arXiv subjects

Prasun Dhang

Publications and source records attributed to Prasun Dhang.

12 recordsLinked to original sources

arXiv2025

The Impact of Plasma Angular Momentum on Magnetically Arrested Flows and Relativistic Jets in Hot Accretion Flows Around Black Holes

In certain scenarios, the accreted angular momentum of plasma onto a black hole could be low; however, how the accretion dynamics depend on the angular momentum content of the plasma is still not fully understood. We present three-dimensional, general relativistic magnetohydrodynamic simulations of low angular momentum accretion flows around rapidly spinning black holes (with spin $a = +0.9$). The initial condition is a Fishbone-Moncrief (FM) torus threaded by a large amount of poloidal magnetic flux, where the angular velocity is a fraction $f$ of the standard value. For $f = 0$, the accretion flow becomes magnetically arrested and launches relativistic jets but only for a very short duration. After that, free-falling plasma breaks through the magnetic barrier, loading the jet with mass and destroying the jet-disk structure. Meanwhile, magnetic flux is lost via giant, asymmetrical magnetic bubbles that float away from the black hole. The accretion then exits the magnetically arrested state. For $f = 0.1$, the dimensionless magnetic flux threading the black hole oscillates quasi-periodically. The jet-disk structure shows concurrent revival and destruction while the gas outflow efficiency at the event horizon changes accordingly. For $f \geq 0.3$, we find that the dynamical behavior of the system starts to approach that of a standard accreting FM torus. Our results thus suggest that the accreted angular momentum is an important parameter that governs the maintenance of a magnetically arrested flow and launching of relativistic jets around black holes.

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arXiv2024

Energy Extraction from a Black Hole by a Strongly Magnetized Thin Accretion Disk

The presence of a strong, large-scale magnetic field in an accretion flow leads to extraction of the rotational energy of the black hole (BH) through the Blandford-Znajek (BZ) process, believed to power relativistic jets in various astrophysical sources. We study rotational energy extraction from a BH surrounded by a highly magnetized thin disk by performing a set of 3D global GRMHD simulations. We find that the saturated flux threading the BH has a weaker dependence on BH spin, compared to highly magnetized hot (geometrically thick) accretion flows. Also, we find that only a fraction ($10-70$ per cent) of the extracted BZ power is channeled into the jet, depending on the spin parameter. The remaining energy is potentially used to launch winds or contribute to the radiative output of the disk or corona. Our simulations reveal that the presence of a strong magnetic field enhances the radiative efficiency of the disk, making it more luminous than its weakly magnetized counterpart or the standard disk model. We attribute this excess luminosity primarily to the enhanced magnetic dissipation in the intra-ISCO region. Our findings have implications for understanding X-ray corona formation and black hole spin measurements, and interpreting black hole transient phenomena.

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arXiv2024

Shedding light on the MRI driven dynamo in a stratified shearing box

We study the magneto-rotational instability (MRI) driven dynamo in a geometrically thin disc ($H/R\ll 1$) using stratified zero net flux (ZNF) shearing box simulations. We find that mean fields and EMFs oscillate with a primary frequency $f_{\rm dyn} = 0.017$ ($\approx 9$ orbital period), but also have higher harmonics at $3f_{\rm dyn}$. Correspondingly, the current helicity, has two frequencies $2f_{\rm dyn}$ and $4f_{\rm dyn}$ respectively, which appear to be the beat frequencies of mean fields and EMFs as expected from the magnetic helicity density evolution equation. Further, we adopt a novel inversion algorithm called the `Iterative Removal Of Sources' (IROS), to extract the turbulent dynamo coefficients in the mean-field closure using the mean magnetic fields and EMFs obtained from the shearing box simulation. We show that an $α-$effect ($α_{yy}$) is predominantly responsible for the creation of the poloidal field from the toroidal field, while shear generates back a toroidal field from the poloidal field; indicating that an $α-Ω$-type dynamo is operative in MRI-driven accretion discs. We also find that both strong outflow ($\bar{v}_z$) and turbulent pumping ($γ_z$ ) transport mean fields away from the mid-plane. Instead of turbulent diffusivity, they are the principal sink terms in the mean magnetic energy evolution equation. We find encouraging evidence that a generative helicity flux is responsible for the effective $α$-effect. Finally, we point out potential limitations of horizontal ($x-y$) averaging in defining the `mean' on the extraction of dynamo coefficients and their physical interpretations.

