arXiv subjects

Indranil Chattopadhyay

Publications and source records attributed to Indranil Chattopadhyay.

At least 19 recordsLinked to original sources

arXiv2026

Magnetohydrodynamic Simulations of Transonic Accretion Flows

Theoretical studies of transonic accretion onto black holes reveal a wide range of possible solutions, broadly classified into smooth flows and flows featuring shocks. Accretion solutions that involve the formation of shocks are particularly intriguing, as they are expected to naturally produce observable variability features. However, despite their theoretical significance, time-dependent studies exploring the stability and evolution of such shocked solutions remain relatively scarce. To address this gap, we perform simulations of transonic accretion flows around a black hole in an ideal magnetohydrodynamic framework. Our simulations are initialized using boundary conditions derived from semi-analytical hydrodynamical models, allowing us to explore the stability of these flows under varying magnetic field strengths. Our results indicate that mildly magnetized flows in a uniform vertical magnetic field alter the accretion dynamics through magnetic pressure, with the resulting force imbalance driving oscillations in the shock front. Variations in the emitted luminosity arising from shock oscillations appear as quasi-periodic oscillations (QPOs), a characteristic feature commonly observed in accreting black holes. We find that the QPO frequency is determined by the radial position of the shock front: oscillations occurring closer to the black hole produce frequencies of tens of hertz, whereas shocks located farther out yield sub-hertz frequencies.

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arXiv2026

Bridging X-ray Polarization with Timing & Spectroscopic Parameters of a galactic black hole: Swift J1727.8-1613

We report the discovery of a correlated energy-dependent time lag and degree of polarization for Swift J1727.8-1613 during its 2023 outburst. The energy-dependent time lag is measured around the type-C quasi-periodic oscillations (QPO) observed by IXPE on 2023-09-07, while the degree of polarization is obtained from energy-resolved polarimetric measurements. The Spearman correlation coefficient was found to be 0.8, with a null hypothesis probability of 4.2\%. Furthermore, the correlation value drops as the quality factor, or Q value, of the observed QPO frequencies decreases. The spectral properties of Swift J1727.8-1613 are analyzed using simultaneous Insight/HXMT data. Thereafter, we present model-independent theoretical arguments to show that processes other than inverse Comptonization also contributes to both the observed polarization and time lags. This correlation may therefore point to additional mechanisms contributing to the connection between the spectral, temporal, and polarimetric properties of black hole binaries in their hard state.

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arXiv2026

Effect of Multi-Species Plasma on Fanaroff-Riley Radio Jets

The Fanaroff-Riley (FR) dichotomy observed in extragalactic radio jets has been attributed to a range of possible mechanisms, including intrinsic jet properties such as the presence of different species in the plasma. Jet material may span from a pure electron-positron pair plasma to mixed plasmas containing electrons, positrons, and protons, or even to hadronic jets made up of electrons and protons only. To investigate this aspect, we present results from three-dimensional simulations of low-power, supersonic, magnetized jets at kiloparsec scales in a magnetohydrodynamic framework. By varying the plasma composition, we show its impact on jet stability and on the development of diffuse structures typical of core-brightened FR type I sources. Our results indicate that the growth of non-axisymmetric instabilities plays a key role in disrupting the jet head.

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arXiv2026

Outflow Behavior from the Transonic Advective Disks: A Hydrodynamical Simulation Study

We investigate the properties of outflows from the transonic advective accretion disk using hydrodynamical numerical simulations. We consider two different disk temperatures with an order-of-magnitude difference. For the hotter disk, we adopt initial conditions for velocity, specific angular momentum, and temperature from analytical solutions. In the colder disk case, the velocity and angular momentum profiles are kept identical, while an order of magnitude reduction in the temperature. The simulations are performed in the presence of viscosity and radiative cooling, considering bremsstrahlung and synchrotron processes. In both disk models, the outflow rate increases with viscosity. We also examine the poloidal velocity structures for both cases. We analyze the influence of viscosity on the mass flux-weighted energy and momentum fluxes of the outflows. Our results show that both energy and momentum fluxes increase with higher viscosity and may play a significant role in accretion feedback mechanisms.

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arXiv2026

QPOs from the Viscous Transonic Accretion Flow Around a Spinning Black Hole

We investigate the dynamics of transonic advective accretion flows around spinning black holes in the presence of viscosity. The spacetime of a Kerr black hole is approximated using a pseudo-potential. We study viscously driven shock oscillations over a range of black hole spin parameters. Our results show that the frequency range of quasi-periodic oscillations (QPOs) obtained from the power density spectra depends strongly on the black hole spin. Low-spin systems predominantly exhibit low-frequency QPOs, whereas rapidly rotating black holes (greater than 0.9 Kerr parameter) produce QPOs spanning a broad range from low to high frequencies, comparable to those observed in black hole X-ray binaries. We further obtain a correlation between the QPO frequency and the power-law photon index by computing the spectrum for a 10 solar mass black hole.

