Baksan Underground Scintillation Telescope observation of a GeV neutrino candidate event at the time of a gamma-ray flare of the blazar PKS 0735+17, a possible source of coinciding IceCube and Baikal high-energy neutrinos

ATel #15143; V. B. Petkov, Yu. F. Novoseltsev and R. V. Novoseltseva (INR RAS) for the Baksan Underground Scintillation Telescope group
on 25 Dec 2021; 17:21 UT
Credential Certification: Sergey Troitsky (st@ms2.inr.ac.ru)

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PKS 0735+17 is currently experiencing a major flare, which attracted much interest because it coincides with a simultaneous detection of two rare high-energy neutrino events by IceCube (172 TeV, GCN #31191) and Baikal-GVD (43 TeV, ATel #15112). The flare was reported in gamma rays (ATel #15099, #15129), X rays (ATel #15102, #15108, #15109, #15130), optical (ATel #15098, #15100, #15136) and radio (ATel #15105) bands. The flux of the blazar in gamma rays and in the optical band hits its historical maximum. The redshift of the blazar is uncertain, z>0.424 (ATel #15132). No additional neutrino events from the direction of the alert have been reported by IceCube (GCN #31195) and ANTARES (ATel #15106) on the same day. Here we report an observation of a lower-energy neutrino candidate event in coincidence with the gamma-ray flare of the blazar, 4 days before the two high-energy neutrino events.
Baksan Underground Scintillation Telescope (BUST) detected an upgoing muon consistent with a neutrino of the energy above 1 GeV from the direction of PKS 0735+17 on December 4, 2021, at 14:52:47.83 UT. The best-fit arrival direction is RA=116.5, DEC=16.6, that is 2.18 deg from the source. The event coincided with the gamma-ray flare observed by Fermi LAT and preceded the IceCube and Baikal high-energy neutrinos by 4.2 days. With the BUST angular resolution of 2.5 deg (50% PSF containment) in the mode used and the average background of 1 upgoing muon per 8 days, the quick-look estimate of the false-alarm probability of a detection of 1 event from PKS 0735+17 during a 10-day gamma-ray flare is 0.0024, corresponding to the 3-sigma detection. More detailed investigation of this and other events from the same direction continues.
PKS 0735+17 is a bright radio blazar and therefore belongs to the class of probable emitters of neutrinos with energies from TeV to PeV, according to Plavin et al. (2021). The present observation of a GeV neutrino by BUST extends this energy range 3 orders of magnitude lower and gives important information about hadronic processes in blazars.
BUST (see e.g. Petkov et al., 2019) is the liquid-scintillator neutrino telescope at the Baksan Neutrino Observatory of INR RAS. Operating for almost 40 years, it is best known for the detection of neutrinos from SN 1987A. The present study uses different selection criteria than those used in the supernova-neutrino search, resulting in the threshold of 1 GeV for upgoing muon neutrinos.