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arXiv2024

Iterative removal of sources to model the turbulent electromotive force

We describe a novel method to compute the components of dynamo tensors from direct magnetohydrodynamic (MHD) simulations. Our method relies upon an extension and generalisation of the standard Högbom CLEAN algorithm widely used in radio astronomy to systematically remove the impact of the strongest beams onto the corresponding image. This generalisation, called the Iterative Removal of Sources (IROS) method, has been adopted here to model the turbulent electromotive force (EMF) in terms of the mean magnetic fields and currents. Analogous to the CLEAN algorithm, IROS treats the time series of the mean magnetic field and current as beams that convolve with the dynamo coefficients which are treated as (clean) images to produce the EMF time series (the dirty image). We apply this method to MHD simulations of galactic dynamos, to which we have previously employed other methods of computing dynamo coefficients such as the test-field method, the regression method, as well as local and non-local versions of the singular value decomposition (SVD) method. We show that our new method reliably recovers the dynamo coefficients from the MHD simulations. It also allows priors on the dynamo coefficients to be incorporated easily during the inversion, unlike in earlier methods. Moreover, using synthetic data, we demonstrate that it may serve as a viable post-processing tool in determining the dynamo coefficients, even when the power of additive noise to the EMF is twice as much the actual EMF.

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arXiv2023

Magnetic Flux Transport in Radiatively Inefficient Accretion Flows and the Pathway towards a Magnetically Arrested Disk

Large-scale magnetic fields play a vital role in determining the angular momentum transport and generating jets/outflows in the accreting systems, yet their origin remains poorly understood. We focus on radiatively inefficient accretion flows (RIAFs) around the black holes (BHs), and conduct three-dimensional general-relativistic magnetohydrodynamic (GRMHD) simulations using the Athena++ code. We first re-confirm that the MRI dynamo in the RIAF alone does not spontaneously form a magnetically arrested disk (MAD), conducive for strong jet formation. We next investigate the other possibility, where the large-scale magnetic fields are advected inward from external sources (e.g. the companion star in X-ray binaries, magnetized ambient medium in AGNs). Although the actual configuration of the external fields could be complex and uncertain, they are likely to be closed. As a first study, we treat them as closed field loops of different sizes, shapes and field strengths. Unlike earlier studies of flux transport, where magnetic flux is injected in the initial laminar flow, we injected the magnetic field loops in the quasi-stationary turbulent RIAF in inflow equilibrium and followed their evolution. We found that a substantial fraction ($\sim15\%-40\%$) of the flux injected at the large radii reaches the BH with a weak dependence on the loop parameters except when the loops are injected at high latitudes, away from the mid-plane. Relatively high efficiency of flux transport observed in our study hints that a MAD might be formed relatively easily close to the BH, provided that a source of the large-scale field exists at the larger radii.

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arXiv2022

Magnetic field structure in the vicinity of a super-massive black hole in low luminosity galaxies: the case of Sgr A*

Observations of $\rm SgrA^*$ have provided a lot of insight on low-luminosity accretion, with a handful of bright flares accompanied with orbital motion close to the horizon. It has been proposed that gas supply comes from stellar winds in the neighborhood of the supermassive black hole. We here argue that the flow at the vicinity of the black hole has a low magnetization and a structure of alternating polarity totally dictated by the well studied and long-ago proposed MRI turbulent process. This can be the case, provided that in larger distances from the black hole magnetic diffusivity is dominant and thus the magnetic field will never reach equipartition values. For $\rm SgrA^*$, we show the immediate consequences of this specific magnetic field geometry, which are: (i) an intermittent flow that passes from quiescent states to flaring activity, (ii) no quasi-steady-state jet, (iii) no possibility of a magnetically arrested configuration. Moreover a further distinctive feature of this geometry is the intense magnetic reconnection events, occurring as layers of opposite magnetic polarity are accreted, in the vicinity of the black hole. Finally, we argue that the absence of a jet structure in such case will be a smoking gun in 43 \& 86 GHz observations.

