[Paper Review] Hard X-ray emission from blazars associated with high-energy neutrinos

A. V. Plavin, R. Burenin+5 more|arXiv (Cornell University)|Jun 1, 2023

Astrophysics and Cosmic Phenomena78 references4 citations

TL;DR

This study identifies a strong observational correlation between hard X-ray emission (≥10 keV) from blazars and high-energy neutrinos detected by IceCube, suggesting that X-rays in the jet region serve as target photons for proton-photon interactions that produce neutrinos. The results imply that neutrino production occurs in compact, relativistically beamed jets, with hard X-rays tracing the site of neutrino generation via synchrotron-self-Compton processes.

ABSTRACT

Bright blazars were found to be prominent neutrino sources, and a number of IceCube events were associated with them. Evaluating high-energy photon emission of such blazars is crucial for better understanding of the processes and regions where neutrinos are produced. Here, we focus on hard X-ray emission observed by the SRG/ART-XC telescope, by the Swift/BAT imager, and by the INTEGRAL/IBIS telescope. Their energy range ~10 keV is well-suited for probing photons that potentially participate in neutrino production by interacting with ultrarelativistic protons. We find that neutrino-associated blazars tend to demonstrate remarkably strong X-ray emission compared to other VLBI blazars in the sky. Both neutrinos and hard X-rays are found to come from blazars at cosmological distances z ~ 1, and are boosted by relativistic beaming that makes it possible to detect them on Earth. Our results suggest that neutrinos are produced within compact blazar jets, with target X-ray photons emitted from accelerated jet regions.

Motivation & Objective

• To investigate the connection between hard X-ray emission from blazars and high-energy neutrinos detected by IceCube.
• To determine whether hard X-rays (≥10 keV) are physically linked to neutrino production, particularly as target photons in pγ interactions.
• To assess whether Doppler-boosted, relativistic jets in blazars host the neutrino production site, using X-ray and radio data as tracers.
• To evaluate whether X-ray emission can serve as a direct electromagnetic proxy for neutrino production processes.
• To constrain models of neutrino production by comparing X-ray fluxes with neutrino arrival directions and energies.

Proposed method

• Utilized all-sky X-ray catalogs from SRG/ART-XC, Swift/BAT, and INTEGRAL/IBIS covering the hard X-ray band (≥10 keV), along with ROSAT for softer X-rays.
• Cross-correlated these X-ray sources with IceCube high-energy neutrino events (E > 200 TeV) and VLBI-selected blazars with compact, radio-bright cores.
• Applied statistical tests to assess the significance of spatial coincidence between neutrino error regions and X-ray-emitting blazars, calculating a chance coincidence probability of p = 0.5%.
• Used relativistic kinematics (E′_p ≈ 20E′_ν and E′_p E′_γ ≈ m²_Δ) to estimate required target photon energies for producing TeV-scale neutrinos.
• Focused on blazars at cosmological redshifts z ~ 1, where relativistic beaming enhances detectability of both X-rays and neutrinos.
• Evaluated the correlation between X-ray flux and neutrino production rate, favoring models where target photons originate in the jet rather than external regions like the accretion disk.

Experimental results

Research questions

• RQ1 Is there a significant spatial correlation between hard X-ray emission from blazars and high-energy neutrino events detected by IceCube?
• RQ2 Do neutrino-associated blazars exhibit stronger hard X-ray emission compared to other VLBI-selected blazars?
• RQ3 Can hard X-rays serve as a physical tracer of the target photon population responsible for pγ interactions in neutrino production?
• RQ4 Is the observed X-ray emission consistent with being Doppler-boosted emission from relativistic jets, and does this support a jet-based origin for neutrinos?
• RQ5 Do the observed X-ray fluxes and neutrino energies support the synchrotron-self-Compton (SSC) model as the source of target photons?

Key findings

• Neutrino-associated blazars show significantly stronger hard X-ray emission (≥10 keV) than other VLBI-selected blazars, with a chance coincidence probability of only 0.5%.
• A strong correlation exists between hard X-ray emission and IceCube neutrino events, while softer X-rays (≤1 keV) show no such correlation.
• The observed X-ray fluxes are consistent with being Doppler-boosted emission from compact, relativistic jets at cosmological distances (z ~ 1).
• The results support a scenario in which target photons for pγ interactions are produced within the jet, likely via synchrotron-self-Compton (SSC) processes.
• Blazars with strong parsec-scale radio emission (from VLBI) are more likely to be associated with high-energy neutrinos, indicating that relativistic beaming enhances detectability of both X-rays and neutrinos.
• The findings suggest that hard X-ray monitoring could serve as a predictive tool for identifying potential high-energy neutrino sources.