Prof Subir Sarkar

Abstract:

We derive lower bounds on the density of sources of ultra-high energy cosmic rays from the lack of significant clustering in the arrival directions of the highest energy events detected at the Pierre Auger Observatory. The density of uniformly distributed sources of equal intrinsic intensity was found to be larger than ∼ (0.06-5) × 10-4 Mpc-3 at 95% CL, depending on the magnitude of the magnetic deflections. Similar bounds, in the range (0.2-7) × 10-4 Mpc-3, were obtained for sources following the local matter distribution. © 2013 IOP Publishing Ltd and Sissa Medialab srl.

Abstract:

We report on results of an all-sky search for high-energy neutrino events interacting within the IceCube neutrino detector conducted between May 2010 and May 2012. The search follows up on the previous detection of two PeV neutrino events, with improved sensitivity and extended energy coverage down to about 30 TeV. Twenty-six additional events were observed, substantially more than expected from atmospheric backgrounds. Combined, both searches reject a purely atmospheric origin for the 28 events at the 4σ level. These 28 events, which include the highest energy neutrinos ever observed, have flavors, directions, and energies inconsistent with those expected from the atmospheric muon and neutrino backgrounds. These properties are, however, consistent with generic predictions for an additional component of extraterrestrial origin.

Abstract:

We study the Galactic radio background produced by synchrotron emission of cosmic ray electrons in Galactic magnetic fields. The angular power spectrum of all-sky maps contains contributions from physical processes on a variety of scales, including the variation of emissivity on large (kpc) scales and the small-scale turbulence in the interstellar medium. Previous studies have focussed on one scale or the other but have not provided a self-consistent picture valid on all scales. We show that the angular power spectrum in the multipole range l ∼ 10 − 100 has a deficit with respect to observations at 408 MHz and suggest that the shells of old supernova remnants (like the well-known ‘radio loops’) can provide the missing power. We model the emissivity from individual shells and find that the Galactic population of O(1000) shells naturally provides the missing power and brings the modelled angular power spectrum into agreement with the observed one. This has important implications for studies of the cosmic microwave background, in particular B-mode polarisation.