Prof Subir Sarkar

Abstract:

Context. Transient neutrino sources such as Gamma-Ray Bursts (GRBs) and Supernovae (SNe) are hypothesized to emit bursts of high-energy neutrinos on a time-scale of \lesssim 100 s. While GRB neutrinos would be produced in high relativistic jets, core-collapse SNe might host soft-relativistic jets, which become stalled in the outer layers of the progenitor star leading to an efficient production of high-energy neutrinos. Aims. To increase the sensitivity to these neutrinos and identify their sources, a low-threshold optical follow-up program for neutrino multiplets detected with the IceCube observatory has been implemented. Methods. If a neutrino multiplet, i.e. two or more neutrinos from the same direction within 100 s, is found by IceCube a trigger is sent to the Robotic Optical Transient Search Experiment, ROTSE. The 4 ROTSE telescopes immediately start an observation program of the corresponding region of the sky in order to detect an optical counterpart to the neutrino events. Results. No statistically significant excess in the rate of neutrino multiplets has been observed and furthermore no coincidence with an optical counterpart was found. Conclusion. The search allows, for the first time, to set stringent limits on current models predicting a high-energy neutrino flux from soft relativistic hadronic jets in core-collapse SNe. We conclude that a sub-population of SNe with typical Lorentz boost factor and jet energy of 10 and 3\times10^{51} erg, respectively, does not exceed 4.2% at 90% confidence.

Abstract:

We present a comprehensive study of the influence of the geomagnetic field on the energy estimation of extensive air showers with a zenith angle smaller than $60^\circ$, detected at the Pierre Auger Observatory. The geomagnetic field induces an azimuthal modulation of the estimated energy of cosmic rays up to the ~2% level at large zenith angles. We present a method to account for this modulation of the reconstructed energy. We analyse the effect of the modulation on large scale anisotropy searches in the arrival direction distributions of cosmic rays. At a given energy, the geomagnetic effect is shown to induce a pseudo-dipolar pattern at the percent level in the declination distribution that needs to be accounted for.

Abstract:

It has been shown that very light or even massless neutralinos are consistent with all current experiments, given non-universal gaugino masses. Furthermore, a very light neutralino is consistent with astrophysical bounds from supernovæ and cosmological bounds on dark matter. Here we study the cosmological constraints on this scenario from Big Bang nucleosynthesis taking gravitinos into account and find that a very light neutralino is even favoured by current observations.

Abstract:

The IceCube Neutrino Observatory at the South Pole has been completed in December 2010. In this paper we describe the final detector and report results on physics and performance using data taken at different stages of the yet incomplete detector. No signals for cosmic neutrinos from point sources and diffuse fluxes have been found. Prospects of these searches, including the setup of multi-messenger programs, are discussed. The limits on neutrinos from GRBs, being far below model predictions, require a reevaluation of GRB model assumptions. Various measurements of cosmic ray properties have been obtained from atmospheric muon and neutrino spectra and from air shower measurements; these results will have an important impact on model developments. IceCube observed an anisotropy of cosmic rays on multiple angular scales, for the first time in the Southern sky. The unique capabilities of IceCube for monitoring transient low energy events are briefly discussed. Finally an outlook to planned extensions is given which will improve the sensitivities both on the low and high energy side.