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Research

My ongoing research is primarily focused on:

• Explaining the radiation mechanism of GRB prompt spectrum 

• Constraining the source properties and radiation mechanism of FRBs 

• EM follow-up of low latency GW triggers

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In the recent past, I have also worked on:

• The suppression of thermal properties in highly magnetised WDs

• Studied the orbits around Schwarzschild BHs in higher dimensional pure Lovelock gravity  

• Developed a formalism to estimate the HI column density from 21 cm emission spectra

FRB population studies and radiation mechanism

FRBs are radio transients of possible cosmological origin with ms duration and Jy-level brightness, mostly detected from high Galactic latitudes. Currently, more than 50 such bursts have been reported and many more bursts are expected to be detected in the near future with the upcoming radio surveys. We:

1. Developed a formalism to estimate the intrinsic properties of FRBs from observations by assuming a fixed DM contribution from a MW-like host galaxy, pulse temporal broadening models for turbulent plasma and a flat FRB energy spectrum.

2. Performed MC simulations to constrain the properties of the FRB source, its host galaxy and scattering in the intervening plasma from the current observations. 

3. Studied the plasma properties and emission conditions for coherent curvature radiation model in order to explain the radiation mechanism of FRBs.

EM follow-up of GW triggers from BHNS mergers

BHNS binaries are amongst promising candidates for the joint detection of EM signals with GWs. At present, six BBH and one BNS mergers have been detected and BHNS merger detections are anticipated in the upcoming aLIGO/Virgo runs at a rate of more than one per year. We:

1. Studied the effect of the BHNS binary parameters on the merger ejecta properties and associated EM signals.

2. Estimated the EM luminosities and the light curves for the early- and late-time emissions from the ultra-relativistic jet, sub-relativistic dynamical ejecta and wind, and the mildly-relativistic cocoon for typical ejecta parameters.

3. Evaluated the low-latency EM follow-up rates of the GW triggers in terms of the GW detection rate for current telescope sensitivities and typical BHNS binary parameters.

Photospheric model for GRB prompt emission

Although the observed spectra for GRB prompt emission is well constrained, no single radiation mechanism can robustly explain its distinctly non-thermal nature. We:

1. Studied the sub-photospheric Comptonization of fast cooled synchrotron photons using our MCRaT code while the Maxwellian electrons and mono-energetic protons are accelerated to relativistic energies by repeated dissipation events.

2. Implemented a realistic photon-to-electron number ratio of 100,000 that is consistent with the typical observed radiation efficiencies of a few percent.

3. Showed that the electron-proton Coulomb coupling is insufficient to inject the required energy into the photons and produce an output photon spectrum that is consistent with observations - episodic events injecting significant energies into the proton and electron populations are needed.

Cooling suppression in highly magnetised WDs

1. We investigated the luminosity and cooling of highly magnetized WDs where cooling occurs by the diffusion of photons by solving the magnetostatic equilibrium and photon diffusion equations to obtain the temperature and density profiles in the surface layers of these white dwarfs. With increase in field strength, the degenerate core shrinks in volume with a simultaneous increase in the core temperature. 

2. We showed that for a given white dwarf age and for a fixed interface radius or temperature, the luminosity decreases significantly as the field strength increases in the surface layers - which is remarkable as it argues that MWDs can remain practically hidden in an observed H-R diagram. We also found that the cooling rates for these MWDs are suppressed significantly.

Particle orbits around BHs in pure Lovelock gravity

1. We studied the motion around Einstein and pure Lovelock BHs in higher dimensional gravity. In higher dimensions, bound orbits exist only for pure Lovelock BH in all even dimensions.

2. We computed the periastron shift and light bending and found that the latter is given by one of transverse spatial components of the Riemann curvature tensor.

3. We also considered pseudo-Newtonian potentials and Kruskal coordinates for these systems.

HI column density from 21 cm emission spectra

The 21 cm transition of HI in ground state is a powerful probe of the neutral gas content of the universe. However, it is not trivial to derive the physically relevant parameters like temperature, density or column density from the emission and absorption observations. We:

1. Developed a formalism to estimate the column density of the HI 21 cm emission spectrum for sightlines with a non-negligible optical depth and a mix of gas at different temperatures. 

2. Based on the observed correlation between the 21 cm brightness temperature and optical depth, we proposed a method to get an unbiased estimate of the HI column density using only the 21 cm emission spectrum.

3. Used this formalism for a large sample to study the spin temperature of the neutral interstellar medium.