Siddhartha Gupta

Energetic charged particles, commonly referred to as cosmic rays, are one of the main contributors to the nonthermal energies in the universe. Although diffusive shock acceleration (DSA) is the promising mechanism for particle acceleration at shocks, the processes that promote electrons to DSA remain unclear. To solve this problem, we have performed first-principles kinetic particle-in-cell simulations of collisionless shocks, exploring a wide range of shock speeds, Alfvénic, and sonic Mach numbers. I will report under which conditions electrons participate in the DSA at quasi-parallel shocks. The results are key to understanding the nonthermal phenomenology of a variety of heliophysical/astrophysical sources that shine in radio, X-rays, and gamma-rays.
Ref.
1. Gupta et al (2023); [NASA/ADS]; ICRC 2023 proceeding; arXiv
2. Gupta et al (2024a); [NASA/ADS]; ApJ 2024; arXiv
3. Gupta et al (2024b); [NASA/ADS]; ApJ 2024; arXiv

A strong super-Alfvénic drift of energetic particles (or cosmic rays) in a magnetized plasma can amplify the magnetic field significantly through nonresonant streaming instability (NRSI). While the traditional analysis is done for an ion current, here we use kinetic particle-in-cell simulations to study how the NRSI behaves when it is driven by electrons or by a mixture of electrons and positrons. In particular, we characterize the growth rate, spectrum, and helicity of the unstable modes, as well the level of the magnetic field at saturation. Our results are potentially relevant for several space/astrophysical environments (e.g., electron strahl in the solar wind, at oblique nonrelativistic shocks, around pulsar wind nebulae), and also in laboratory experiments.
Ref.
1. Gupta et al (2021); [NASA/ADS]; ApJ; arXiv
2. Gupta et al (2022); [NASA/ADS]; ICRC 2021 proceeding; arXiv

Cosmic rays (CRs) are frequently modelled as an additional fluid in hydrodynamic (HD) and magnetohydrodynamic (MHD) simulations of astrophysical flows. The standard CR two-fluid model is described in terms of three conservation laws (expressing conservation of mass, momentum, and total energy) and one additional equation (for the CR pressure) that cannot be cast in a satisfactory conservative form. The presence of non-conservative terms with spatial derivatives in the model equations prevents a unique weak solution behind a shock. We investigate a number of methods for the numerical solution of the two-fluid equations and find that, in the presence of shock waves, the results generally depend on the numerical details (spatial reconstruction, time stepping, the CFL number, and the adopted discretization). All methods converge to a unique result if the energy partition between the thermal and non-thermal fluids at the shock is prescribed using a subgrid prescription. This highlights the non-uniqueness problem of the two-fluid equations at shocks. From our numerical investigations, we report a robust method for which the solutions are insensitive to the numerical details even in absence of a subgrid prescription, although we recommend a subgrid closure at shocks using results from kinetic theory. The subgrid closure is crucial for a reliable post-shock solution and also its impact on large-scale flows because the shock microphysics that determines CR acceleration is not accurately captured in a fluid approximation. Critical test problems, limitations of fluid modelling, and future directions are discussed.
Ref.
1. Gupta et al (2020); [NASA/ADS]; MNRAS; arXiv

Cosmic ray (CR) sources leave signatures in the isotopic abundances of CRs. Current models of Galactic CRs that consider supernovae (SNe) shocks as the main sites of particle acceleration cannot satisfactorily explain the higher 22Ne/20Ne ratio in CRs compared to the interstellar medium. Although stellar winds from massive stars have been invoked, their contribution relative to SNe ejecta has been taken as a free parameter. Here, we present a theoretical calculation of the relative contributions of wind termination shocks (WTSs) and SNe shocks in superbubbles, based on the hydrodynamics of winds in clusters, the standard stellar mass function, and stellar evolution theory. We find that the contribution of WTSs towards the total CR production is at least 25 percent, which rises to ≳ 50 per cent for young (≲10 Myr) clusters, and explains the observed 22Ne/20Ne ratio. We argue that since the progenitors of apparently isolated supernovae remnants (SNRs) are born in massive star clusters, both WTS and SNe shocks can be integrated into a combined scenario of CRs being accelerated in massive clusters. This scenario is consistent with the observed ratio of SNRs to γ-ray bright (Lγ ≳ 1e35 erg/s) star clusters, as predicted by star cluster mass function. Moreover, WTSs can accelerate CRs to PeV energies, and solve other long-standing problems of the standard SN paradigm of CR acceleration.
Ref.
1. Gupta et al (2020) MNRAS 493, 3159; [NASA/ADS]; MNRAS; arXiv

Winds and supernovae drive shocks and produce all necessary conditions to accelerate high energy particles (e.g. Cosmic rays (CRs)). Despite the observations over a century, there are many aspects of CRs that are still unknown, e.g. from where do they come from? are they accelerated in supernova shocks or in stellar winds? To understand this problem, we have developed an analytical model and performed 1D and 3D simulations. We have calculated various observables which will be useful to understand high energy activity in star clusters.

Paper I: We develop a two-fluid model of an interstellar bubble (ISB) where cosmic rays are considered as the second fluid. We show that diffusive shock acceleration can be an effective process in changing the thermal properties of an ISB. Movie 2 shows the time evolution of thermal and CR pressure profiles in a two-fluid interstellar bubble. The enhancement of CR pressure at the reverse shock (wind termination shock) represents the diffusive shock acceleration. Once the CR pressure becomes larger than thermal pressure, the shock becomes very smooth representing the globally smooth solution of the two-fluid model. This simulation is performed using a code which is written by me. Details of the code can be found here.

Paper II: In continuation with Paper I, we estimate gamma-ray, x-ray, and radio (thermal and nonthermal) luminosities and compare them with observations. In this study, we show how a compact star cluster can produce diffuse gamma-ray. We also discuss the role of thermal and non-thermal radio, and X-rays in constraining the high energy activity in young star clusters. Movie 3 displays the surface brightness map of an ISB in multi-waveband.
Ref:
1. Gupta et al. (2018) MNRAS, 473, 1537; [NASA/ADS]; MNRAS; arXiv
2. Gupta et al (2018) MNRAS 479, 5220; [NASA/ADS]; MNRAS; arXiv

Star clusters are one of the most energetic sources in a galaxy. They emit wind and radiation which are so energetic that they can expel gas from their vicinity. Observers have found such gas expulsion, however, the driving mechanism was not clear.

We have studied this problem by considering the time evolution of a star cluster. We use a realistic Initial Mass Function (IMF) and include stellar winds (source of mechanical energy) and radiation in our analysis. After ~3 Myr when one-by-one massive stars die through supernova explosion, they also provide mechanical energy. However, the radiation power decreases with time. Therefore, the contribution from wind and radiation depend on the age of the star cluster. Using 1D numerical simulations, we show that radiation pressure can push the gas before 1 Myr. After 1 Myr, the expansion is controlled by mechanical power and radiation heating. Movie 1, shown here, displays the time evolution of various profiles due to interactions of stellar wind and radiation with the ambient medium.

Ref:
Gupta et al. (2016) MNRAS, 462, 4532; [NASA/ADS]; MNRAS; arXiv

My MSc thesis was focused on the applications of a modified Newtonian gravity in various astrophysical objects. This study involved a phenomenological approach which combines strong field and weak field gravities. With my advisor, Prof. Sayan Kar at IIT Kgp, we investigated the non-singular collapse of gas cloud and galaxy rotation curve.
Ref.
1. Gupta, S. 2014, MScThesis, IIT Kharagpur.