arXiv subjects

Prakriti Pal Choudhury

Publications and source records attributed to Prakriti Pal Choudhury.

10 recordsLinked to original sources

arXiv2026

Collisionless whistler heat-flux instability in ultra-high-$β$ plasmas

Kinetic instabilities, notably the whistler heat-flux instability (WHFI), are known to suppress thermal transport significantly in the moderate- to high-$β$ plasmas relevant to many astrophysical systems. This paper explores WHFI-regulated heat transport in a new regime: ultra-high-$β$ plasmas with $β_{e} \gtrsim L_{\mathrm{T}}/ρ_e$. Extrapolating previous theories of the WHFI to ultra-high-$β$ plasmas, we propose that the magnetic energy in unstable whistler fluctuations becomes comparable to that of the background magnetic field at saturation. We corroborate this hypothesis using 1D3V and 2D3V kinetic simulations using the particle-in-cell code OSIRIS. We find that, in ultra-high-$β$ plasmas, the heat flux is localised and no longer regulated primarily by resonant pitch-angle scattering of electrons; instead, thermal energy is transported predominantly by advection at the whistler phase velocity. Heat-flux suppression is observed in 1D3V and 2D3V simulations; however, we show that the saturation of the WHFI and the regulation of heat flux are sensitive to dimensionality in the ultra-high-$β$ regime. The amplitude and phase velocity of the heat-flux-regulating whistler waves scale differently with $β_e$, yielding parallel heat fluxes, normalised to the free-streaming value, of $q_{e\parallel} / q_\mathrm{fs} \approx 4.7 β_{e}^{-1}$ and $q_{e\parallel} / q_\mathrm{fs} \approx 0.3 β_{e}^{-1/2}$ in 2D3V and 1D3V simulations, respectively. We perform 2D3V simulations with background magnetic fields inclined to the temperature gradient, showing perpendicular heat transport remains negligible. We develop a heuristic theory from kinetic equations that explains these phenomena. Our work extends our understanding of how the WHFI modifies thermal transport to regimes applicable to high-energy-density physics and the reionised intergalactic medium.

physics.plasm-ph↗

arXiv2026

Latent thermal instability

Multiscale temperature fluctuations are abundant in the intracluster medium (ICM) outside of galaxy cluster cores ($\sim 100~{\rm kpc}$). Their origin is often attributed to turbulent stirring by subhalos or accreting baryons crossing the virial radius. However, their apparent resistance to mixing and thermal conduction in a collisional medium has not been explained. We propose a new mechanism by which steady-state temperature fluctuations can form and persist outside the cluster core. Local thermal instability, or Field instability, is used to explain filamentary condensates in cluster cores but is usually dismissed outside them because thermal conduction should suppress instability. In weakly collisional or collisionless plasmas, however, thermal conduction can be anomalously suppressed by heat-flux-driven plasma instabilities triggered in presence of a local magnetic field, leading to two effects: (i) condensates form in a new parameter regime that overlaps with conditions outside the core, and (ii) condensates reach a steady state as in the hydrodynamic limit. This extends the regime of instability-driven fluctuations to over $\gtrsim50\%$ (depending on hot plasma temperature) of the cluster. We use one-dimensional hydrodynamic simulations of condensates to test our analytical ideas.

