Prof Subir Sarkar

Abstract:

The Advanced LIGO observatories detected gravitational waves from two binary black hole mergers during their first observation run (O1). We present a high-energy neutrino follow-up search for the second gravitational wave event, GW151226, as well as for gravitational wave candidate LVT151012. We find 2 and 4 neutrino candidates detected by IceCube, and 1 and 0 detected by ANTARES, within $\pm500$ s around the respective gravitational wave signals, consistent with the expected background rate. None of these neutrino candidates are found to be directionally coincident with GW151226 or LVT151012. We use non-detection to constrain isotropic-equivalent high-energy neutrino emission from GW151226 adopting the GW event's 3D localization, to less than $2\times 10^{51}-2\times10^{54}$ erg.

Abstract:

Previous studies have found our velocity in the rest frame of radio galaxies at high redshift to be much larger than that inferred from the dipole anisotropy of the cosmic microwave background. We construct a full sky catalogue, NVSUMSS, by merging the NRAO VLA Sky Survey and the Sydney University Molonglo Sky Survey catalogues and removing local sources by various means including cross-correlating with the 2MASS Redshift Survey catalogue. We take into account both aberration and Doppler boost to deduce our velocity from the hemispheric number count asymmetry, as well as via a three-dimensional linear estimator. Both its magnitude and direction depend on cuts made to the catalogue, e.g. on the lowest source flux; however these effects are small. From the hemispheric number count asymmetry we obtain a velocity of 1729 ± 187 km s−1, i.e. about four times larger than that obtained from the cosmic microwave background dipole, but close in direction, towards RA=149° ± 2°, Dec. = −17° ± 12°. With the three-dimensional estimator, the derived velocity is 1355 ± 174 km s−1 towards RA = 141° ± 11°, Dec. = −9° ± 10°. We assess the statistical significance of these results by comparison with catalogues of random distributions, finding it to be 2.81σ (99.75 per cent confidence).

Abstract:

We present a search for a light sterile neutrino using three years of atmospheric neutrino data from the DeepCore detector in the energy range of approximately 10-60 GeV. DeepCore is the low-energy subarray of the IceCube Neutrino Observatory. The standard three-neutrino paradigm can be probed by adding an additional light (Δm412∼1 eV2) sterile neutrino. Sterile neutrinos do not interact through the standard weak interaction and, therefore, cannot be directly detected. However, their mixing with the three active neutrino states leaves an imprint on the standard atmospheric neutrino oscillations for energies below 100 GeV. A search for such mixing via muon neutrino disappearance is presented here. The data are found to be consistent with the standard three-neutrino hypothesis. Therefore, we derive limits on the mixing matrix elements at the level of |Uμ4|2 < 0.11 and |Uτ4|2 < 0.15 (90% C.L.) for the sterile neutrino mass splitting Δm412=1.0 eV2.

Abstract:

© 2017 COSPAR. The core mission of the IceCube neutrino observatory is to study the origin and propagation of cosmic rays. IceCube, with its surface component IceTop, observes multiple signatures to accomplish this mission. Most important are the astrophysical neutrinos that are produced in interactions of cosmic rays, close to their sources and in interstellar space. IceCube is the first instrument that measures the properties of this astrophysical neutrino flux and constrains its origin. In addition, the spectrum, composition, and anisotropy of the local cosmic-ray flux are obtained from measurements of atmospheric muons and showers. Here we provide an overview of recent findings from the analysis of IceCube data, and their implications to our understanding of cosmic rays.