Gamma Ray Burst Prompt Emission Variability in Synchrotron and Synchrotron Self-Compton Lightcurves

Gamma Ray Burst prompt emission is believed to originate from electrons accelerated in a highly relativistic outflow. "Internal shocks" due to collisions between shells ejected by the central engine is a leading candidate for electron acceleration. While synchrotron radiation is generally invoked to interpret prompt gamma-ray emission within the internal shock model, synchrotron self-Compton (SSC) is also considered as a possible candidate of radiation mechanism. In this case, one would expect a synchrotron emission component at low energies, and the naked-eye GRB 080319B has been considered as such an example. In the view that the gamma-ray lightcurve of GRB 080319B is much more variable than its optical counterpart, in this paper we study the relative variability between the synchrotron and SSC components. We develop a "top-down" formalism by using observed quantities to infer physical parameters, and subsequently to study the temporal structure of synchrotron and SSC components of a GRB. We complement the formalism with a "bottom-up" approach where the synchrotron and SSC lightcurves are calculated through a Monte-Carlo simulations of the internal shock model. Both approaches lead to the same conclusion. Small variations in the synchrotron lightcurve can be only moderately amplified in the SSC lightcurve. The SSC model therefore cannot adequately interpret the gamma-ray emission properties of GRB 080319B.Comment: 13 pages, 4 figures, accepted for publication in MNRA

Intra and interspecific morphological variations of three species of Capsicum (Solanaceae)

The widely cultivated pepper, Capsicum spp., important as a vegetable and spice crop worldwide, is one of the most diverse crops. Considerable morphological variation, especially in fruit shape, color, and size exist in the genus. The present study analysed the intra and interspecific morphological variations of three species of Capsicum (Solanaceae) using thirty nine morphological characters. Morphological variations with respect to vegetative, floral and fruit characteristics were observed among the members of the same species. Morphological data was used to obtain UPGMA derived dendrogram and the cluster analysis indicated genetic divergence among the three species. Present observations imply a great potential for chilli breeding through a hybridization programme or direct use of the varieties for the successful production of promising cultivars

Optical Observations and Multiband Modelling of the Afterglow of GRB 041006: Evidence of A Hard Electron Energy Spectrum

We present the CCD Cousins R band photometric observations of the afterglow of GRB 041006. The multiband afterglow evolution is modelled using an underlying `hard' electron energy spectrum with a $p_1 \sim 1.3$. The burst appears to be of very low energy ($E \sim 10^{48}$ ergs) confined to a narrow cone of opening angle $\theta \sim 2.3^{\circ}$. The associated supernova is compared with SN1998bw and is found to be brighter.Comment: Accepted for publication in Bull. Astr. Soc. India (BASI

Rates of Short-GRB afterglows in association with Binary Neutron Star mergers

Assuming all binary Neutron Star mergers produce Short Gamma Ray Bursts (SGRBs), we combine the merger rates of binary Neutron Stars (BNS) from population synthesis studies, the sensitivities of advanced Gravitational Wave (GW) interferometer networks, and of the electromagnetic (EM) facilities in various wave bands, to compute the detection rate of associated afterglows in these bands. Using the inclination angle measured from GWs as a proxy for the viewing angle and assuming a uniform distribution of jet opening angle between 3 to 30 degrees, we generate light curves of the counterparts using the open access afterglow hydrodynamics package BoxFit for X-ray, Optical and Radio bands. For different EM detectors we obtain the fraction of EM counterparts detectable in these three bands by imposing appropriate detection thresholds. In association with BNS mergers detected by five (three) detector network of advanced GW interferometers, assuming a BNS merger rate of $0.6-774{\rm Gpc}^{-3}{\rm yr}^{-1}$~\citep{dominik2012double}, we find the afterglow detection rates (per year) to be $0.04-53$ ($0.02-27$), $0.03-36$ ($0.01-19$) and $0.04-47$ ($0.02-25$) in the X-ray, optical and radio bands respectively. Our rates represent maximum possible detections for the given BNS rate since we ignore effects of cadence and field of view in EM follow up observations.Comment: Published in MNRA

