arXiv subjects

Mukul Bhattacharya

Publications and source records attributed to Mukul Bhattacharya.

30 records · Page 2Linked to original sources

arXiv2020

Fast Radio Burst Dispersion Measure Distribution as a Probe of Helium Reionization

Fast radio burst (FRB) discoveries are occurring rapidly, with thousands expected from upcoming surveys. The dispersion measures (DM) observed for FRB include important information on cosmological distances and the ionization state of the universe from the redshift of emission until today. Rather than considering the DM--redshift relation, we investigate the statistical ensemble of the distribution of dispersion measures. We explore the use of this abundance information, with and without redshift information, to probe helium reionization. Carrying out Monte Carlo simulations of FRB survey samples, we examine the effect of different source redshift distributions, host galaxy models, sudden vs gradual reionization, and covariance with cosmological parameters on determination of helium reionization properties. We find that a fluence limited survey with 10$^4$ FRBs can discriminate different helium reionization histories at $\sim6σ$ using the DM-distribution of bursts, without redshift information (and $\sim10σ$ with redshifts).

astro-ph.CO↗

arXiv2020

Population modelling of FRBs from intrinsic properties

We present a method to estimate the source 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. We then perform Monte Carlo simulations to constrain the properties of the FRB source, its host galaxy and scattering in the intervening plasma from the observational data of FRBs detected with Parkes. The typical scatter broadening of the intrinsic pulse is found to be considerably small $\lesssim 10^{-2}-1\ {\rm ms}$ from physical models, with the ISM contribution suppressed significantly relative to IGM. The intrinsic width for non-repeating FRBs is broadened by a factor $\sim 2-3$ on average primarily due to dispersive smearing. From the simulations, we find that the host galaxy DM contribution is likely to be comparable to the Galactic contribution and the FRB energy decreases significantly at high frequencies with a negative spectral index. The FRB spatial density is found to increase up to redshift $\sim 2.0$ and then drops significantly at larger distances. We obtain the energy distribution for FRB 121102 with repetition rate $\sim 0.1-0.3\ {\rm hr^{-1}}$ and exponential energy cutoff that is significantly smaller compared to typical FRB energies. We find that the probability of observing none of the other FRBs to be repeating at Parkes is $\sim 0.8-0.9$ with the current follow-up data insufficient to suggest more than one class of FRB progenitors.

astro-ph.HE↗

arXiv2019

Explaining GRB prompt emission with sub-photospheric dissipation and Comptonization

Even though the observed spectra for GRB prompt emission is well constrained, no single radiation mechanism can robustly explain its distinct non-thermal nature. Here we explore the radiation mechanism with the photospheric emission model using our Monte Carlo Radiative Transfer (MCRaT) code. We study the sub-photospheric Comptonization of fast cooled synchrotron photons while the Maxwellian electrons and mono-energetic protons are accelerated to relativistic energies by repeated dissipation events. Unlike previous simulations, we implement a realistic photon to electron number ratio $N_γ/N_{e} \sim 10^5$ consistent with the observed radiative efficiency of a few percent. We show that it is necessary to have a critical number of episodic energy injection events $N_{rh,cr} \sim {\rm few}\ 10{\rm s}-100$ in the jet in addition to the electron-proton Coulomb coupling in order to inject sufficient energy $E_{inj,cr} \sim 2500-4000\ m_e c^2$ per electron and produce an output photon spectrum consistent with observations. The observed GRB spectrum can be generated when the electrons are repeatedly accelerated to highly relativistic energies $γ_{e,in} \sim {\rm few}\ 10{\rm s}-100$ in a jet with bulk Lorentz factor $Γ\sim 30-100$, starting out from moderate optical depths $τ_{in} \sim 20-40$. The shape of the photon spectrum is independent of the initial photon energy distribution and baryonic energy content of the jet and hence independent of the emission mechanism, as expected for photospheric emission.

astro-ph.HE↗

arXiv2019

Constraining FRB progenitors from flux distribution

We present a generic formalism to constrain the cosmic rate density history and the source luminosity function of FRB progenitors from the statistical properties of the apparent flux density of non-repeating FRBs detected with Parkes. We include the pulse multipath propagation effects to evaluate the flux density distribution for a generalised spatial density model and luminosity function. We perform simulations to investigate the effects of the telescope beam pattern and temporal resolution on the observed flux density. We find that the FRB progenitors are likely to be younger stars with a relatively flat energy spectrum and host galaxy DM contribution similar to the MW. Our analysis can be extended to larger FRB samples detected with multiple surveys to place stronger constraints on the FRB progenitor properties.

