the EOS suggested in Ryu et al. (2006) as the fiducial setup for simulations of ultra-relativistic jets. The results of our study of ultra-relativistic jets using the code is reported in an accompanying paper (Seo et al. 2021, Paper II)., 21 pages, 10 figures, accepted for publication in ApJ

We study the evolution of one-dimensional relativistic jets, using the exact solution of the Riemann problem for relativistic flows. For this purpose, we solve equations for the ideal special relativistic fluid composed of dissimilar particles in flat space-time and the thermodynamics of fluid is governed by a relativistic equation of state. We obtain the exact solution of jets impinging on denser ambient media. The time variation of the cross-section of the jet-head is modeled and incorporated. We present the initial condition that gives rise to a reverse shock. If the jet-head cross-section increases in time, the jet propagation speed slows down significantly and the reverse-shock may recede opposite to the propagation direction of the jet. We show that the composition of jet and ambient medium can affect the jet solution significantly. For instance, the propagation speed depends on the composition and is maximum for a pair-dominated jet, rather than a pure electron-positron or electron-proton jet. The propagation direction of the reverse-shock may also strongly depend on the composition of the jet., Accepted for publication in MNRAS; 19 pages, 18 figures

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., Comment: 31 pages, 13 figures, accepted for publication in MNRAS

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 $a_s>0.99$., 22 pages, 14 figures; Accepted on May 4, 2017, for publication in MNRAS. Corrections incorporated during proof check

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., Comment: Accepted for publication in MNRAS; 12 pages and 10 figures

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., Comment: 29 pages in referee version, 17 figures. Accepted for publication in the journal Astronomy & Astrophysics. The abstract in the announcement page is shortened in order to fit arxiv requirement

We study relativistic magnetized outflows using relativistic equation of state having variable adiabatic index ($\Gamma$) and composition parameter $(\xi)$. We study the outflow in special relativistic magneto-hydrodynamic regime, from sub-Alfv\'enic 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 $(\xi)$ on such magnetized outflows. We showed that relativistic outflows are affected by the location of the Alfv\'en point, the polar angle at the Alfv\'en 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., Comment: 25 pages; 10 figures. Accepted for publication in MNRAS

In this paper, we study magnetized winds described by variable adiabatic index equation of state in Paczy\'{n}ski \& 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, Alfv\'en 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., Comment: Accepted for publication in MNRAS, 22 pages, 12 figures

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., 26 pages, 9 figures. Accepted for publication in IJMPD

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 $E1$. 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., 12 pages, 13 figures, accepted for publication in MNRAS

We examine the dynamical behavior of accretion flow around XTE J1859+226 during the 1999 outburst by analyzing the entire outburst data ($\sim$ 166 days) from RXTE Satellite. Towards this, we study the hysteresis behavior in the hardness intensity diagram (HID) based on the broadband ($3 - 150$ keV) spectral modeling, spectral signature of jet ejection and the evolution of Quasi-periodic Oscillation (QPO) frequencies using the two-component advective flow model around a black hole. We compute the flow parameters, namely Keplerian accretion rate (${\dot m}_d$), sub-Keplerian accretion rate (${\dot m}_h$), shock location ($r_s$) and black hole mass ($M_{bh}$) from the spectral modeling and study their evolution along the q-diagram. Subsequently, the kinetic jet power is computed as $L^{\rm obs}_{\rm jet}\sim 3 - 6 \times 10^{37}$ erg~s$^{-1}$ during one of the observed radio flares which indicates that jet power corresponds to $8-16\%$ mass outflow rate from the disc. This estimate of mass outflow rate is in close agreement with the change in total accretion rate ($\sim 14\%$) required for spectral modeling before and during the flare. Finally, we provide a mass estimate of the source XTE J1859+226 based on the spectral modeling that lies in the range of $5.2 - 7.9 M_{\odot}$ with 90\% confidence., 12 pages, 8 figures, 3 tables, Accepted for publication in Astrophysics and Space Science

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., 8 pages, 6 figures, published in a special issue of JApA

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., Comment: 8 pages, 8 figures, accepted for publication in a special issue of JoAA

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$^{\prime}$H\^opital'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 $\gamma_{\rm \small T} \sim 3$. Moderate terminal speed $v_{\rm \small T} \sim 0.5$ is obtained for moderately luminous discs. Lepton dominated jets may achieve $\gamma_{\rm \small T} \sim 10$. 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., Comment: 14 pages, 13 figures; accepted for publication in A&A

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 ($\theta=0$) 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., Comment: 36 pages, 11 figures, accepted for publication in MNRAS on 20th April 2017

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., 24pages, 18figures. Accepted for publication in the Astrophysical Journal

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 $��\gsim0.06$} 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., 37 pages, 15 figures. Accepted for publication in MNRAS

We investigate a relativistic fluid jet driven by radiation from a shocked accretion disc around a non-rotating black hole approximated by Paczy\'nski-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., Comment: Accepted for publication in MNRAS on August 4, 2015. The paper consists of 42 pages, 15 figures

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., 33 pages, 11 figures. Accepted for publication in MNRAS. Now replaced with proof corrections

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., 9 pages, 7 figures, Accepted in MNRAS

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., 26 pages, 12 pages, accepted in MNRAS on September 19, 2013

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., 5 pages, 4 figures. Published in Observational Evidence for Black Holes in the Universe: Proceedings of the 2nd Kolkata Conference on Observational Evidence for Black Holes in the Universe held in Kolkata India, 10-15 February 2008 and the Satellite Meeting on Black Holes, Neutron Stars, and Gamma-Ray Bursts held 16-17 February 2008. AIP Conference Proceedings, Volume 1053, pp. 353-356 (2008)

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., Comment: 18 pages, 11 figures. Accepted for publication in MNRAS on 31 October 2013

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., Pages 3, 2 figures. Proceedings of the Twelfth Marcel Grossmann Meeting on General Relativity, edited by Thibault Damour, Robert T. Jantzen and Remo Ruffini. ISBN 978-981-4374-51-4. Singapore: World Scientific, 2012, p.982