20 research outputs found
Prompt emission polarimetry of Gamma Ray Bursts with ASTROSAT CZT-Imager
X-ray and Gamma-ray polarization measurements of the prompt emission of
Gamma-ray bursts (GRBs) are believed to be extremely important for testing
various models of GRBs. So far, the available measurements of hard X-ray
polarization of GRB prompt emission have not significantly constrained the GRB
models, particularly because of the difficulty of measuring polarization in
these bands. The CZT Imager (CZTI) onboard {\em AstroSat} is primarily an X-ray
spectroscopic instrument that also works as a wide angle GRB monitor due to the
transparency of its support structure above 100 keV. It also has experimentally
verified polarization measurement capability in the 100 300 keV energy
range and thus provides a unique opportunity to attempt spectro-polarimetric
studies of GRBs. Here we present the polarization data for the brightest 11
GRBs detected by CZTI during its first year of operation. Among these, 5 GRBs
show polarization signatures with 3, and 1 GRB shows
2 detection significance. We place upper limits for the remaining 5
GRBs. We provide details of the various tests performed to validate our
polarization measurements. While it is difficult yet to discriminate between
various emission models with the current sample alone, the large number of
polarization measurements CZTI expects to gather in its minimum lifetime of
five years should help to significantly improve our understanding of the prompt
emission.Comment: Accepted for Publication in ApJ ; a figure has been update
Fabrication of Cd target for the astrophysical p-process studies
The detailed process of preparing enriched Cd targets on mylar and
copper backing using the vacuum evaporation technique is described. These
targets were employed in an experiment to measure the proton capture
cross-section at energies significantly below the Coulomb barrier, for the
astrophysical p-process studies. Due to the low melting point and high vapor
pressure of cadmium, some adjustments were implemented in the Telemark
multipocket e-beam setup. The target thickness was determined through the
measurement of alpha particle energy loss from a triple alpha source and also
by RBS measurements. The thickness of the Cd films varies between 290
to 660 g/cm, with a non-uniformity of approximately 10. X-ray
Photoelectron Spectroscopy (XPS) and X-ray Fluorescence (XRF) analyses were
conducted to examine the presence of impurities and to assess surface
morphology, phase, and chemical composition
Lindhard and RPA susceptibility computations in extended momentum space in electron doped cuprates
We present an approximation for efficient calculation of the Lindhard
susceptibility in a periodic system through the use of
simple products of real space functions and the fast Fourier transform (FFT).
The method is illustrated by providing results for the
electron doped cuprate NdCeCuO extended over several
Brillouin zones. These results are relevant for interpreting inelastic X-ray
scattering spectra from cuprates.Comment: 6 pages, 6 figures, accepted in Physical Review
Dark Matter in Minimal Model
We study the gauge extension of the Standard Model which contains a singlet scalar and three right-handed neutrinos. The third generation right-handed neutrino is qualified as the dark matter candidate, as an artifact of -charge assignment. Relic abundance of the dark matter is consistent with WMAP9 and PLANCK data, only near scalar resonances. Requiring correct relic abundance, we restrict the parameter space of the scalar mixing angle and mass of the heavy scalar boson of this model
Galactic center gamma-ray excess and higgs-portal dark matter
From astronomical observations, we know that dark matter exists and makes up ∼25 % of our Universe. Recently the study of anomalous gamma-ray emission in the regions surrounding the galactic center has drawn a lot of attention. It has been pointed out that the excess of 1–3 GeV gamma-ray in the low latitude is consistent with the emission expected from annihilating dark matter. I will discuss the Higgs-portal dark matter models which can explain these phenomena because of the presence of scalar resonance. In addition, the parameter space of these models also satisfy constraints from the LHC Higgs searches, relic abundance and direct detection experiments. The gauged U(1)B−L model is very well suited with the FERMI-LAT observation along with other constraints.by Tanmoy Mondal and Tanushree Basa