Prof Subir Sarkar

Abstract:

We present a measurement of the atmospheric νe spectrum at energies between 0.1 and 100 TeV using data from the first year of the complete IceCube detector. Atmospheric νe originate mainly from the decays of kaons produced in cosmic-ray air showers. This analysis selects 1078 fully contained events in 332 days of live time, and then identifies those consistent with particle showers. A likelihood analysis with improved event selection extends our previous measurement of the conventional νe fluxes to higher energies. The data constrain the conventional νe flux to be 1.3-0.3+0.4 times a baseline prediction from a Honda's calculation, including the knee of the cosmic-ray spectrum. A fit to the kaon contribution (ξ) to the neutrino flux finds a kaon component that is ξ=1.3-0.4+0.5 times the baseline value. The fitted/measured prompt neutrino flux from charmed hadron decays strongly depends on the assumed astrophysical flux and shape. If the astrophysical component follows a power law, the result for the prompt flux is 0.0-0.0+3.0 times a calculated flux based on the work by Enberg, Reno, and Sarcevic.

Abstract:

The "standard" model of cosmology is founded on the basis that the expansion rate of the universe is accelerating at present --- as was inferred originally from the Hubble diagram of Type Ia supernovae. There exists now a much bigger database of supernovae so we can perform rigorous statistical tests to check whether these "standardisable candles" indeed indicate cosmic acceleration. Taking account of the empirical procedure by which corrections are made to their absolute magnitudes to allow for the varying shape of the light curve and extinction by dust, we find, rather surprisingly, that the data are still quite consistent with a constant rate of expansion.

Abstract:

© 2015. The American Astronomical Society. All rights reserved.. We present constraints derived from a search of four years of IceCube data for a prompt neutrino flux from gamma-ray bursts (GRBs). A single low-significance neutrino, compatible with the atmospheric neutrino background, was found in coincidence with one of the 506 observed bursts. Although GRBs have been proposed as candidate sources for ultra-high-energy cosmic rays, our limits on the neutrino flux disfavor much of the parameter space for the latest models. We also find that no more than ∼1% of the recently observed astrophysical neutrino flux consists of prompt emission from GRBs that are potentially observable by existing satellites.