Showing papers by "Pawan Kumar published in 2011"

Implications of electron acceleration for high-energy radiation from gamma-ray bursts

Abstract: In recent work, we suggested that photons of energy >100 MeV detected from gamma-ray bursts (GRBs) by the Fermi satellite are produced via synchrotron emission in the external forward shock with a weak magnetic field - consistent with shock-compressed upstream magnetic field of a few tens of μG. Here we investigate whether electrons can be accelerated to energies such that they radiate synchrotron photons with energy up to about 10 GeV in this particular scenario. We do this using two methods: (i) we check if these electrons can be confined to the shock front; and (ii) we calculate radiative losses while they are being accelerated. We find that these electrons remain confined to the shock front, as long as the upstream magnetic field is ≳ 10 μG, and do not suffer substantial radiative losses, the only condition required is that the external reverse shock emission be not too bright: peak flux less than 1 Jy in order to produce photons of 100 MeV and less than ∼100 mJy for producing 1-GeV photons. We also find that the acceleration time for electrons radiating at 100 MeV is a few seconds (in observer frame) and the acceleration time is somewhat longer for electrons radiating at a few GeV. This could explain the lack of > 100 MeV photons for the first few seconds after the trigger time for long GRBs reported by the Fermi satellite and also the slight lag between photons of GeV and 100 MeV energies. We model the onset of the external forward shock light curve in this scenario and find it consistent with the sharp rise observed in the 100-MeV light curve of GRB 080916C and similar bursts.

Constraints on Cold Magnetized Shocks in Gamma-Ray Bursts

Abstract: We consider a model in which the ultrarelativistic jet in a gamma-ray burst (GRB) is cold and magnetically accelerated. We assume that the energy flux in the outflowing material is partially thermalized via internal shocks or a reverse shock, and we estimate the maximum amount of radiation that could be produced in such magnetized shocks. We compare this estimate with the available observational data on prompt γ -ray emission in GRBs. We find that, even with highly optimistic assumptions, the magnetized jet model is radiatively too inefficient to be consistent with observations. One way out is to assume that much of the magnetic energy in the post-shock, or even pre-shock, jet material is converted to particle thermal energy by some unspecified process, and then radiated. This can increase the radiative efficiency sufficiently to fit observations. Alternatively, jet acceleration may be driven by thermal pressure rather than magnetic fields. In this case, which corresponds to the traditional fireball model, sufficient prompt GRB emission could be produced either from shocks at a large radius or from the jet photosphere closer to the centre.

Constraints on Cold Magnetized Shocks in Gamma-Ray Bursts

Abstract: We consider a model in which the ultra-relativistic jet in a gamma-ray burst (GRB) is cold and magnetically accelerated. We assume that the energy flux in the outflowing material is partially thermalized via internal shocks or a reverse shock, and we estimate the maximum amount of radiation that could be produced in such magnetized shocks. We compare this estimate with the available observational data on prompt gamma-ray emission in GRBs. We find that, even with highly optimistic assumptions, the magnetized jet model is radiatively too inefficient to be consistent with observations. One way out is to assume that much of the magnetic energy in the post-shock, or even pre-shock, jet material is converted to particle thermal energy by some unspecified process, and then radiated. This can increase the radiative efficiency sufficiently to fit observations. Alternatively, jet acceleration may be driven by thermal pressure rather than magnetic fields. In this case, which corresponds to the traditional fireball model, sufficient prompt GRB emission could be produced either from shocks at a large radius or from the jet photosphere closer to the center.

Magnetic jet model for GRBs and the delayed arrival of >100 MeV photons

Abstract: Photons of energy larger than 100 MeV from long-GRBs arrive a few seconds after 100 MeV. The delay depends almost linearly on burst redshift, and on the distance from the central compact object where the jet is launched (R_0). Therefore, the delay in arrival of >10^2 MeV photons can be used to estimate burst redshift if the magnetic jet model for gamma-ray generation is correct and R_0 is roughly the same for long-GRBs.