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arXiv2026

How Plasma Properties of the Fanaroff-Riley Jet can Shape its Morphology

Extragalactic jets are broadly classified into two categories based on radio observations: core-brightened jets, known as Fanaroff-Riley Type I (FR I), and edge-brightened jets, classified as Type II (FR II). This FR dichotomy may arise due to variation in the ambient medium and/or the properties of the jet itself, such as injection speed, temperature, composition, magnetization, etc. To investigate this, we perform large-scale three-dimensional magnetohydrodynamic (3D-MHD) simulations of low-power, supersonic jets extending to kiloparsec scales. We inject a jet beam carrying an initially toroidal magnetic field into a denser, unmagnetized, and stratified ambient medium through a cylindrical nozzle. Our simulations explore jets with varying injection parameters to investigate their impact on morphology and emission properties. Furthermore, we examine jets with significantly different plasma compositions, such as hadronic and mixed electron-positron-proton configurations, to study the conditions that may drive transitions between FR I and FR II morphologies. We find that, under the same injection parameters, mixed plasma composition jets tend to evolve into FR I structures. In contrast, electron-proton jets exhibit a transition between FR I and FR II morphologies at different stages of their evolution.

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arXiv2025

Dynamical properties of oscillating, viscous, transonic accretion disks around black holes

We investigate the time evolution of sub-Keplerian transonic accretion flow onto a non-rotating black hole using axisymmetric viscous hydrodynamic simulations. We simulate the accretion flow using boundary values from semi-analytical analysis and set up three different models. Two of the models do not predict accretion shocks from the semi-analytic analysis, while one of them does. We also consider radiative cooling along with viscosity in the simulation. Our two-dimensional simulation deviated from the one-dimensional semi-analytical solution and admitted shocks in all three models. Viscous dissipation tends to push the shock front outward, and radiative cooling will push it in. Additionally, gravity is attractive. Depending on the competing strengths of all three processes, it may trigger shock oscillation. Different rates of angular-momentum transport in various layers may trigger eddies, which will enhance the shock oscillation. We show that any simple power law cannot approximate these solutions. We find that hot and higher angular-momentum flow requires higher viscosity to produce oscillatory shocks. From the temporal variation of the luminosity, shock oscillations generate QPOs in the range of sub-Hertz to a few Hertz frequencies if a ten solar mass black hole is assumed.

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arXiv2025

Astrophysics with Compact Objects: An Indian Perspective, Present Status and Future Vision

Astrophysical compact objects, viz., white dwarfs, neutron stars, and black holes, are the remnants of stellar deaths at the end of their life cycles. They are ideal testbeds for various fundamental physical processes under extreme conditions that are unique in nature. Observational radio astronomy with uGMRT and OORT facilities has led to several important breakthroughs in studies of different kinds of pulsars and their emission mechanisms. On the other hand, accretion processes around compact objects are at the core of Indian astronomy research. In this context, AstroSat mission revolutionized spectro-temporal observations and measurements of accretion phenomena, quasi-periodic oscillations, and jet behaviour in binary systems hosting compact objects. Moreover, recently launched XPoSat mission is set to provide an impetus to these high-energy phenomena around compact objects by enabling us to conduct polarization measurements in the X-ray band. Further, during the past decade, numerous gravitational wave signals have been observed from coalescing black holes and neutron stars in binary systems. Recent simultaneous observation of the GW170817 event in both gravitational waves and electromagnetic channels has ushered in the era of multi-messenger astronomy. In the future, synergistic efforts among several world-class observational facilities, e.g., LIGO-India, SKA, TMT, etc., within the Indian astrophysics community will provide a significant boost to achieve several key science goals that have been delineated here. In general, this article plans to highlight scientific projects being pursued across Indian institutions in this field, the scientific challenges that this community would be focusing on, and the opportunities in the coming decade. Finally, we have also mentioned the required resources, both in the form of infrastructural and human resources.

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arXiv2024

On Disk Formation around Isolated Black Holes via Stream Accretion

We investigate accretion onto an isolated black hole from uniform winds. If the winds are directed towards the black hole, then the accretion process can be well described by the classical Bondi-Hoyle Lyttleton or BHL accretion. If the wind is not directed towards the black hole and flows past it, then a smaller fraction of the flow can be attracted by the black hole, and this type of accretion cannot be described by the classical BHL, and we coin the second kind as the lateral BHL. We show that the classical BHL cannot form an accretion disk, while lateral BHL can form transient accretion disks. To describe the thermodynamics of the flow, we have used a variable adiabatic index equation of state which depends on the temperature of the flow as well as the composition of the gas. We show that the electron-proton gas forms an accretion disk, which disappears and forms a shock cone, only to form the disk again at a later time, while for flows with less protons, the accretion disk, once lost, does not reappear again. Only when the flow is pair-dominated does it form a persistent accretion disk. We also show that a shock cone is less luminous than the accretion disk.

