SAAO Weekly Colloquium 03 July 2025 by Dr. Sasha (Alexander) Plavin

Speaker: Dr. Sasha (Alexander) Plavin
Postdoctoral researcher at Harvard University, Black Hole Initiative

Title: Hunting neutrino blazars in the multimessenger era, from statistical surveys to focused studies

Abstract: High-energy neutrinos traverse the universe unimpeded, offering a direct probe of the engines that accelerate particles to extreme energies. Blazars – active galactic nuclei whose relativistic jets point toward Earth – are prime candidates for such engines, yet their role in the neutrino sky has long been debated. I present results from a collection of recent studies that combine IceCube neutrino data with mas- or pc-resolution Very Long Baseline Interferometry (VLBI), demonstrating that blazars as a population are a major source of TeV–PeV neutrinos. As for electromagnetic radiation, relativistic beaming proves crucial: Doppler-boosted jets dramatically enhance the neutrino flux that reaches us. Key evidence comes from broad, flux-limited surveys that provide the statistical power to isolate a collective neutrino signal. Complementary insight is gained through deep VLBI observations of the most notable candidates, which expose the inner-jet structures responsible for producing that emission. Together these efforts provide a coherent pathway for pinpointing – and ultimately understanding – the neutrino-emitting blazars and their jet properties. The results highlight the importance of both broad-sample surveys and focused observations for multimessenger studies of astrophysical sources.