[Paper Review] Searches for neutrinos in the direction of radio-bright blazars with the ANTARES telescope

ANTARES Collaboration, A. Albert+163 more|arXiv (Cornell University)|Sep 13, 2023

Astrophysics and Cosmic Phenomena68 references4 citations

TL;DR

This study searches for high-energy neutrinos from radio-bright blazars using the ANTARES neutrino telescope, employing directional and time-dependent analyses. It finds tentative evidence of neutrino flares correlated with electromagnetic activity in two blazars—J1355−6326 (3.7σ) and J1826+1831 (3.3σ)—and a significant temporal coincidence between neutrino emission and a multiwavelength flare in J0242+1101 (p = 0.5%), suggesting a potential link between neutrino and electromagnetic flares in blazars.

ABSTRACT

Active galaxies, especially blazars, are among the most promising neutrino source candidates. To date, ANTARES searches for these objects considered GeV-TeV $γ$-ray bright blazars. Here, a statistically complete radio-bright blazar sample is used as the target for searches of origins of neutrinos collected by the ANTARES neutrino telescope over 13 years of operation. The hypothesis of a neutrino-blazar directional correlation is tested by pair counting and by a complementary likelihood-based approach. The resulting post-trial $p$-value is $3.0\%$ ($2.2σ$ in the two-sided convention), possibly indicating a correlation. Additionally, a time-dependent analysis is performed to search for temporal clustering of neutrino candidates as a mean of detecting neutrino flares in blazars. None of the investigated sources alone reaches a significant flare detection level. However, the presence of 18 sources with a pre-trial significance above $3σ$ indicates a $p=1.4\%$ ($2.5σ$ in the two-sided convention) detection of a time-variable neutrino flux. An extit{a posteriori} investigation reveals an intriguing temporal coincidence of neutrino, radio, and $γ$-ray flares of the J0242+1101 blazar at a $p=0.5\%$ ($2.9σ$ in the two-sided convention) level. Altogether, the results presented here suggest a possible connection of neutrino candidates detected by the ANTARES telescope with radio-bright blazars.

Motivation & Objective

• To investigate whether radio-bright blazars are sources of high-energy neutrinos detected by the ANTARES telescope.
• To test for temporal correlations between neutrino events and electromagnetic flares in blazars, particularly in radio and gamma-ray bands.
• To constrain the neutrino flux from blazars under different spectral assumptions, especially in relation to IceCube's HESE measurements.
• To assess the significance of observed neutrino flares after correcting for multiple trials and trial factors.
• To explore the potential role of blazars as a dominant component of the diffuse high-energy neutrino background.

Proposed method

• Directional searches for neutrinos from 107 radio-bright blazars identified in the VLBI catalogue, using ANTARES data from 2007 to 2017.
• Application of a likelihood-based method to test for excess neutrino flux in the direction of these sources, assuming a power-law energy spectrum with index E−γ.
• Time-dependent analysis using an unbinned maximum likelihood method to identify flaring activity in both track-like and shower-like neutrino events.
• Calculation of post-trial p-values to correct for multiple trials when assessing significance of flaring candidates.
• Comparison of neutrino light curves with multiwavelength data, including radio (OVRO 40-m) and gamma-ray (Fermi-LAT) light curves, to assess temporal coincidences.
• Computation of upper limits on the neutrino flux for different spectral indices (E−γ), particularly constraining the contribution to the IceCube HESE flux at ~100 TeV.

Experimental results

Research questions

• RQ1 Are radio-bright blazars a significant source of high-energy neutrinos detectable by ANTARES?
• RQ2 Is there a statistically significant temporal correlation between neutrino flares and electromagnetic flares in blazars?
• RQ3 What is the upper limit on the neutrino flux from radio-bright blazars, and how does it compare to the IceCube HESE flux at 100 TeV?
• RQ4 Can a cumulative effect of multiple flaring sources be detected, indicating a population-level neutrino emission pattern?
• RQ5 Is there a physical connection between neutrino emission and multiwavelength flares, such as in radio and gamma-ray bands?

Key findings

• A cumulative effect of neutrino flares from multiple blazars is detected with a post-trial p-value of 0.014 (2.5σ), suggesting a possible population-level correlation.
• The two most significant individual flares are from J1355−6326 (3.7σ pre-trial) and J1826+1831 (3.3σ pre-trial), though neither remains significant after correction for multiple trials.
• A notable temporal coincidence is found between a neutrino flare from J0242+1101 and a prominent multiwavelength electromagnetic flare, with a chance probability of only 0.5%.
• The analysis constrains the neutrino flux from VLBI blazars, with upper limits indicating they could contribute up to ~50% of the IceCube HESE flux at ~100 TeV for a spectral index of E−2.3.
• The likelihood-based method yields a p-value of 0.03 for a flux proportional to VLBI flux density, supporting a tentative but not yet conclusive correlation.
• The results suggest that blazar neutrino emission is highly variable and may be linked to electromagnetic flares, though other source classes, including a possible Galactic component, likely contribute to the diffuse neutrino background.