Evidence for mild deviation from power-law distribution of electrons in relativistic shocks: GRB 090902B

Abstract: Many previous studies have determined that the long lasting emission at X-ray, optical and radio wavelengths from gamma-ray bursts (GRBs), called the afterglow, is likely produced by the external forward shock model. In this model, the GRB jet interacts with the circum-stellar medium and drives a shock that heats the medium, which radiates via synchrotron emission. In this work, we carried out a detailed analysis of the late time afterglow data of GRB 090902B using a very careful accounting of the Inverse Compton losses. We find that in the context of the external forward shock model, the only viable option to explain the X-ray and optical data of GRB 090920B is to have the electron energy distribution deviate from a power-law shape and exhibit some slight curvature immediately downstream of the shock front (we explored other models that rely on a single power-law assumption, but they all fail to explain the observations). We find the fraction of the energy of shocked plasma in magnetic field to be ∼10 −6 using late time afterglow data, which is consistent with the value obtained using early gamma-ray data. Studies like the present one might be able to provide a link between GRB afterglow modelling and numerical simulations of particle acceleration in collisionless shocks. We also provide detailed calculations for the early (10 3 s) high-energy (>100 MeV) emission and confirm that it is consistent with origin in the external forward shock. We investigated the possibility that the ∼10 keV excess observed in the spectrum during the prompt phase also has its origin in the external shock and found the answer to be negative.

Evidence for mild deviation from power-law distribution of electrons in relativistic shocks: GRB 090902B

Abstract: Many previous studies have determined that the long lasting emission at X-ray, optical and radio wavelengths from gamma-ray bursts (GRBs), called the afterglow, is likely produced by the external forward shock model. In this model, the GRB jet interacts with the circum-stellar medium and drives a shock that heats the medium, which radiates via synchrotron emission. In this work, we carried out a detailed analysis of the late time afterglow data of GRB 090902B using a very careful accounting of the Inverse Compton losses. We find that in the context of the external forward shock model, the only viable option to explain the X-ray and optical data of GRB 090920B is to have the electron energy distribution deviate from a power-law shape and exhibit some slight curvature immediately downstream of the shock front (we explored other models that rely on a single power-law assumption, but they all fail to explain the observations). We find the fraction of the energy of shocked plasma in magnetic field to be ~10^{-6} using late time afterglow data, which is consistent with the value obtained using early gamma-ray data. Studies like the present one might be able to provide a link between GRB afterglow modeling and numerical simulations of particle acceleration in collisionless shocks. We also provide detailed calculations for the early ( 100 MeV) emission and confirm that it is consistent with origin in the external forward shock. We investigated the possibility that the ~10 keV excess observed in the spectrum during the prompt phase also has its origin in the external shock and found the answer to be negative.

Simulations of Accretion Powered Supernovae in the Progenitors of Gamma Ray Bursts

Abstract: Observational evidence suggests a link between long duration gamma ray bursts (LGRBs) and Type Ic supernovae. Here, we propose a potential mechanism for Type Ic supernovae in LGRB progenitors powered solely by accretion energy. We present spherically-symmetric hydrodynamic simulations of the long-term accretion of a rotating gamma-ray burst progenitor star, a "collapsar," onto the central compact object, which we take to be a black hole. The simulations were carried out with the adaptive mesh refinement code FLASH in one spatial dimension and with rotation, an explicit shear viscosity, and convection in the mixing length theory approximation. Once the accretion flow becomes rotationally supported outside of the black hole, an accretion shock forms and traverses the stellar envelope. Energy is carried from the central geometrically thick accretion disk to the stellar envelope by convection. Energy losses through neutrino emission and nuclear photodisintegration are calculated but do not seem important following the rapid early drop of the accretion rate following circularization. We find that the shock velocity, energy, and unbound mass are sensitive to convective efficiency, effective viscosity, and initial stellar angular momentum. Our simulations show that given the appropriate combinations of stellar and physical parameters, explosions with energies ~5x10^50 ergs, velocities 3000 km/s, and unbound material masses >6 solar masses are possible in a rapidly rotating 16 solar mass main sequence progenitor star. Further work is needed to constrain the values of these parameters, to identify the likely outcomes in more plausible and massive LRGB progenitors, and to explore nucleosynthetic implications.

Formulation and Evaluation of Fast Dissolving Tablet of Telmisartan

Abstract: Fast dissolving drug delivery system offers a solution for those patients having difficulty in swallowing tablet. In the present study, an attempt has been made to prepare fast dissolving tablets of the drug telmisartan using meglumine as an organic base. Telmisartan, in the presence of acidic pH rapidly ionized and which leads to decrease absorption. This problem can be overcome by using meglumine. Meglumine is an organic base used as a pH-adjusting agent. Fast dissolving tablets of telmisartan were prepared by direct compression method using three superdisintegrants crospovidone, sodium starch glycolate and croscarmellose sodium in different concentrations. Ten formulations having superdisintegrants at different concentration levels were prepared to assess their efficiency and critical concentration level. Different types of evaluation parameters were used. Tablets containing crospovidone showed superior organoleptic properties, along with excellent in vitro and a vivo dispersion time and drug release, as compared to other formulations.