astro-ph.HE↗

arXiv2019

Mergers of black hole-neutron star binaries and rates of associated electromagnetic counterparts

Black hole-neutron star (BHNS) binaries are amongst promising candidates for the joint detection of electromagnetic (EM) signals with gravitational waves (GWs) and are expected to be detected in the near future. Here we study the effect of the BHNS binary parameters on the merger ejecta properties and associated EM signals. We estimate the remnant disk and unbound ejecta masses for BH mass and spin distributions motivated from the observations of transient low-mass X-ray binaries (LMXBs) and specific NS equation of state (EoS). The amount of r-process elements synthesised in BHNS mergers is estimated to be a factor of $\sim 10^{2}-10^{4}$ smaller than BNS mergers, due to the smaller dynamical ejecta and merger rates for the former. We compute the EM luminosities and 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. We then evaluate the low-latency EM follow-up rates of the GW triggers in terms of the GW detection rate $\dot{N}_{GW}$ for current telescope sensitivities and typical BHNS binary parameters to find that most of the EM counterparts are detectable for high BH spin, small BH mass and stiffer NS EoS when NS disruption is significant. Based on the relative detection rates for given binary parameters, we find the ease of EM follow-up to be: ejecta afterglow $>$ cocoon afterglow $\gtrsim$ jet prompt $>$ ejecta macronova $>$ cocoon prompt $>$ jet afterglow $>>$ wind macronova $>>$ wind afterglow.

astro-ph.HE↗

arXiv2018

Luminosity and cooling suppression in magnetized white dwarfs

We investigate the luminosity and cooling of highly magnetized white dwarfs where cooling occurs by the diffusion of photons. We solve 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. For a given white dwarf age and for a fixed interface radius or temperature, the luminosity decreases significantly from $\sim 10^{-6}\, L_{\odot}$ to $10^{-9}\, L_{\odot}$ as the field strength increases from $\sim 10^9$ to $10^{12}\,$G in the surface layers. This is remarkable as it argues that magnetized white dwarfs can remain practically hidden in an observed H--R diagram. We also find that the cooling rates for these highly magnetized white dwarfs are suppressed significantly.

astro-ph.SR↗

arXiv2018

On estimating the atomic hydrogen column density from the H I 21 cm emission spectra

The 21 cm hyperfine transition of the atomic hydrogen (HI) in ground state is a powerful probe of the neutral gas content of the universe. This radio frequency transition has been used routinely for decades to observe, both in emission and absorption, HI in the Galactic interstellar medium as well as in extragalactic sources. In general, however, it is not trivial to derive the physically relevant parameters like temperature, density or column density from these observations. Here, we have considered the issue of column density estimation from the HI 21 cm emission spectrum for sightlines with a non-negligible optical depth and a mix of gas at different temperatures. The complicated radiative transfer and a lack of knowledge about the relative position of gas clouds along the sightline often make it impossible to uniquely separate the components, and hinders reliable estimation of column densities in such cases. Based on the observed correlation between the 21 cm brightness temperature and optical depth, we propose a method to get an unbiased estimate of the HI column density using only the 21 cm emission spectrum. This formalism is further used for a large sample to study the spin temperature of the neutral interstellar medium.

astro-ph.GA↗

arXiv2018

Luminosity and cooling of highly magnetized white dwarfs: suppression of luminosity by strong magnetic fields

We investigate the luminosity and cooling of highly magnetized white dwarfs with electron-degenerate cores and non-degenerate surface layers where cooling occurs by diffusion of photons. We find the temperature and density profiles in the surface layers or envelope of white dwarfs by solving the magnetostatic equilibrium and photon diffusion equations in a Newtonian framework. We also obtain the properties of white dwarfs at the core-envelope interface, when the core is assumed to be practically isothermal. With the increase in magnetic field, the interface temperature increases whereas the interface radius decreases. For a given age of the white dwarf and for fixed interface radius or interface temperature, we find that the luminosity decreases significantly from about $10^{-6}\, L_{\odot}$ to $10^{-9}\, L_{\odot}$ as the magnetic field strength increases from about $10^9$ to $10^{12}\,$G at the interface and hence the envelope. This is remarkable because it argues that magnetized white dwarfs are fainter and can be practically hidden in an observed Hertzsprung--Russell diagram. We also find the cooling rates corresponding to these luminosities. Interestingly, the decrease in temperature with time, for the fields under consideration, is not found to be appreciable.

