## 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.

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.

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.

Faculty Member : Dr. Indranil Chattopadhyay

Post Doctoral Researcher : Kuldeep Singh

Research Scholars : Shilpa Sarkar, Raj Kishor Joshi

- Raj Kishor Joshi, Indranil Chattopadhyay, Dongsu Ryu, Lallan Yadav;
*Exact solution of one dimensional relativistic jet with relativistic equation of state*: 2021, MNRAS,**502**, 5227.**(arXiv : 2102.02835)** - Sananda Raychaudhuri, Mukesh K Vyas, Indranil Chattopadhyay;
*Simulations of radiation-driven winds from Keplerian discs*: 2021, MNRAS,**501**, 4850.**(arXiv : 2012.08886)** - Shilpa Sarkar, Indranil Chattopadhyay, Philippe Laurent;
*Two-temperature solutions and emergent spectra from relativistic accretion discs around black holes*: 2020, A&A,**642**, A209.**(arXiv : 2007.00919)** - Kuldeep Singh, Indranil Chattopadhyay;
*Study of relativistic magnetized outflows with relativistic equation of state*: 2019, MNRAS,**488**, 5713.**(arXiv : 1907.12547)** - Mukesh K. Vyas, Indranil Chattopadhyay;
*Radiation driving and heating of general relativistic jets under Compton scattering regime*: 2019, MNRAS,**482**, 4203.**(arXiv : 1810.11183)** - Indranil Chattopadhyay, Rajiv Kumar;
*Estimation of mass outflow rates from viscous relativistic accretion discs around black holes*: 2016, MNRAS,**459**, 3792 (2016).**(arXiv : 1605.00752)** - Indranil Chattopadhyay, Dongsu Ryu;
*Effect of fluid composition on spherical flows around black holes*: 2009, ApJ,**694**, 492.**(arXiv : 0812.2607)**