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arXiv2021

Advective accretion onto a non-spherical accretor in white dwarf and neutron star binaries: a new scenario of shock formation

Numerous studies on hydrodynamics of the Keplerian as well as the sub-Keplerian accretion disc around a compact object (e.g., white dwarf (WD), neutron star (NS), or a black hole (BH)) attempted to explain the observed UV, soft and hard X-ray spectra. Although, when the compact object (e.g., a WD or an NS) has a finite surface, its rapid rotation, the stellar magnetic field could cause deformation of the spherical symmetry. Earlier studies for Keplerian disc showed that a deviation from the spherical symmetry of the compact object could affect the observed light curve and spectra at high frequencies. Here, we have explored the effect of the non-spherical nature of a compact object on the hydrodynamics of an optically thin, geometrically thick sub-Keplerian advective flow. We find that due to non-spherical shape of the central accretor, there is a possibility to trigger Rankine-Hugoniot shock in the sub-Keplerian advective flow close to the accretor without considering any general relativistic effect or presence of the hard surface of the star. Our results are more relevant for accretion onto WD as hardly any general relativistic effect will come in the picture. We propose that some observational features e.g., high significance of fitting the spectra with multi-temperature plasma models rather than single temperature models, and variable efficiency of X-ray emission (X-ray luminosity in comparison with optical and UV luminosity of the disk) in nonmagnetic cataclysmic variables can be explained by the presence of a shock in the sub-Keplerian advective flow.

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arXiv2020

Characterising the Dynamo in a Radiatively Inefficient Accretion Flow

We explore the MRI driven dynamo in a radiatively inefficient accretion flow (RIAF) using the mean field dynamo paradigm. Using singular value decomposition (SVD) we obtain the least squares fitting dynamo coefficients $α$ and $γ$ by comparing the time series of the turbulent electromotive force and the mean magnetic field. Our study is the first one to show the poloidal distribution of these dynamo coefficients in global accretion flow simulations. Surprisingly, we obtain a high value of the turbulent pumping coefficient $γ$ which transports the mean magnetic flux radially outward. This would have implications for the launching of magnetised jets which are produced efficiently in presence a large-scale poloidal magnetic field close to the compact object. We present a scenario of a truncated disc beyond the RIAF where a large scale dynamo-generated poloidal magnetic field can aid jet-launching close to the black hole. Magnitude of all the calculated coefficients decreases with radius. Meridional variations of $α_{ϕϕ}$, responsible for toroidal to poloidal field conversion, is very similar to that found in shearing box simulations using the `test field' (TF) method. By estimating the relative importance of $α$-effect and shear, we conclude that the MRI driven large-scale dynamo, which operates at high latitudes beyond a disc scale height, is essentially of the $α-Ω$ type.

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arXiv2018

3D global simulations of RIAFs: convergence, effects of azimuthal extent and dynamo

We study the long-term evolution of non-radiative geometrically thick ($H/R\approx 0.5$) accretion flows using 3D global ideal MHD simulations and a pseudo-Newtonian gravity. We find that resolving the scale height with 42 grid points is adequate to obtain convergence with the product of quality factors $\langle \langle Q_θ \rangle \rangle \langle \langle Q_ϕ \rangle \rangle \geq 300$ and magnetic tilt angle $θ_B \sim 13^{\circ}-14^{\circ}$. Like previous global isothermal thin disk simulations, we find stronger mean magnetic fields for the restricted azimuthal domains. Imposing periodic boundary conditions with the azimuthal extent smaller than $2π$ make the turbulent field at low $m$ appear as a mean field in the runs with smaller azimuthal extent. But unlike previous works, we do not find a monotonic trend in turbulence with the azimuthal extent. We conclude that the minimum azimuthal extent should be $\geq π/2$ to capture the flow structure, but a full $2 π$ extent is necessary to study the dynamo. We find an intermittent dynamo cycle, with $α$-quenching playing an important role in the nonlinear saturated state. Unlike previous local studies, we find almost similar values of kinetic and magnetic $α$-s, giving rise to an irregular distribution of dynamo-$α$. The effects of dynamical quenching are shown explicitly for the first time in global simulations of accretion flows.

