Active galactic nuclei (AGNs) are leading candidates for the sources of high-energy astrophysical neutrinos—tiny, nearly massless particles—detected by the IceCube Neutrino Observatory at the South Pole. This is demonstrated by the real-time multimessenger detection of the blazar TXS 0506+056 and recent evidence of neutrino emission from NGC 1068 from a separate time-averaged study. However, the exact processes by which AGNs produce these neutrinos are not well understood, a gap that could be bridged through studies correlating neutrino detections with observations of photons.
Currently, such studies, especially involving time-dependent information, are hindered by the limited overlap between photon and neutrino observations, which reduces their effectiveness. This challenge can be addressed by identifying more neutrino sources and conducting thorough parallel studies with photon data sets.
In a study accepted for publication by The Astrophysical Journal, the IceCube Collaboration presents a search for neutrino production in AGNs via photo-hadronic interactions. These interactions are expected to result in correlations not only with high-energy photons but also with photons emitted at radio wavelengths. No significant correlation was found for both time-averaged and time-dependent analyses, and upper limits were reported.
The research team leveraged time-dependent photon information published in the MOJAVE (Monitoring Of Jets in Active galactic nuclei with VLBA Experiments) XV catalog containing 15 GHz radio observations of AGN sources over a 20-year period. Using 10 years of IceCube data, they performed a stacking analysis that combined data from multiple sources, increasing the signal-to-noise ratio and sensitivity to detect weak signals and mitigate biases. This was used to test the hypothesis that there is a 1:1 correlation between the 15-GHz radio flux and the high-energy neutrino flux.
When compared to the IceCube total astrophysical neutrino flux, at 100 TeV and for a spectral index of 2.5, the upper limits derived are roughly 3% and 9% for the time-averaged and time-dependent cases, respectively.
“Including time-dependent light curves enhances the statistical power of stacking analyses when compared with the time-averaged study,” says Abhishek Desai, a former John Bachall postdoctoral fellow at the Wisconsin IceCube Particle Astrophysics Center at the University of Wisconsin–Madison (UW–Madison), who led the study. Desai is now a NASA Postdoctoral Program fellow at the NASA Goddard Space Flight Center.
Adds Desai, “Although both analyses show similar sensitivity, the time-dependent study, incorporating temporal information, yields higher best-fit test statistic values and alters the outcomes.”
The results showcase the power of adding time-dependent information to IceCube stacking analyses and highlights the importance of performing a study that includes all available neutrino and photon data. Despite the computational intensity of large-scale light curve stacking analyses, these findings motivate future research to employ time-dependent information for enhanced statistical power.
“Using the full archive of IceCube data, including both the highest energy neutrinos IceCube detects and those at lower energies, provides excellent sensitivity in searching for neutrino sources” says UW–Madison associate professor Justin Vandenbroucke. “This systematic search did not find any evidence for neutrinos associated with radio-bright AGN, enabling useful constraints on the physics of these extreme environments.”
+ info “Probing the connection between IceCube neutrinos and MOJAVE AGN,” IceCube Collaboration: R. Abbasi et al. Accepted by The Astrophysical Journal, arxiv.org/abs/2407.01351