Leptohadronic multi-messenger modeling of 324 gamma-ray blazars

TL;DR: The multi-messenger modeling of 324 gamma-ray blazars provides insights into the relationship between neutrino production and the multiwavelength spectrum of active galaxies. The results suggest that blazars that are brighter in gamma rays have a higher neutrino production efficiency but a lower best-fit baryonic loading.

Abstract: The origin of the diffuse astrophysical neutrino flux observed by the IceCube experiment is still under debate. Multiple associations have been reported between high-energy neutrino events and individual blazars, such as the source TXS 0506+056, which are active galaxies with relativistic jets pointing toward Earth. From a theoretical perspective, the properties of these sources as neutrino emitters are not yet well understood. By systematically modeling the effect of cosmic-ray protons on the multiwavelength data from the largest sample of bright gamma-ray blazars to date, we expect to learn about the multi-messenger nature of the active galaxy population as a whole, as well as the relationship between neutrino production and the multiwavelength spectrum of these sources. We predict the emitted multiwavelength and neutrino spectrum using a self-consistent numerical radiation model applied individually to each source in the sample. We then study the properties of the full population and identify empirical relations. We focus on public multiwavelength data from the radio to the gamma-ray bands from a sample of 324 blazars detected by the Fermi Large Area Telescope (LAT), most of which are flat-spectrum radio quasars (FSRQs). This amounts to 34 of all FSRQs in the latest Fermi catalog. We demonstrate that the optical and gigaelectronvolt gamma-ray broadband features are generally well described by electron emission, which helps for the location of the emission region relative to the central black hole to be constrained. For 33 of the blazars in our sample, a description of the observed X-ray spectrum benefits from an additional component from proton interactions, in agreement with recent studies of individual IceCube candidate blazars. We show that, on average, blazars that are brighter in gigaelectronvolt gamma rays have a higher neutrino production efficiency but a lower best-fit baryonic loading. The predicted neutrino luminosity shows a positive correlation both with the observed flux of gigaelectronvolt gamma rays and with the predicted flux of megaelectronvolt gamma rays. We also estimate the diffuse neutrino flux from gamma-ray blazars by extrapolating the result to the Fermi population, and we show that it may be at the level of sim 20 of the diffuse neutrino flux observed by IceCube, in agreement with current limits from stacking analyses. We discuss the implications of our results for future neutrino searches and suggest promising sources for potential detections.