astro-ph.GA↗

arXiv2026

Modeling transport in weakly collisional plasmas using thermodynamic forcing

How momentum, energy, and magnetic fields are transported in the presence of macroscopic gradients is a fundamental question in plasma physics. Answering this question is especially challenging for weakly collisional, magnetized plasmas, where macroscopic gradients influence the plasma's microphysical structure. In this paper, we introduce thermodynamic forcing, a new method for systematically modeling how macroscopic gradients in magnetized or unmagnetized plasmas shape the distribution functions of constituent particles. In this method, we propose to apply an anomalous force to those particles inducing the anisotropy that would naturally emerge due to macroscopic gradients in weakly collisional plasmas in which thermal pressure is much larger than magnetic pressure. We implement thermodynamic forcing in particle-in-cell (TF-PIC) simulations using a modified Vay particle pusher and validate it against analytic solutions of the equations of motion. We then carry out a series of simulations of electron-proton plasmas with periodic boundary conditions using TF-PIC. First, we confirm that the properties of two electron-scale kinetic instabilities - one driven by a temperature gradient and the other by bulk-velocity gradient - are consistent with previous results. Then, we demonstrate that in the presence of both macroscopic gradients, heat-flux saturation is mediated by the bulk-velocity-gradient-driven electron firehose instability rather than the temperature-gradient-driven whistler instability. This suggests that saturation mechanisms may differ from our current understanding in the presence of multiple free energy sources. This work enables, for the first time, systematic and self-consistent transport modeling in weakly collisional plasmas, with broad applications in astrophysics, laser-plasma physics, and inertial confinement fusion.

physics.plasm-ph↗

arXiv2026

The Advanced X-ray Imaging Satellite (AXIS) Community Science Book

The AXIS Community Science Book represents the collective effort of 592 scientists worldwide to define the transformative science enabled by the Advanced X-ray Imaging Satellite (AXIS), a next-generation X-ray mission selected by NASA's Astrophysics Probe Program for Phase A study. AXIS will advance the legacy of high-angular-resolution X-ray astronomy with ~1.5'' imaging over a wide 24' field of view and an order of magnitude greater collecting area than Chandra in the 0.3-12 keV band. Combining sharp imaging, high throughput, and rapid response capabilities, AXIS will open new windows on virtually every aspect of modern astrophysics, exploring the birth and growth of supermassive black holes, the feedback processes that shape galaxies, the life cycles of stars and exoplanet environments, and the nature of compact stellar remnants, supernova remnants, and explosive transients. This book compiles 138 community-contributed science cases developed by five Science Working Groups focused on AGN and supermassive black holes, galaxy evolution and feedback, compact objects and supernova remnants, stellar physics and exoplanets, and time-domain and multi-messenger astrophysics. Together, these studies establish the scientific foundation for next-generation X-ray exploration in the 2030s and highlight strong synergies with facilities of the 2030s, such as JWST, Roman, Rubin/LSST, SKA, ALMA, ngVLA, and next-generation gravitational-wave and neutrino networks.

astro-ph.HE↗

arXiv2025

Cold fronts in galaxy clusters I: A case for the large-scale global eigen modes in unmagnetized and weakly magnetized cluster core

Galaxy clusters show large-scale azimuthal X-ray surface brightness fluctuations known as cold fronts. Cold fronts are argued to originate due to sloshing driven by sub-halo passage at close proximity to the cluster center. While this causes large-scale perturbations, the physical mechanisms that can sustain spiral density structures are not clear. In this work, we explore whether long wavelength thermal instability is an explanation for cold front formation in a cluster core which is perturbed by sub-halos or AGN activity. Using global linear perturbation analysis, we show that unstable internal gravity waves form large-scale three-dimensional spirals, akin to observed cold fronts. We explore if the presence of magnetic field (along spherical $\hatϕ$) may support such structures (by suppressing small scale Kelvin-Helmholtz modes) or disrupt them (by promoting additional thermal instability). We find that latter happens at shorter wavelengths and above characteristic Brunt Väisälä frequency ($>N_{\rm BV}$). Our work implies that large-scale spirals are sustained over a long timescale ($>N^{-1}_{\rm BV}$) even in presence of aligned magnetic fields that is otherwise supportive against mixing at the interface. Secondly, short-wavelength (but relatively longer along the field) unstable compressive modes may form within or in the vicinity of such spirals. The instability is an overstable slow wave, and grows in 2D at timescales $\gtrsim 2-3$ times longer than the spiral growth timescale (via thermal instability). Thus this instability cannot destroy the large scale coherence.

astro-ph.GA↗

arXiv2022

Acoustic waves and g-mode turbulence as energy carriers in a viscous intracluster medium

