Multiwavelength variability of the high-energy neutrino candidate PKS 0735+178 over three decades

arXiv:2606.22190 · astro-ph.HE, astro-ph.GA · Submitted 2026-06-20 · Read on arXiv

T. Mufakharov, Yu. Sotnikova, V. Vlasyuk, D. Kudryavtsev, A. Pushkarev, A. Mikhailov, M. Khabibullina, Yu. Kovalev, Y. Kovalev, A. Popkov, A. Erkenov, O. Spiridonova, T. Semenova, P. Tsybulev, D. Nezamov

Special Astrophysical Observatory of the RAS · Institute for Nuclear Research of the RAS · Special Astrophysical Observatory of the RAS · Special Astrophysical Observatory of the RAS · Astro Space Center, Lebedev Physical Institute of the RAS · Special Astrophysical Observatory of the RAS · Special Astrophysical Observatory of the RAS · Institute for Nuclear Research of the RAS · Max-Planck-Institut für Radioastronomie · Astro Space Center, Lebedev Physical Institute of the RAS · Special Astrophysical Observatory of the RAS · Special Astrophysical Observatory of the RAS · Special Astrophysical Observatory of the RAS · Institute for Nuclear Research of the RAS · Kazan Federal University

astro-ph.HE, astro-ph.GA

Submitted: 2026-06-20

Comments: Accepted for publication in MNRAS

Journal ref: MNRAS 550 (2026) stag1179

DOI: 10.1093/mnras/stag1179

Code: https://github.com/skiehl/wwz

License: http://creativecommons.org/licenses/by/4.0/

The gist: We present the multiwavelength variability of the BL Lac object PKS 0735+178, associated with the high-energy neutrino event IC211208A.

Terminology

Abstract

We present the multiwavelength variability of the BL Lac object PKS 0735+178, associated with the high-energy neutrino event IC211208A. The light curves cover the radio (1-230 GHz), optical, and Fermi-LAT gamma-ray bands over a about 30 years time-scale. The light curves are correlated, with delays from 0 to 1200 days increasing towards lower frequencies, consistent with emission from an opacity-stratified jet. The bright flare after the IC211208A event indicates emission from a compact, optically thick region with enhanced activity and more efficient particle acceleration. Short optical and gamma-ray bursts have been detected very close to the neutrino event, within a few days. The about 12-day lag between the gamma-ray and optical emissions is detected for the first time, suggesting that the emission regions are not co-spatial. A characteristic variability time-scale of about 10-11 yr is robustly detected in the radio-mm bands (at least3 sigma), while the optical and gamma-ray data show weaker, shorter-period signals. The independent constraints from the VLBI core-shift measurements and radio time delays yield consistent estimates of the jet geometry and disturbance propagation, supporting variability governed by jet propagation effects. The long-term modulation is consistent with a slow variation in energy release at the jet base, while individual flares arise from shocks propagating downstream. Jet precession may contribute to the long-term modulation; however, the required viewing angles are inconsistent with the VLBI constraints, indicating that precession alone cannot explain the observed variability.

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