Prof Subir Sarkar

Abstract:

We investigate the possibility that blazars in the Roma-BZCAT Multifrequency Catalogue of Blazars (5BZCAT) are sources of the high-energy astrophysical neutrinos detected by the IceCube Neutrino Observatory, as recently suggested by Buson et al. (2022a,b). Although we can reproduce their $\sim 4.6\, \sigma$ result, which applies to 7 years of neutrino data in the Southern sky, we find no significant correlation with 5BZCAT sources when extending the search to the Northern sky, where IceCube is most sensitive to astrophysical signals. To further test this scenario, we use a larger sample consisting of 10 years of neutrino data recently released by the IceCube collaboration, this time finding no significant correlation in either the Southern or the Northern sky. These results suggest that the strong correlation reported by Buson et al. (2022a,b) using 5BZCAT could be due to a statistical fluctuation and possibly the spatial and flux non-uniformities in the blazar sample. We perform some additional correlation tests using the more uniform, flux-limited, and blazar-dominated Radio Fundamental Catalogue (RFC) and find a $\sim 3.2\sigma$ equivalent p-value when correlating it with the 7-year Southern neutrino sky. However, this correlation disappears completely when extending the analysis to the Northern sky and when analyzing 10 years of all-sky neutrino data. Our findings support a scenario where the contribution of the whole blazar class to the IceCube signal is relevant but not dominant, in agreement with most previous studies.Comment: Accepted for publication in ApJ

Abstract:

In view of the growing tension between the dipole anisotropy of number counts of cosmologically distant sources and of the cosmic microwave background (CMB), we investigate the number count dipole induced by primordial perturbations with wavelength comparable to or exceeding the Hubble radius today. First, we find that neither adiabatic nor isocurvature superhorizon modes can generate an intrinsic number count dipole. However a superhorizon isocurvature mode does induce a relative velocity between the CMB and the (dark) matter rest frames and thereby affects the CMB dipole. We revisit the possibility that it has an intrinsic component due to such a mode, thus enabling consistency with the galaxy number count dipole if the latter is actually kinematic in origin. Although this scenario is not particularly natural, there are possible links with other anomalies and it predicts a concommitant galaxy number count quadrupole which may be measurable in future surveys. We also investigate the number count dipole induced by modes smaller than the Hubble radius, finding that subject to CMB constraints this is too small to reconcile the dipole tension.