astro-ph.SR↗

arXiv2018

Monte Carlo simulations of Photospheric emission in relativistic outflows

We study the spectra of photospheric emission from highly relativistic gamma-ray burst outflows using a Monte Carlo (MC) code. We consider the Comptonization of photons with a fast cooled synchrotron spectrum in a relativistic jet with photon to electron number ratio $N_γ/N_e = 10^5$. For all our simulations, we use mono-energetic protons which interact with thermalised electrons through the Coulomb interaction. The photons, electrons and protons are cooled adiabatically as the jet expands outwards. We find that the initial energy distribution of the protons and electrons do not have any appreciable effect on the photon peak energy and the power-law spectrum above the peak energy. We also find that the Coulomb interaction between the electrons and the protons does not affect the output photon spectrum significantly as the energy of the electrons is elevated only marginally. The peak energy and the spectral indices for the low and high energy power-law tails of the photon spectrum remain practically unchanged even in the presence of electron-proton coupling. Increasing the initial optical depth $τ_{in}$ results in shallower photon spectrum below the peak energy ($f_ν \propto ν^{1.1}$ for $τ_{in} = 2$ to $f_ν \propto ν^{0.3}$ for $τ_{in} = 16$) and fewer photons at the high-energy tail, although $f_ν \propto ν^{-0.5}$ above the peak energy up to $\sim 1$ MeV, independent of $τ_{in}$. The peak energy of the seed photon spectrum $E_{γ,peak}$ determines the peak energy and the shape of the output photon spectrum. Lastly, we find that our simulation results are quite sensitive to $N_γ/N_e$, for $N_{e} = 10^3$. For almost all our simulations, we obtain an output photon spectrum with power-law tail above $E_{γ,peak}$ extending up to $\sim 1$ MeV.

astro-ph.HE↗

arXiv2017

Fast radio burst source properties and curvature radiation model

We use the observed properties of fast radio bursts (FRBs) and a number of general physical considerations to provide a broad-brush model for the physical properties of FRB sources and the radiation mechanism. We show that the magnetic field in the source region should be at least 10^{14} Gauss. This strong field is required to ensure that the electrons have sufficiently high ground state Landau energy so that particle collisions, instabilities, and strong electric and magnetic fields associated with the FRB radiation do not perturb electrons' motion in the direction transverse to the magnetic field and destroy their coherent motion; coherence is required by the high observed brightness temperature of FRB radiation. The electric field in the source region required to sustain particle motion for a wave period is estimated to be of order 10^{11} esu. These requirements suggest that FRBs are produced near the surface of magnetars perhaps via forced reconnection of magnetic fields to produce episodic, repeated, outbursts. The beaming-corrected energy release in these bursts is estimated to be ~10^{36} ergs, whereas the total energy in the magnetic field is at least ~10^{45} ergs. We provide a number of predictions for this model which can be tested by future observations. One of which is that short duration FRB-like bursts should exist at much higher frequencies, possibly up to optical.

astro-ph.HE↗

arXiv2016

Significantly super-Chandrasekhar limiting mass white dwarfs and their consequences

Since 2012, we have initiated a new idea showing that the mass of highly magnetized or modified Einstein's gravity induced white dwarfs could be significantly super-Chandrasekhar with a different mass-limit. This discovery has several important consequences, including explanation of peculiar, over-luminous type Ia supernovae, soft gamma-ray repeaters and anomalous X-ray pulsars without invoking extraordinarily strong, yet unobserved, magnetic fields. It further argues for a possible second standard candle. Based on simpler calculations, these white dwarfs are also shown to be much less luminous than their standard counter-parts (of low magnetic fields). This discovery altogether initiates a new field of research.

astro-ph.SR↗

arXiv2015

Study of motion around a static black hole in Einstein and Lovelock gravity

We study motion around a static Einstein and pure Lovelock black hole in higher dimensions. It is known that in higher dimensions, bound orbits exist only for pure Lovelock black hole in all even dimensions, D=2N+2, where N is degree of Lovelock polynomial action. In particular, we compute periastron shift and light bending and the latter is given by one of transverse spatial components of Riemann curvature tensor. We also consider pseudo-Newtonian potentials and Kruskal coordinates.

gr-qc↗