A seminar on computational astrophysics and numerical simulations was recently organized by Singularity: The Astronomy Club of IISER Kolkata on the 9th of October 2025 at MN Saha Auditorium, the speaker being Prof. Sudip Kr. Garain, Assistant Professor, Department of Physical Sciences and Centre of Excellence in Space Sciences India (CESSI). The event gathered students and scholars to listen to Prof. Garain talk about advanced computational techniques and their applications in understanding high energy astrophysical phenomena.
Professor Garain outlined extensive research with a primary focus on understanding the dynamics and radiative properties of matter surrounding high energy central objects like black holes. He detailed how he developed and utilized numerical simulation codes applying finite volume method algorithms for fluid dynamics alongside Monte Carlo based algorithms for radiative transfer. Applications discussed included the dynamical study of accretion flows onto compact objects, turbulence in astrophysics, magnetic reconnection in two fluid plasmas, and computational electrodynamics.
Observational data set the stage for these simulations. Accreting matter onto black holes produced detectable electromagnetic radiation across the spectrum from radio to high energy gamma rays alongside gravitational waves. Prof. Garain highlighted typical X-ray observations from astronomical instruments which captured highly variable light curves from black hole binaries. Spectral and temporal analyses of these observations revealed key components such as a disk blackbody and thermal Comptonization. Black hole binaries exhibited distinct spectral states typically transitioning between steady hard and steady soft states indicating the presence of an optically thick emitting disk and an optically thin Compton cloud. The immense energy radiated by these systems was driven primarily by the release of gravitational potential energy as matter spiraled toward the event horizon which vastly exceeded the energy released by nuclear fusion.
To model these complex astrophysical phenomena Prof. Garain employed various sets of governing equations ranging from ideal hydrodynamics and ideal magnetohydrodynamics to two fluid isothermal magnetohydrodynamics for partially ionized plasmas and relativistic two fluid electrodynamics. Furthermore general relativistic fluid dynamics equations were utilized to account for the extreme gravitational environments near black holes. These conservation equations were solved using the finite volume method where the computational domain was discretized into one, two, or three dimensional meshes. Given the massive computational requirements of three dimensional simulations the codes were highly parallelized using domain decomposition techniques allowing them to scale efficiently across thousands of cores on supercomputers.
The numerical simulations provided deep insights into the behavior of accretion disks. Three dimensional simulations of advective thick accretion disks onto non rotating black holes revealed the formation of non axisymmetric structures and density variations. The research also explored the generation of outflows and jets from shocked accretion flows. When magnetic fields were introduced into the simulations they were found to contribute significantly to collimating and accelerating the jets. Furthermore magnetic flux tubes aided in transporting angular momentum within the disk and advecting matter continuously brought frozen in magnetic field loops into the system. The simulations also captured the dynamic escape of magnetic flux from the accretion disk over time.
Professor also conducted three dimensional general relativistic hydrodynamics simulations comparing non rotating black holes with highly spinning black holes which possessed a spin parameter of zero point nine five. To understand the observational signatures of these simulated flows the fluid dynamics codes were coupled with radiative transfer codes based on Monte Carlo methods. This two step coupling created a self consistent radiation hydrodynamics code. The Monte Carlo approach was highly parallelizable as it tracked individual photon paths which were mutually exclusive. This technique successfully simulated the spectral and timing properties of the accretion flows reproducing features such as low frequency quasi periodic oscillations and demonstrating how the energy spectrum softened with an increase in the Keplerian disk accretion rate.
Finally the seminar outlined future and ongoing work aimed at expanding these computational capabilities. Planned developments in Monte Carlo based radiative transfer included ray tracing in background spacetime, tracking the polarization of individual photons, producing direct simulated images of thick accretion disks, and refining the self consistent coupling of radiation and fluid modules. Concurrently the development of the general relativistic fluid dynamics solver was projected to progress to include hydrodynamics and magnetohydrodynamics solvers on geodesic meshes as well as advanced three dimensional simulations of misaligned accretion disks.