Prof Subir Sarkar

Abstract:

The energy at which cosmic rays from extra-galactic sources begin to dominate over those from galactic sources is an important open question in astroparticle physics. A natural candidate is the energy at the 'ankle' in the approximately power-law energy spectrum which is indicative of a cross-over from a falling galactic component to a flatter extra-galactic component. The transition can occur without such flattening but this requires some degree of conspiracy of the spectral shapes and normalizations of the two components. Nevertheless it has been argued that extra-galactic sources of cosmic ray protons which undergo interactions on the CMB can reproduce the energy spectrum below the ankle if the cross-over energy is as low as the 'second knee' in the spectrum. This low cross-over model is constrained by direct measurements by the Pierre Auger Observatory which indicate a heavier composition at these energies. We demonstrate that upper limits on the cosmic diffuse neutrino flux provide a complementary constraint on the proton fraction in ultra-high energy extra-galactic cosmic rays and forthcoming data from IceCube will provide a definitive test of this model.

Abstract:

We present new results of searches for neutrino point sources in the northern sky, using data recorded in 2007-2008 with 22 strings of the IceCube detector (approximately one-fourth of the planned total) and 275.7 days of live time. The final sample of 5114 neutrino candidate events agrees well with the expected background of atmospheric muon neutrinos and a small component of atmospheric muons. No evidence of a point source is found, with the most significant excess of events in the sky at 2.2σ after accounting for all trials. The average upper limit over the northern sky for point sources of muon-neutrinos with E -2 spectrum is E -2 φv le; 1.4 × 10-11 TeV cm?2 ≤ s?1, in the energy range from 3TeV to 3PeV, improving the previous best average upper limit by the AMANDA-II detector by a factor of 2. © 2009. The American Astronomical Society.

Abstract:

We report on a search with the IceCube detector for high-energy muon neutrinos from GRB080319B, one of the brightest gamma-ray bursts (GRBs) ever observed. The fireball model predicts that a mean of 0.1 events should be detected by IceCube for a bulk Lorentz boost of the jet of 300. In both the direct on-time window of 66s and an extended window of about 300s around the GRB, no excess was found above background. The 90% CL upper limit on the number of track-like events from the GRB is 2.7, corresponding to a muon neutrino fluence limit of 9.5 × 10-3 erg cm-2 in the energy range between 120 TeV and 2.2 PeV, which contains 90% of the expected events. © 2009 The American Astronomical Society. All rights reserved.

Abstract:

Recent measurements of the spectrum and composition of ultrahigh energy cosmic rays suggest that their extragalactic sources may be accelerating heavy nuclei in addition to protons. This can suppress the cosmogenic neutrino flux relative to the usual expectation for an all-proton composition. Cosmic neutrino detectors may therefore need to be even larger than currently planned but conversely they will also be able to provide valuable information concerning astrophysical accelerators. Moreover measurement of ultrahigh energy cosmic neutrino interactions can provide an unique probe of QCD dynamics at high parton density.