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arXiv2024

Radiative acceleration of relativistic jets from accretion discs around black holes

Matter falling onto black holes, {also called} accretion discs, emit intense high-energy radiation. Accretion discs during {hard to hard intermediate} spectral states also emit bipolar outflows. Radiation drag was supposed to impose the upper limit on the terminal speed. It was later shown that a radiation field around an advective accretion disc imposes no upper limit on speed, about a few hundred of Schwarzschild radius from the disc surface. We {study radiatively driven electron-proton and electron-positron jets, for gemeotrically thick and slim transonic discs} by using numerical simulation. We show that pair-dominated jets can reach ultra-relativistic speeds by radiation driving. We also discuss at what limits radiative acceleration may fail.

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arXiv2024

Numerical Simulation of Radiatively driven Transonic Relativistic Jets

We perform the numerical simulations of axisymmetric, relativistic, optically thin jets under the influence of the radiation field of an accretion disk. We show that starting from a very low injection velocity at the base, jets can be accelerated to relativistic terminal speeds when traveling through the radiation field. The jet gains momentum through the interaction with the radiation field. We use a relativistic equation of state for multi-species plasma, which self-consistently calculates the adiabatic index for the jet material. All the jet solutions obtained are transonic in nature. In addition to the acceleration of the jet to relativistic speeds, our results show that the radiation field also acts as a collimating agent. The jets remain well collimated under the effect of radiation pressure. We also show that if the jet starts with a rotational velocity, the radiation field will reduce the angular momentum of the jet beam.

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arXiv2024

Oscillating shocks in the transonic, viscous, variable $Γ$, accretion flows around black holes

We investigate the time evolution of the transonic-viscous accretion flow around a non-rotating black hole. The input parameters used for the simulation are obtained from semi-analytical solutions. This code is based on the TVD routine and correctly handles the angular momentum transport due to viscosity. The thermodynamic properties of the flow are described by a variable adiabatic index equation of state. We regenerate the inviscid and viscous steady-state solutions, including shocks, using the simulation code and compare them with the semi-analytical solutions. The angular momentum piles up across a shock due to shock-jump conditions and viscous transport of angular momentum. This will push the shock-front outward and can result in shock oscillation or a complete destabilization of shock. We study how shocks behave in the presence of viscosity. As the viscosity parameter ($α$) crosses a critical value, the previously steady shock becomes time-dependent, eventually leading to oscillations. The value of this critical viscosity depends on the injection angular momentum ($\λ_{ou}$) and the specific energy ($ε$). We estimated the posteriori bremsstrahlung and synchrotron cooling, and the net radiative output also oscillates with the frequency of the shock. We also study the variation of frequency, amplitude, and mean position of oscillation with $α$. Considering a black hole with a mass of $10M_{\odot}$, we observed that the power spectrum exhibits a prominent peak at the fundamental frequency of a few to about tens of Hz, accompanied by multiple harmonics. This characteristic is frequently observed in numerous accreting black hole candidates.

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arXiv2023

Two-temperature accretion flows around strongly magnetized stars and their spectral analysis

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.

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arXiv2023

The Morphology and Dynamics of Relativistic Jets with Relativistic Equation of State

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.

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arXiv2022

Shocks in radiatively driven time dependent, relativistic jets around black holes

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.

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arXiv2021

Radiatively driven, time dependent bipolar outflows

We study the radiatively driven fluid jets around a non-rotating black hole. The radiation arising from the inner compact corona and outer sub-Keplerian part of the disc accelerates the jets. We obtain the steady state, semi-analytical, radiatively driven outflow solutions. The thermodynamics of the outflow is described by a variable adiabatic index equation of state. We develop a TVD routine to investigate the time dependent behaviour of the radiatively driven bipolar outflow. We inject with flow variables from the steady state outflow solutions in the TVD code and allow the code to settle into steady state and match the numerical results with the steady state solution. The radiation arising out of the accretion disc can provide a wide range of jet solutions, depending upon parameters like the intensity of disc, location of the inner corona etc. We induce the time dependence of the radiation field by inducing oscillation of the inner corona of the accretion disc. The radiation field then makes the bipolar outflow time dependent. We show that a non-steady radiation field arising out of disc oscillations can generate the internal shocks closer to the jet base. Depending on the disc geometry, there might be transient shocks in the jet and there might be multiple non-stationary shocks in the jet, which are of much interest in jet physics.

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arXiv2021

A Simulation Study of Ultra-Relativistic Jets -- I. A New Code for Relativistic Hydrodynamics

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).

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arXiv2021

Exact solution of one dimensional relativistic jet with relativistic equation of state

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.

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