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 $\gtrapprox$3$\sigma$, and 1 GRB shows $\>$2$\sigma$ 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

Lindhard and RPA susceptibility computations in extended momentum space in electron doped cuprates

We present an approximation for efficient calculation of the Lindhard susceptibility $\chi^{L}(q,\omega)$ 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 $\chi^{L}(q,\omega)$ results for the electron doped cuprate Nd$_{2-x}$Ce$_{x}$CuO$_{4}$ 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 U(1)B−LU(1)_{B-L} Model

We study the $B-L$ 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 $Z_2$-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

Class of higgs-portal dark matter models in the light of gamma-ray excess from galactic center

Recently the study of anomalous gamma-ray emission in the regions surrounding the galactic center has drawn a lot of attention as it points out that the excess of ∼1–3 GeV∼1–3 GeV gamma-ray in the low latitude is consistent with the emission expected from annihilating dark matter. The best-fit to the gamma-ray spectrum corresponds to dark matter (DM) candidate having mass in the range ∼31–40 GeV∼31–40 GeV annihilating into View the MathML sourcebb¯-pair with cross-section View the MathML source〈σv〉=(1.4–2.0)×10−26 cm3s−1. We have shown that the Higgs-portal dark matter models in presence of scalar resonance (in the annihilation channel) are well-suited for explaining these phenomena. In addition, the parameter space of these models also satisfies constraints from the LHC Higgs searches, relic abundance and direct detection experiments. We also comment on real singlet scalar Higgs-portal DM model which is found to be incompatible with the recent analysis.by Tanmoy Mondal and Tanushree Basa