Showing papers by "Pawan Kumar published in 2015"
TL;DR: A comprehensive review of major developments in our understanding of gamma-ray bursts, with particular focus on the discoveries made within the last fifteen years when their true nature was uncovered, can be found in this paper.
864 citations
TL;DR: In this article, the authors performed a systematic study to address the question: how bad/good are the external forward shock models of gamma-ray bursts (GRBs) afterglow data of 85 GRBs well-monitored X-ray and optical lightcurves.
Abstract: The external forward shock (EFS) models have been the standard paradigm to interpret the broad-band afterglow data of gamma-ray bursts (GRBs). One prediction of the models is that some afterglow temporal breaks at different energy bands should be achromatic. Observations in the Swift era have revealed chromatic afterglow behaviors at least in some GRBs, casting doubts on the EFS origin of GRB afterglows. In this paper, we perform a systematic study to address the question: how bad/good are the external forward shock models? Our sample includes 85 GRBs well-monitored X-ray and optical lightcurves. Based on how well the data abide by the EFS models, we categorize them as: Gold sample: (Grade I and II) include 45/85 GRBs. They show evidence of, or are consistent with having, an achromatic break. The temporal/spectral behaviors in each afterglow segment are consistent with the predictions (closure relations) of the EFS models. Silver sample: (Grade III and IV) include 37/85 GRBs. They are also consistent with having an achromatic break, even though one or more afterglow segments do not comply with the closure relations. Bad sample: (Grade V), 3/85 shows direct evidence of chromatic behaviors, suggesting that the EFS models are inconsistent with the data. These are included in the Bad sample. We further perform statistical analyses of various observational properties ($\alpha$, $\beta$, $t_b$ and model parameters (energy injection index q, p, $\theta_j$, $\eta_\gamma$, etc) of the GRBs in the Gold Sample, and derive constraints on the magnetization parameter $\epsilon_B$ in the EFS. Overall, we conclude that the simplest EFS models can account for the multi-wavelength afterglow data of at least half of the GRBs. When more advanced modeling (e.g., long-lasting reverse shock, structured jets) is invoked, up to $>90 \%$ of the afterglows may be interpreted within the framework of the ESF models.
152 citations
TL;DR: In this paper, the authors proposed a new model that explains how an X-ray afterglow can be sustained by the product of the merger and how the Xray emission is produced before the corresponding emission in the gamma-band, though it is observed to follow it.
Abstract: The merger of a binary of neutron stars provides natural explanations for many of the features of short gamma-ray bursts (SGRBs), such as the generation of a hot torus orbiting a rapidly rotating black hole, which can then build a magnetic jet and provide the energy reservoir to launch a relativistic outflow. However, this scenario has problems explaining the recently discovered long-term and sustained X-ray emission associated with the afterglows of a subclass of SGRBs. We propose a new model that explains how an X-ray afterglow can be sustained by the product of the merger and how the X-ray emission is produced before the corresponding emission in the gamma-band, though it is observed to follow it. Overall, our paradigm combines in a novel manner a number of well-established features of the emission in SGRBs and results from simulations. Because it involves the propagation of an ultra-relativistic outflow and its interaction with a confining medium, the paradigm also highlights a unifying phenomenology between short and long GRBs.
84 citations
TL;DR: In this article, the flux and maximum frequency of radiation produced when the magnetic field in a relativistic, highly magnetized, jet is dissipated and particles are accelerated using general considerations.
Abstract: We provide estimates for the flux and maximum frequency of radiation produced when the magnetic field in a relativistic, highly magnetized, jet is dissipated and particles are accelerated using general considerations. We also provide limits on the jet Lorentz factor and magnetization parameter from the observed flux. Furthermore, using the Lorentz invariance of scalar quantities produced with electromagnetic tensor, we provide constraints on particle acceleration, and general features of the emergent radiation. We find that the spectrum below the peak softens with decreasing frequency. This spectral feature might be one way of identifying a magnetic jet.
23 citations
TL;DR: In this paper, the inverse-Compton scattering of EPF by the hot electrons in the GRB jet was calculated, taking fully into account the equal-arrival time surfaces and possible pair-production absorption with the prompt gamma-rays.
