Showing papers by "Pawan Kumar published in 2014"

The Physics of Gamma-Ray Bursts and Relativistic Jets

Abstract: We provide 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. We describe the observational properties of photons from the radio to multi-GeV bands, both in the prompt emission and the afterglow phases. Mechanisms for the generation of these photons in GRBs are discussed and confronted with observations to shed light on the physical properties of these explosions, their progenitor stars and the surrounding medium. After presenting observational evidence that a powerful, collimated, jet moving at close to the speed of light is produced in these explosions, we describe our current understanding regarding the generation, acceleration, and dissipation of the jet and compare these properties with jets associated with AGNs and pulsars. We discuss mounting observational evidence that long duration GRBs are produced when massive stars die, and that at least some short duration bursts are associated with old, roughly solar mass, compact stars. The question of whether a black-hole or a strongly magnetized, rapidly rotating neutron star is produced in these explosions is also discussed. We provide a brief summary of what we have learned about relativistic collisionless shocks and particle acceleration from GRB afterglow studies, and discuss the current understanding of radiation mechanism during the prompt emission phase. We discuss theoretical predictions of possible high-energy neutrino emission from GRBs and the current observational constraints. Finally, we discuss how these explosions may be used to study cosmology, e.g. star formation, metal enrichment, reionization history, as well as the formation of first stars and galaxies in the universe.

Magnetic Fields in Relativistic Collisionless Shocks

Abstract: We present a systematic study on magnetic fields in gamma-ray burst (GRB) external forward shocks (FSs). There are 60 (35) GRBs in our X-ray (optical) sample, mostly from Swift. We use two methods to study B (fraction of energy in magnetic field in the FS): (1) for the X-ray sample, we use the constraint that the observed flux at the end of the steep decline is ≥ X-ray FS flux; (2) for the optical sample, we use the condition that the observed flux arises from the FS (optical sample light curves decline as ~t –1, as expected for the FS). Making a reasonable assumption on E (jet isotropic equivalent kinetic energy), we converted these conditions into an upper limit (measurement) on B n 2/(p + 1) for our X-ray (optical) sample, where n is the circumburst density and p is the electron index. Taking n = 1 cm–3, the distribution of B measurements (upper limits) for our optical (X-ray) sample has a range of ~10–8-10–3 (~10–6-10–3) and median of ~few × 10–5 (~few × 10–5). To characterize how much amplification is needed, beyond shock compression of a seed magnetic field ~10 μG, we expressed our results in terms of an amplification factor, AF, which is very weakly dependent on n (AF∝n 0.21). The range of AF measurements (upper limits) for our optical (X-ray) sample is ~1-1000 (~10-300) with a median of ~50 (~50). These results suggest that some amplification, in addition to shock compression, is needed to explain the afterglow observations.

A novel paradigm for short gamma-ray bursts with extended X-ray emission

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. Yet, 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, although 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.

Some implications of inverse-Compton scattering of hot cocoon radiation by relativistic jets in gamma-ray bursts

Abstract: Long Gamma-Ray Bursts (GRB) relativistic jets are surrounded by hot cocoons which confine jets during their punch out from the progenitor star These cocoons are copious sources of X-ray photons that can be and are inverse-Compton (IC) scattered to MeV--GeV energies by electrons in the relativistic jet We provide detailed estimates for IC flux resulting from various interactions between X-ray photons and the relativistic jet, and describe what we can learn about GRB jets and progenitor stars from the detection (or an upper limit) of these IC scattered photons

A Small, Rapid Optical-IR Response Gamma-Ray Burst Space Observatory (The NGRG)

Abstract: Here we propose a new gamma-ray burst (GRB) mission, the Next Generation Rapid-Response GRB Observatory (NGRG) As with Swift, GRBs are initially located with a coded-mask X-ray camera However, the NGRG has two new features: First, a beam-steering system to begin optical observations within ~ 1 s after location; second, a near-IR (NIR) camera viewing the same sky, for sensitivity to extinguished bursts These features allow measurement of the rise phase of GRB optical-NIR emission Thus far, the rise time and transition between prompt and afterglow in the optical and NIR are rarely measured Rapid-response measurements explore many science topics including optical emission mechanisms (synchrotron vs SSC, photospheric emission) and jet characteristics (reverse vs forward shock emission, baryon-dominated vs magnetic dominated) Rapid optical-NIR response can measure dynamic evolution of extinction due to vaporization of dust, and separate star system and galaxy dust extinction We discuss these measurements, giving reliable detection rate estimates from analysis of Swift data and scaled Swift performance The NGRG will explore optical/NIR emission measured earlier than ever before, and potentially fainter, more extinguished GRBs than ever before In the current era, costs are important Our proposed modest NGRG can still produce new GRB science, while providing rapid GRB alerts for the entire community for post-Swift GRB science We show that an X-ray instrument barely 1/5 the area of Swift BAT will yield a significant fraction of Swift's detection rate: more than 65 X-ray, and with a 30 cm optical-IR telescope and modern cameras, more than 19 NIR and 14 optical detections each year In addition, active feedback control of the beam-steering would remove the need for arc sec stabilization of the spacecraft, for a substantial cost saving

