1,094 research outputs found
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 . The burst
appears to be of very low energy ( ergs) confined to a narrow
cone of opening angle . 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 ~\citep{dominik2012double}, we find the afterglow
detection rates (per year) to be (), ()
and () 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 -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 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 ~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 ~MHz.
Results. The spectral energy distribution of the source remains optically
thin even at ~MHz, with a spectral index of
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
~MHz. Like Galactic plerions, the energy in the persistent source is
carried predominantly by leptons. The emitting plasma has a ~G, and
its age is . 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 ~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
GRB 030329: 3 years of radio afterglow monitoring
Radio observations of gamma-ray burst (GRB) afterglows are essential for our
understanding of the physics of relativistic blast waves, as they enable us to
follow the evolution of GRB explosions much longer than the afterglows in any
other wave band. We have performed a three-year monitoring campaign of GRB
030329 with the Westerbork Synthesis Radio Telescopes (WSRT) and the Giant
Metrewave Radio Telescope (GMRT). Our observations, combined with observations
at other wavelengths, have allowed us to determine the GRB blast wave physical
parameters, such as the total burst energy and the ambient medium density, as
well as investigate the jet nature of the relativistic outflow. Further, by
modeling the late-time radio light curve of GRB 030329, we predict that the
Low-Frequency Array (LOFAR, 30-240 MHz) will be able to observe afterglows of
similar GRBs, and constrain the physics of the blast wave during its
non-relativistic phase.Comment: 5 pages, 2 figures, Phil. Trans. R. Soc. A, vol.365, p.1241,
proceedings of the Royal Society Scientific Discussion Meeting, London,
September 200
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