Many recent works on the observed galaxy clusters in the X-rays highlight broadly two classes of exclusive energy carriers - sound waves and turbulence. In order to understand this dichotomy, we design an idealized three-dimensional hydrodynamic simulation of a cluster, to assess which of these carriers can dissipate energy in and around the core ($\gtrsim 100$ kpc) . Specifically, we explore how gentle (long-duration outbursts) and intermediate (shorter duration outbursts) feedback modes can function efficiently mediated by compressible (sound waves) and incompressible (g-modes/instabilities/turbulence) disturbances. Since g-modes are confined tightly to the central core, we attempt to maximise the flux of fast sound waves to distribute the feedback energy over a large distance. We find that the contribution to heat dissipation from sound and turbulence varies on the basis of the aforementioned feedback modes, namely: turbulence contributes relatively more than sound in the slow-piston regime and vice versa for the intermediate regime. For the first time in a 3D simulation, we show that up to $\lesssim 20\%$ (in some directions) of the injected power can be carried away by sound flux in the intermediate feedback but it reduces to $\lesssim 10 \%$ (in some directions) in the slow-piston regime. Lastly, we find that sound waves can be elusive if we deduce the equation-of-state (isobaric/isentropic) of the fluctuations from X-ray observations.

astro-ph.GA↗

arXiv2021

Shatter or not: role of temperature and metallicity in the evolution of thermal instability

We test how metallicity variation (a background gradient and fluctuations) affects the physics of local thermal instability using analytical calculations and idealized, high-resolution 1D hydrodynamic simulations. Although the cooling function ($Λ[T,Z]$) and the cooling time ($t_{\rm cool}$) depend on gas temperature and metallicity, we find that the growth rate of thermal instability is explicitly dependent only on the derivative of the cooling function relative to temperature ($\partial \ln Λ/\partial \ln T$) and not on the metallicity derivative ($\partial \ln Λ/ \partial \ln Z$). For most of $10^4~{\rm K} \lesssim T \lesssim 10^7~{\rm K}$, both the isobaric and isochoric modes (occurring at scales smaller and larger than the sonic length covered in a cooling time [$c_s t_{\rm cool}$], respectively) grow linearly, and at higher temperatures ($\gtrsim 10^7~{\rm K}$) the isochoric modes are stable. We show that even the nonlinear evolution depends on whether the isochoric modes are linearly stable or unstable. For the stable isochoric modes, we observe the growth of small-scale isobaric modes but this is distinct from the nonlinear fragmentation of a dense cooling region. For unstable isochoric perturbations we do not observe large density perturbations at small scales. While very small clouds ($\sim {\rm min}[c_st_{\rm cool}]$) form in the transient state of nonlinear evolution of the stable isochoric thermal instability, most of them merge eventually.

astro-ph.GA↗

arXiv2019

Multiphase gas in the circumgalactic medium: relative role of $t_{\rm cool}/t_{\rm ff}$ and density fluctuations

We perform a suite of simulations with realistic gravity and thermal balance in shells to quantify the role of the ratio of cooling time to the free-fall time ($t_{\rm cool}/t_{\rm ff}$) and the amplitude of density perturbations ($δρ/ρ$) in the production of multiphase gas in the circumgalactic medium (CGM). Previous idealized simulations, focussing on small amplitude perturbations in the intracluster medium (ICM), found that cold gas can condense out of the hot ICM in global thermal balance when the background $t_{\rm cool}/t_{\rm ff} \lesssim 10$. Recent observations suggest the presence of cold gas even when the background profiles have somewhat large values of ${t_{\rm cool}/t_{\rm ff}}$. This partly motivates a better understanding of additional factors such as large density perturbations that can enhance the propensity for cooling and condensation even when the background ${t_{\rm cool}/t_{\rm ff}}$ is high. Such large density contrasts can be seeded by galaxy wakes or dense cosmological filaments. From our simulations, we introduce a condensation curve in the $(δρ/ρ)$ - min$(t_{\rm cool}/t_{\rm ff})$ space, that defines the threshold for condensation of multiphase gas in the CGM. We show that this condensation curve corresponds to ${(t_{\rm cool}/t_{\rm ff})}_{\rm blob} \lesssim 10$ applied to the overdense blob instead of the background for which $t_{\rm cool}/t_{\rm ff}$ can be higher. We also study the modification in the condensation curve by varying entropy stratification. Steeper (positive) entropy gradients shift the condensation curve to higher amplitudes of perturbations (i.e., make condensation difficult). A constant entropy core, applicable to the CGM in smaller halos, shows condensation over a larger range of radii as compared to the steeper entropy profiles in the ICM.