Exploring Short-GRB afterglow parameter space for observations in coincidence with gravitational waves

Short duration Gamma Ray Bursts(SGRB) and their afterglows are among the most promising electro-magnetic (EM) counterparts of Neutron Star (NS) mergers. The afterglow emission is broadband, visible across the entire electro-magnetic window from $\gamma$-ray to radio frequencies. The flux evolution in these frequencies is sensitive to the multi-dimensional afterglow physical parameter space. Observations of gravitational wave (GW) from BNS mergers in spatial and temporal coincidence with SGRB and associated afterglows can provide valuable constraints on afterglow physics. We run simulations of GW-detected BNS events and assuming all of them are associated with a GRB jet which also produces an afterglow, investigate how detections or non-detections in X-ray, optical and radio frequencies can be influenced by the parameter space. We narrow-down the regions of afterglow parameter space for a uniform top-hat jet model which would result in different detection scenarios. We list inferences which can be drawn on the physics of GRB afterglows from multi-messenger astronomy with coincident GW-EM observations.Comment: Published in MNRA

Observations of the Optical Afterglow of GRB 050319 : Wind to ISM transition in view

The collapse of a massive star is believed to be the most probable progenitor of a long GRB. Such a star is expected to modify its environment by stellar wind. The effect of such a circum-stellar wind medium is expected to be seen in the evolution of a GRB afterglow, but has so far not been conclusively found. We claim that a signature of wind to constant density medium transition of circum-burst medium is visible in the afterglow of GRB 050319. Along with the optical observations of the afterglow of GRB 050319 we present a model for the multiband afterglow of GRB 050319. We show that the break seen in optical light curve at $\sim$ 0.02 day could be explained as being due to wind to constant density medium transition of circum-burst medium, in which case, to our knowledge, this could be the first ever detection of such a transition at any given frequency band. Detection of such a transition could also serve as a confirmation of massive star collapse scenario for GRB progenitors, independent of supernova signatures.Comment: 11 pages, 3 tables, 1 figure

Implications of the lowest frequency detection of the persistent counterpart of FRB121102

Context. The repeating FRB121102 is so far the only extra-galactic Fast Radio Burst found to be associated with a counterpart, a steady radio source with a nearly flat spectral energy distribution (SED) in centimeter wavelengths. Aims. Previous observations of the persistent source down to $1.6$~GHz has shown no sign of a spectral turn-over. Absorption is expected to eventually cause a turn-over at lower frequencies. Better constraints on the physical parameters of the emitting medium can be derived by detecting the self-absorption frequency. Methods. We used the Giant Metre-Wave Radio Telescope (GMRT) during the period of July to December 2017 to observe the source at low radio frequencies down to $400$~MHz. Results. The spectral energy distribution of the source remains optically thin even at $400$~MHz, with a spectral index of $\nu^{-(0.07 \pm 0.03)}$ similar to what is seen in Galactic plerions. Using a generic synchrotron radiation model, we obtain constraints on properties of the non-thermal plasma and the central engine powering it. Conclusions. We present low-frequency detections of the persistent source associated with FRB121102. Its characteristic flat SED extends down to $400$~MHz. Like Galactic plerions, the energy in the persistent source is carried predominantly by leptons. The emitting plasma has a $B< 0.01$~G, and its age is $> 524 \left(\frac{B}{0.01 {\rm G}} \right)^{-3/2}$. We show that the energetics of the persistent source requires an initial spin period shorter than 36~ms, and the magnetic field of the neutron star must exceed $4.5\times 10^{12}$~G. This implies that the persistent source does not necessarily require energetic input from a magnetar.Comment: Submitted to A&A, 8 pages, 4 figure