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Baikal-GVD observation of a high-energy neutrino candidate event from the blazar PKS 0735+17 at the day of the IceCube-211208A neutrino alert from the same direction

ATel #15112; Zh.-A. Dzhilkibaev and O. Suvorova (INR RAS, Moscow) for the Baikal-GVD collaboration
on 14 Dec 2021; 18:44 UT
Credential Certification: Sergey Troitsky (st@ms2.inr.ac.ru)

Subjects: Neutrinos, AGN, Blazar

Referred to by ATel #: 15132, 15136, 15143, 15290

IceCube collaboration has recently reported the detection of an event IceCube-211208A with high probability (signalness 50.2%) explained by an astrophysical neutrino with estimated energy of 172 TeV (GCN #31191). A prominent radio and gamma-ray blazar PKS 0735+17 is located just outside the 90% PSF containment of the IceCube event, within the expected systematic uncertainty of the determination of the arrival direction. The blazar was found to experience a strong flare in gamma rays (ATel #15099>), X rays (ATel #15102, #15108, #15109), optical (ATel #15098, #15100) and radio (ATel #15099) bands. No additional neutrino events from the direction of the alert have been reported by IceCube (GCN #31195) and ANTARES (ATel #15106). Here we report an observation of a high-energy neutrino in coincidence with this flaring blazar and IceCube-211208A by the Baikal-GVD experiment.
During the alert follow-up examination of data (for details of the procedure, see e.g. Dik et al., 2021), we found a cascade event with the estimated energy of 43 TeV, arrived 3.95 hours after the IceCube event from the direction (RA=119.44 deg, DEC=18.00 deg), that is 4.68 degrees from PKS 0735+17 and 5.30 degrees from the best-fit direction of IceCube-211208A. The Baikal-GVD PSF, estimated by Monte-Carlo simulations for the conditions of this cascade event, has 5.5 deg (50%), 8.1 deg (68%) containment. One expects only 0.0044 events per 24 h in the 5.5 deg circle around this source, and the Poisson probability of the chance coincidence is 0.0044 (2.85 sigma significance). More detailed investigation of this and other Baikal-GVD events from the same direction continues.
Historically, PKS 0735+17 was observed to reach 5 Jy at centimeter wavelengths in its flaring state which makes it one of the brightest radio blazars in the sky and as such - a probable neutrino emitter according to Plavin et al. (2020). Continuing VLBI monitoring observations at 15 GHz and 43 GHz show a clear core-dominated parsec-scale jet morphology. Its RATAN-600 spectrum from 2 to 22 GHz is currently undergoing a slow flare and is observed at the level 0.6-0.9 Jy.
Baikal-GVD (see e.g. Belolaptikov et al., 2021) is the largest neutrino telescope in the Northern hemisphere, operating with an effective volume of ~0.4 km3 for the cascade detection by a collaboration of more than 60 scientists from Russia, Slovakia, Czech Republic, Poland and Germany.