astro-ph.GA↗

arXiv2019

A 1-dimensional hydrodynamic model for accretion, cooling and heating of gas in dark matter halos from $z=6$ to $z=0$

We study an idealized 1D model for the evolution of hot gas in dark matter halos for redshifts $z=[0,6]$. We introduce a numerical setup incorporating cosmological accretion of gas, along with the growth of the halo, based on the Van den Bosch model for the average growth of halos as a function of cosmic time. We evolve one-dimensional Lagrangian shells with radiative cooling of the gas and heating due to feedback from the gas cooling and moving in toward the center. A simple Bondi accretion model on to a central black hole is used to include feedback heating. The setup captures some of the key characteristics of spherically symmetric accretion onto the halos: formation of virial shocks slightly outside $r_{200}$ and long-term thermal balance in the form of cooling and heating cycles. The gas density outside our initial halos at $z=6$ is constrained by requiring that the baryon fraction within the virial radius for non-radiative evolution be equal to the universal value at almost all times. The total mass in the cold phase (taken to be $\sim 10^4$ K) within $40$ kpc is tracked as a function of the halo mass and redshift. We compare the evolution of the cold gas mass to the observed stellar-mass versus halo mass relations, following which, we can constrain the feedback energy required for different halo masses and redshifts. We also compare and match the hot gas density and temperature profiles for our most massive halo to those of clusters observed upto redshift $2$. Our model is thus an improvement over the semi-analytic models in which isothermal condition and $ρ\propto r^{-2}$ are assumed.

astro-ph.GA↗

arXiv2016

Cold gas in cluster cores: Global stability analysis and non-linear simulations of thermal instability

We perform global linear stability analysis and idealized numerical simulations in global thermal balance to understand the condensation of cold gas from hot/virial atmospheres (coronae), in particular the intracluster medium (ICM). We pay particular attention to geometry (e.g., spherical versus plane-parallel) and the nature of the gravitational potential. Global linear analysis gives a similar value for the fastest growing thermal instability modes in spherical and Cartesian geometries. Simulations and observations suggest that cooling in halos critically depends on the ratio of the cooling time to the free-fall time ($t_{cool}/t_{ff}$). Extended cold gas condenses out of the ICM only if this ratio is smaller than a threshold value close to 10. Previous works highlighted the difference between the nature of cold gas condensation in spherical and plane-parallel atmospheres; namely, cold gas condensation appeared easier in spherical atmospheres. This apparent difference due to geometry arises because the previous plane-parallel simulations focussed on {\em in situ} condensation of multiphase gas but spherical simulations studied condensation {\em anywhere} in the box. Unlike previous claims, our nonlinear simulations show that there are only minor differences in cold gas condensation, either in situ or anywhere, for different geometries. The amount of cold gas condensing depends on the shape of the gravitational potential well; gas has more time to condense if gravitational acceleration decreases toward the center. In our idealized simulations with heating balancing cooling in each layer, there can be significant mass/energy/momentum transfer across layers that can trigger condensation and drive $t_{cool}/t_{ff}$ far beyond the critical value close to 10. Triggered condensation is very prominent in plane-parallel simulations, in which a large amount of cold gas condenses out.

astro-ph.GA↗