Abstract: Long Gamma-Ray Bursts (GRBs) are produced by ultra-relativistic jets launched from core collapse of massive stars Most massive stars form in binaries and/or in star clusters, which means that there may be a significant external photon field (EPF) around the GRB progenitor We calculate the inverse-Compton scattering of EPF by the hot electrons in the GRB jet Three possible cases of EPFs are considered: the progenitor is (I) in a massive binary system, (II) surrounded by a Wolf-Rayet-star wind, and (III) in a dense star cluster Typical luminosities of 10^46 - 10^50 erg/s in the 1 - 100 GeV band are expected, depending on the stellar luminosity, binary separation (I), wind mass loss rate (II), stellar number density (III), etc We calculate the lightcurve and spectrum in each case, taking fully into account the equal-arrival time surfaces and possible pair-production absorption with the prompt gamma-rays Observations can put constraints on the existence of such EPFs (and hence on the nature of GRB progenitors) and on the radius where the jet internal dissipation process accelerates electrons
6 citations
TL;DR: In this article, the authors consider the case of a Poynting-flux-dominated relativistic outflow and show that it suffers strong inverse-Compton scattering drag near the stellar surface and the jet is slowed down to sub-relativistic speed if its initial magnetization parameter (σ 0) is larger than about 105.
Abstract: The collimation and evolution of relativistic outflows in γ-ray bursts are determined by their interaction with the stellar envelope through which they travel before reaching the much larger distance where the energy is dissipated and γ-rays are produced. We consider the case of a Poynting-flux-dominated relativistic outflow and show that it suffers strong inverse-Compton (IC) scattering drag near the stellar surface and the jet is slowed down to sub-relativistic speed if its initial magnetization parameter (σ0) is larger than about 105. If the temperature of the cocoon surrounding the jet were to be larger than about 10 keV, then an optically thick layer of electrons and positrons forms at the interface of the cocoon and the jet, and one might expect this pair screen to protect the interior of the jet from IC drag. However, the pair screen turns out to be ephemeral, and instead of shielding the jet it speeds up the IC drag on it. Although a high σ0 jet might not survive its passage through the star, a fraction of its energy is converted to 1-100 MeV radiation that escapes the star and appears as a bright flash lasting for about 10 s.
3 citations
01 Jan 2015
TL;DR: In this article, the feasibility of using natural gums for development of sustained release matrix tablets of Itopride in view to improve patient compliance and therapeutic action was checked by using natural polymers like xanthan gum, guar gum, karaya gum, locust bean gum, neem gum as matrix forming agent and excipients such as magnesium stearate, MCC, PVP and talc were used.
Abstract: The objective of this research work was to check feasibility of using natural gums for development of sustained release matrix tablets of Itopride in view to improve patient compliance and therapeutic action. Matrix tablets were prepared by direct compression method by using natural polymers like xanthan gum, guar gum, karaya gum, locust bean gum, neem gum as matrix forming agent and excipients such as magnesium stearate, MCC, PVP and talc were used. The dissolution medium consisted of 900 ml of 0.1 N HCl for first 2 hours and then 7.4 phosphate buffer for remaining 10 hours. All the tablet formulations showed acceptable pharmacokinetic properties and complied with in-house specifications for tested parameters. Stability studies were performed for optimised formulation as per ICH guidelines which show that formulations were stable after three months of short term stability studies. The formulation was optimized on the basis of acceptable tablet properties and in-vitro drug release. Formulation F-6 was successfully sustained the release of drug upto 12 hours. The kinetic treatment of selected optimized formulation shows that the regression coefficient for zero-order kinetics were found to be higher when compared with those of the first-order kinetics, indicating that drug release from all the formulations followed zero-order kinetics and the ‘n’ value lies between 0.76-0.85 (Korsmeyer-Peppas model) demonstrating that the mechanism controlling the drug release was Anomalous (non-Fickian) diffusion. Optimized formulation was tested for their compatibility with Itopride by FT-IR studies, which revealed that there is no chemical interaction occurred with polymer and other excipients. The drug release profile of the best formulation was well controlled and uniform throughout the dissolution studies.
1 citations