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Sayan Chakraborti

Publications and source records attributed to Sayan Chakraborti.

26 records · Page 2Linked to original sources
arXiv2011

Engine-driven Relativistic Supernovae as Sources of Ultra High Energy Cosmic Rays

Understanding the origin of the highest energy cosmic rays is a crucial step in probing new physics at energies unattainable by terrestrial accelerators. Their sources remain an enigma half a century after their discovery. They must be accelerated in the local universe as otherwise interaction with cosmic background radiations would severely deplete the flux of protons and nuclei at energies above the Greisen-Zatsepin-Kuzmin (GZK) limit. Hypernovae, nearby GRBs, AGNs and their flares have all been suggested and debated in the literature as possible sources. Type Ibc supernovae have a local sub-population with mildly relativistic ejecta which are known to be sub-energetic GRBs or X-Ray Flashes for sometime and more recently as those with radio afterglows but without detected GRB counterparts, such as SN 2009bb. In this work we measure the size-magnetic field evolution, baryon loading and energetics of SN 2009bb using its radio spectra obtained with VLA and GMRT. We show that the engine-driven SNe lie above the Hillas line and they can explain the characteristics of post-GZK UHECRs.

astro-ph.HE↗
arXiv2011

Supernova Feedback Keeps Galaxies Simple

Galaxies evolve continuously under the influence of self-gravity, rotation, accretion, mergers and feedback. The currently favored cold dark matter cosmological framework, suggests a hierarchical process of galaxy formation, wherein the present properties of galaxies are decided by their individual histories of being assembled from smaller pieces. However, recent studies have uncovered surprising correlations among the properties of galaxies, to the extent of forming a one-parameter set lying on a single fundamental line. It has been argued in the literature that such simplicity is hard to explain within the paradigm of hierarchical galaxy mergers. One of the puzzling results, is the simple linear correlation between the neutral hydrogen mass and the surface area, implying that widely different galaxies share very similar neutral hydrogen surface densities. In this work we show that self-regulated star formation, driven by the competition between gravitational instabilities and mechanical feedback from supernovae, can explain the nearly constant neutral hydrogen surface density across galaxies. We therefore recover the simple scaling relation observed between the neutral hydrogen mass and surface area. This result furthers our understanding of the surprising simplicity in the observed properties of diverse galaxies.

astro-ph.CO↗
arXiv2011

Baryon Loaded Relativistic Blastwaves in Supernovae

We provide a new analytic blastwave solution which generalizes the Blandford-McKee solution to arbitrary ejecta masses and Lorentz factors. Until recently relativistic supernovae have been discovered only through their association with long duration Gamma Ray Bursts (GRB). The blastwaves of such explosions are well described by the Blandford-McKee (in the ultra relativistic regime) and Sedov-Taylor (in the non-relativistic regime) solutions during their afterglows, as the ejecta mass is negligible in comparison to the swept up mass. The recent discovery of the relativistic supernova SN 2009bb, without a detected GRB, opens up the possibility of highly baryon loaded mildly relativistic outflows which remains in nearly free expansion phase during the radio afterglow. In this work, we consider a massive, relativistic shell, launched by a Central Engine Driven EXplosion (CEDEX), decelerating adiabatically due to its collision with the pre-explosion circumstellar wind profile of the progenitor. We compute the synchrotron emission from relativistic electrons in the shock amplified magnetic field. This models the radio emission from the circumstellar interaction of a CEDEX. We show that this model explains the observed radio evolution of the prototypical SN 2009bb and demonstrate that SN 2009bb had a highly baryon loaded, mildly relativistic outflow. We discuss the effect of baryon loading on the dynamics and observational manifestations of a CEDEX. In particular, our predicted angular size of SN 2009bb is consistent with VLBI upper limits on day 85, but is presently resolvable on VLBI angular scales, since the relativistic ejecta is still in the nearly free expansion phase.

astro-ph.HE↗
arXiv2010

Ultra High Energy Cosmic Ray Acceleration in Engine-driven Relativistic Supernovae

The origin of the highest energy cosmic rays remains an enigma. They offer a window to new physics, including tests of physical laws relevant to their propagation and interactions, at energies unattainable by terrestrial accelerators. They must be accelerated locally, as otherwise background radiations would severely suppress the flux of protons and nuclei, at energies above the Greisen-Zatsepin-Kuzmin (GZK) limit. Nearby Gamma Ray Bursts (GRBs), Hypernovae, Active Galactic Nuclei (AGNs) and their flares, have all been suggested and debated as possible sources. A local sub-population of type Ibc supernovae (SNe) with mildly relativistic outflows have been detected as sub-energetic GRBs or X-Ray Flashes (XRFs) and recently as radio afterglows without detected GRB counterparts. We measure the size-magnetic field evolution, baryon loading and energetics, using the observed radio spectra of SN 2009bb. We place such engine-driven SNe above the Hillas line and establish that they can readily explain the post-GZK UHECRs.

astro-ph.HE↗
arXiv2010

An Expanding Neutral Hydrogen Supershell Evacuated by Multiple Supernovae in M101

Several neutral hydrogen (HI) cavities have been detected in the Milky Way and other nearby star forming galaxies. It has been suggested that at least a fraction of them may be expanding supershells driven by the combined mechanical feedback from multiple supernovae occurring in an OB association. Yet most extragalactic HI holes have neither a demonstrated expansion velocity, nor an identified OB association inside them. In this work, we report on the discovery of an unbroken expanding HI supershell in the nearby spiral galaxy M101, with an UV emitting OB association inside it. We measure its size (500 pc) and expansion velocity (20 km/s) by identifying both its approaching and receding components in the position-velocity space, using 21 cm emission spectroscopy. This provides us with an ideal system to test the theory of supershells driven by the mechanical feedback from multiple supernovae. The UV emission of the cluster inside the supershell is compared with simulated spectral energy distribution of synthetic clusters of the appropriate age (~15 Myr). The observed UV flux is found to be consistent with an association of the appropriate mass (~10^5 Solar Mass) and age required by the energy budget of the supershell. Properties of this supershell and another previously reported in the same galaxy are used to infer its neutral hydrogen scale height and mean neutral hydrogen density in the disk. The presence of another UV emitting stellar association in over-dense swept up gas is discussed in the context of propagating star formation.

astro-ph.HE↗
arXiv2007

Mass radius scaling near the Chandrasekhar Limit

The mass radius relationship of white dwarfs, near the Chandrasekhar Limit, is derived for a toy model of uniform density, using the variational principle. A power law scaling, reminiscent of those found in 2nd order phase transitions, is obtained. The derived exponent is shown to explain the relationship obtained by numerically integrating the TOV equations with the equation of state for a relativistic Fermi gas of electrons.

astro-ph↗
arXiv2007

A Semiclassical Model for Molecular Localization in Ammonia

The pedagogic two stste system of the ammonia molecule is used to illustrate the phenomenon of environment induced molecular localization in pyramidal molecules. A semiclassical model is used to describe a gas of pyramidal molecules interacting via hard ball collisions. This modifies the tunnelling dynamics between the classical equilibrium configurations of an isolated molecule. For sufficiently high pressures, the model explains molecular localization in these classical configurations. The decrease in the inversion line frequency of ammonia, noted upon increase in pressure, is also studied.

cond-mat.other↗