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arXiv2018

Magnetized SASI: its mechanism and possible connection to some QPOs in XRBs

The presence of a surface at the inner boundary, such as in a neutron star or a white dwarf, allows the existence of a standing shock in steady spherical accretion.The standing shock can become unstable in 2D or 3D; this is called the {\em standing accretion shock instability} (SASI).Two mechanisms -- advective-acoustic and purely acoustic -- have been proposed to explain SASI. Using axisymmetric hydrodynamic (HD) and magnetohydrodynamic (MHD) simulations, we find that the advective-acoustic mechanism better matches the observed oscillation timescales in our simulations. The global shock oscillations present in the accretion flow can explain many observed high frequency ($\gtrsim 100$ Hz) quasi-periodic oscillations (QPOs) in X-ray binaries (XRBs). The presence of a moderately strong magnetic field adds more features to the shock oscillation pattern, giving rise to low frequency modulation in the computed light curve. This low frequency modulation can be responsible for $\sim 100$ Hz QPOs (known as hHz QPOs). We propose that the appearance of hHz QPO determines the separation of twin peak QPOs of higher frequencies.

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arXiv2018

Correlating nonlinear properties with spectral states of RXTE data: Possible observational evidences for four different accretion modes around compact objects

By analyzing the time series of RXTE/PCA data, the nonlinear variabilities of compact sources have been repeatedly established. Depending on the variation in temporal classes, compact sources exhibit different nonlinear features. Sometimes they show low correlation/fractal dimension, but in other classes or intervals of time they exhibit stochastic nature. This could be because the accretion flow around a compact object is a nonlinear general relativistic system involving magnetohydrodynamics. However, the more conventional way of addressing a compact source is the analysis of its spectral state. Therefore, the question arises: What is the connection of nonlinearity to the underlying spectral properties of the flow when the nonlinear properties are related to the associated transport mechanisms describing the geometry of the flow? The present work is aimed at addressing this question. Based on the connection between observed spectral and nonlinear (time series) properties of two X-ray binaries: GRS 1915+105 and Sco X-1, we attempt to diagnose the underlying accretion modes of the sources in terms of known accretion classes, namely, Keplerian disc, slim disc, advection dominated accretion flow (ADAF) and general advective accretion flow (GAAF). We explore the possible transition of the sources from one accretion mode to others with time. We further argue that the accretion rate must play an important role in transition between these modes.

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arXiv2016

Spherical accretion: the influence of inner boundary and quasi-periodic oscillations

Bondi accretion assumes that there is a sink of mass at the center -- which in case of a black hole (BH) corresponds to the advection of matter across the event horizon. Other stars, such as a neutron star (NS), have surfaces and hence the infalling matter has to slow down at the surface. We study the initial value problem in which the matter distribution is uniform and at rest at $t=0$. We consider different inner boundary conditions for BHs and NSs: outflow boundary condition (mimicking mass sink at the center) valid for BHs; and {\em reflective} and steady-shock (allowing gas to cross the inner boundary at subsonic speeds) boundary conditions for NSs. We also obtain a similarity solution for cold accretion on to BHs and NSs. 1-D simulations show the formation of an outward propagating and a standing shock in NSs for reflective and steady-shock boundary conditions, respectively. Entropy is the highest at the bottom of the subsonic region for reflective boundary conditions. In 2-D this profile is convectively unstable. Using steady-shock inner boundary conditions, the flow is unstable to the standing accretion shock instability (SASI) in 2-D, which leads to global shock oscillations and may be responsible for quasi-periodic oscillations (QPOs) seen in the lightcurves of accreting systems. For steady accretion in the quiescent state, spherical accretion rate on to a NS can be suppressed by orders of magnitude compared to that on to a BH.

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