A Small, Rapid Optical-IR Response Gamma-Ray Burst Space Observatory Concept: The NGRG

Abstract: After Swift, there is no sure plan to furnish a replacement for the rapidly disseminated, high-precision GRB positions it provides, nor a new type of observatory to probe new GRB parameter space. We propose a new GRB mission concept, the Next Generation Rapid Optical–NIR (near infrared) Response GRB Observatory (NGRG) concept, and demonstrate, through analysis of Swift BAT data, studies of new GRB samples, and extinction predictions, that a relatively modest size observatory will produce valuable new measurements and good GRB detection rates. As with Swift, GRBs are initially located with a coded-mask X-ray camera. However, the NGRG has two distinguishing features: first, a beam-steering system to begin optical observations within ∼1 s after location; second, in addition to the optical camera, a separate near-IR (NIR) camera viewing the same field, greatly increasing sensitivity to extinguished bursts. These features yield the unique capability of exploring the rise phase of GRB optical-NIR emission. Thus far, among GRBs with optical afterglow detections, a peak is measured in only ∼26–40% of the light curves. The rise time for prompt, or pre-afterglow, optical emission is rarely measured, as is the transition to afterglow emission. Prompt or pre-afterglow NIR emission is even less frequently measured. Rapid-response measurements give new tools for exploration of many science topics, including optical emission mechanisms (synchrotron vs. SSC, photospheric emission) and jet characteristics (reverse vs. forward shock emission, baryon-dominated vs. magnetic dominated). The rapid-response capability also allows measurement of dynamic evolution of extinction due to vaporization of progenitor system dust. This dynamic dust measurement is the only tool we know of to separate the effects of star-system-scale dust and galactic-structure-scale dust; it is remarkable that this probe of small-scale phenomena can be used at the high redshifts where GRBs are observed. In this paper, we discuss techniques and the feasibility of these measurements, and give detection rate estimates using only measured Swift performance (without extrapolations). The NGRG will explore two new frontiers: optical and NIR GRB emission measured earlier than ever before, via rapid-response, and potentially fainter, more extinguished GRBs than ever before, via sensitive, early NIR measurements. In an era with little funding for new extragalactic science space missions, costs are important. Our modest NGRG concept will produce new GRB science, while providing crucial access to rapid GRB alerts for the community. An X-ray instrument barely 1/5 the detecting area of Swift BAT, 1024 cm^2, will yield a significant fraction of BAT’s GRB detection rate: more than 65 X-ray detections per year. With a 30 cm optical-IR telescope and modern cameras, more than 19 NIR and 14 optical band detections would be produced each year for community follow-up. In addition, active control of the beam-steering system, via feedback from a fast-read optical camera, would remove the need for arcsec pointing stabilization of the spacecraft platform, for a substantial cost saving and a wider range of potential space platforms.

Formulation and in vitro evaluation of sustained release matrix tablets of roxatidine acetate HCl by using natural and synthetic polymers

Abstract: The work focuses mainly on sustaining the release of Roxatidine acetate Hcl, formulating them in to matrix tablets by using various matrix materials like Aloe barbadensis miller mucilage, HPMC K100 and Eudragit RSPO. Plasma half–life of Roxatidine acetate Hcl after oral administration, about 5 to 6 hrs and its bioavailability is 80%, So Roxatidine acetate Hcl is suitable for sustained drug delivery system, which may improve bioavailability. The granules were evaluated for angle of repose, loose bulk density, tapped bulk density and compressibility index. The tablets were subjected to various tests for physical parameters such as thickness, hardness, friability, drug content, and in vitro release studies. The prepared tablets were found to have better pharmacopoeial standard values. The drug release data fit well to the zero order. Korsmeyer’s plot indicated that the drug release mechanism from the matrix tablet followed was Anomalous (non-Fickian) diffusion.