Interest

Astrophysical fluid and plasmas
Cosmic rays, high energy astrophysics
Computational Physics
Star cluster, Stellar-wind bubble
Astrophysical shocks, stellar outflow
Supernova remnants as the origin of cosmic rays.

Collaborators

Prateek Sharma (Ph.D. advisor), Indian Institute of Science, Bangalore.

Biman Nath (Ph.D. co-advisor), Raman Research Institute, Bangalore

Siddhartha Gupta, University of Chicago, IL, USA.

Satyendra Thoudam Khalifa University, Abu Dhabi, UAE

Currently, I am working on high-energy astrophysics, especially on Cosmic rays and gamma rays. We are mainly focusing our study on the origin and accelerations of cosmic rays in our Galaxy. Cosmic Rays (CRs) are mainly highly energetic nuclei and electrons that are believed to be mostly accelerated in supernovae (SNe) shocks in the interstellar medium (ISM).

1. My current study focuses on the understanding of star clusters as potential sites of cosmic ray acceleration in the PeV energy range. We show that massive young star clusters may be possible candidates that can accelerate Galactic cosmic rays in the range of 10–1000 PeV (between the ‘knee’ and ‘ankle’). Considering a realistic spatial distribution of star clusters in our Galaxy and appropriate elemental abundances in the stellar wind, we present a model for producing different nuclei in CRs from these star clusters. Our recent work shows that stellar winds from young massive star clusters can explain the Galactic CRs in the abovementioned energy range. Young massive star clusters have continuous mass outflow (stellar wind), which will create a tind termination shock (WTS) around the cluster. The star cluster WTS is strong enough to accelerate particles in this energy range, which is difficult to explain in the standard paradigm of CR acceleration in supernova shocks. (10.3847/1538-4357/ad1605)

2. We study the effect of cosmic ray (CR) acceleration in the massive compact star cluster Westerlund 1 in light of its recent detection in γ-rays. Recent observations reveal a 1/r radial distribution of the CR energy density. Here we theoretically investigate whether or not this profile can help to distinguish between (1) continuous CR acceleration in the star cluster stellar wind-driven shocks and (2) discrete CR acceleration in multiple supernovae shocks – which are often debated in the literature. Using idealized two-fluid simulations and exploring different acceleration sites and diffusion coefficients, we obtain the CR energy density profile and luminosity to find the best match for the γ-ray observations. We find that the inferred CR energy density profiles from observations of γ-ray luminosity and mass can be much different from the true radial profile. CR acceleration at either the cluster core region or the wind termination shock can explain the observations if a fraction of ~ 10% of the shock power/post-shock pressure is deposited into the CR component. We also study the possibility of discrete supernovae (SN) explosions being responsible for CR acceleration and find that with an injection rate of 1 SN in every ∼ 0.03 Myr, one can explain the observed γ-ray profile. This multiple SN scenario is consistent with X-ray observations only if the thermal conductivity is close to the Spitzer value. (10.1093/mnras/stac023)

3. In another study, we are now focusing on the observed spectral bump in the cosmic ray proton and helium spectra at around TeV energy which has remained unexplained. Recent results from DAMPE and CALET experiments reveal that the cosmic ray energy spectrum experiences a hardening in the TeV range, followed by a softening at hundreds of TeV. We show that the observed spectral hardening can be attributed to the influence of nearby supernova remnants within a distance of approximately 1 kpc. These nearby supernova remnants have some additional contribution to the diffuse background CR flux which can explain the observed spectral hardening. Our model comprehensively accounts for proton and helium observations, as well as heavier elements. Additionally, we explain the observed all-particle cosmic ray spectra with our model. (Ongoing project)

Fig: Our model prediction for the all-particle spectrum using the Galactic star cluster CR model as the second galactic component. For the star cluster component, the considered injection fraction is ∼5% and the cutoff is at 50 Z PeV. The thick dashed maroon line represents the total SNR-CRs, the thick solid maroon line represents star cluster CRs, the thick maroon dotted line represents the extragalactic CR component, and the thick solid blue line represents the total all-particle spectrum.

Fig: Results of simulations with the 1/r ambient density profile and different injection scenarios are displayed. We plot the radial density and CR pressure profiles (a), the projected γ-ray luminosity above 1 TeV (b), projected mass (c), and inferred CR energy density above 10 TeV (d) for different injection sites of CRs. Black data points with error bars represent observational data and the blue, red, and cyan dashed lines show the simulation results for luminosity, mass, and CR density, respectively. The vertical lines in panel (a) represent different projection bins. All profiles are shown at 4.5 Myr. The uppermost row shows the case of central injection, the middle row shows the case of CR injection at shocks, and the bottom row shows the case of combined injection of CRs.