Prof Subir Sarkar

Abstract:

Astrophysical transient events like Gamma Ray Bursts (GRBs) have always been promising candidates for multi-messenger astronomy, with electromagnetic and gravitational wave signals having already been observed in GRBs such as GRB 170817A. The neutrino signatures of these bursts have been long-awaited as well, with many models predicting different spectra. Most of these searches have been in the hundreds of GeV to PeV range. However, as different models indicate a possible lower energy neutrino signal, we intend to expand this search to the lowest limits of IceCube (0.5-5 GeV) as well. With the plan to look at more transient events, we present the result of the first IceCube search for < 5 GeV astrophysical neutrinos emitted from a GRB, for GRB 221009A; the brightest GRB ever observed. Furthermore, we present plans to improve the observations of < 5 GeV neutrinos in IceCube, with which we plan to probe more transient events in the future. These improvements include the addition of direction reconstruction at these energies, and optimization of the noise rejection. With these improvements, GRB 221009A is just the start of the low-energy neutrino search from transient events with IceCube.

Abstract:

Active galactic nuclei (AGN) are prime candidate sources of the high-energy, astrophysical neutrinos detected by IceCube. This is demonstrated by the real-time multimessenger detection of the blazar TXS 0506+056 and the recent evidence of neutrino emission from NGC 1068 from a separate time-averaged study. However, the production mechanism of the astrophysical neutrinos in AGN is not well established, which can be resolved via correlation studies with photon observations. For neutrinos produced due to photohadronic interactions in AGN, in addition to a correlation of neutrinos with high-energy photons, there would also be a correlation of neutrinos with photons emitted at radio wavelengths. In this work, we perform an in-depth stacking study of the correlation between 15 GHz radio observations of AGN reported in the MOJAVE XV catalog, and 10 yr of neutrino data from IceCube. We also use a time-dependent approach, which improves the statistical power of the stacking analysis. No significant correlation was found for both analyses, and upper limits are reported. When compared to the IceCube diffuse flux, at 100 TeV and for a spectral index of 2.5, the upper limits derived are ∼3% and ∼9% for the time-averaged and time-dependent cases, respectively.