Numerical Astrophysics

Numerical Astrophysics

The most powerful laboratory of relativity is in the vast Cosmos, where matter either hurl onto the compact objects like black holes and neutron stars, or flow away from them as relativistic jets. Neutron stars and white dwarfs are the 'dead bodies’ of lighter stars. Black holes are dead bodies of heavier stars, and the masses range from few to few tens of solar mass. However, at the centers of galaxies super massive black holes in the range of million to billion solar masses exist. These objects are observed from the radiations that we receive from the accreting matter. Interestingly, not all the matter that falls onto these object actually manage to do so, but a part of them eject as relativistic jets. These jets also interact with the ambient medium and radiate at lower frequencies. Even energetic events like GRBs are thought to be high energy radiation produced by such astrophysical jets from collapsing massive stars to black holes or merging binary neutron stars. In other words, investigations of these enigmatic objects can throw light on such diverse subject like accretion in X-ray binaries and AGNs to astrophysical jet morphology and studies in instabilities in relativistic plasma, GRBs etc.

A fluid is said to relativistic on the account of its bulk speed and if its thermal energy is comparable or greater than its rest mass energy. ARIES theory group spends a significant time studying relativistic fluid solving various Riemann problem etc. These exercise are the stepping stone of building better simulation codes. The extreme conditions like extremely high shock strengths, or very high compressibility, and the relativistic nature of these flows needs the development of novel numerical methods. Emphasis is put on high-resolution shock-capturing finite-volume schemes based on Riemann solvers. In order to trust the results of the numerical simulation code and avoid the pitfalls while carrying the simulations, the numerical solutions should be matched with known analytical solutions wherever available so the group also spends a significant amount of time to study and understand the analytical solutions thoroughly.

Comparison between the exact solutions (solid lines) and solutions obtained from TVD simulation code (open circles), for a one-dimensional relativistic jet. The density jumps in the first panel show the positions of reverse shock, contact discontinuity (jet-head), and forward shock (from left to right, respectively). The TVD code resolves shocks and contact discontinuity with fair accuracy. CR EoS with electron-proton composition has been used here. (2021, MNRAS, 502, 5227)
Simulation of QPO and jet generation from viscous accretion disc around black holes (2016, ApJ, 831, 33)

The ARIES theory group studies accretion disc models with relativistic equation of state to describe the thermodynamics of relativistic plasma that makes up the accretion disc. Viscous accretion disc around black holes has been studied both analytically and numerically. Numerical simulations show multiple colliding shocks in the disc which can be interpreted as the source of quasi periodic oscillation as a source of episodic bipolar outflows. They also obtained self-consistent, global accretion column solutions on to magnetized compact stars like neutron stars. The group has also invented an ingenious method to solve the problem of degenerate solution of accretion discs around black holes and are intending it to extend it to accretion columns around magnetized compact stars.

Relativistic magnetized outflow streamlines which pass through the Alfvenic and fast points (2019, MNRAS, 488, 5713)

Acceleration mechanism of jets have been studied where the jet is driven by accretion disc radiation. Jets can be accelerated up to few Lorentz factors. More interestingly, even for jets, general relativity and consideration of Compton scattering cross-section is important to get correct inner boundary condition of the jet. In contrast, magnetic driving could produce jets with terminal Lorentz factors around a 100. It has also been shown that one need to use relativistic equation of state to get correct temperature distribution of jets. In all these solutions, it was shown that the jet solutions depend on the composition of the flow. Simulations of relativistic jets also show that jet composition affects jet structure.

Movie for radiatively driven wind above a Keplerian disc around a non-rotating black hole with an accretion rate = 1.8 Eddington rate (left) and 4 Eddington rate (right) . In the smaller accretion rate case, the wind is quenched by radiation drag while for larger accretion rate the wind blows. (2021, MNRAS, 501, 4850)

Faculty Member : Dr. Indranil Chattopadhyay
Post Doctoral Researcher : Kuldeep Singh
Research Scholars : Shilpa Sarkar, Raj Kishor Joshi