Radio blazar 1801+253 is associated with IceCube-210811A and flares immediately after the neutrino event

ATel #15022; S. V. Troitsky (INR), Yu. V. Sotnikova (SAO), A. K. Erkenov (SAO), Yu. A. Kovalev (ASC Lebedev), Y. Y. Kovalev (ASC Lebedev, MIPT, MPIfR), A. V. Plavin (ASC Lebedev, MIPT), A. V. Popkov (MIPT, ASC Lebedev)
on 8 Nov 2021; 20:05 UT
Credential Certification: Yuri Y. Kovalev (yyk@asc.rssi.ru)

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We have recently shown that VLBI-selected radio-bright AGN are highly probable neutrino associations (Plavin et al., 2020, 2021) with neutrinos being generated within their central parsec-scale region.
IceCube collaboration has detected a new high-energy neutrino event with a high probability of its astrophysical origin and issued a "GOLD" alert on August 11, 2021. The closest (5' from the best-fit neutrino arrival direction) radio blazar within the error region to the IceCube-210811A event is 1801+253 (J1803+2521, J2000.0 VLBI positions according to the RFC catalog: 18:03:12.4064 +25:21:18.741), redshift is unknown. See details on the source in NASA NED. Its archival VLBI observations in 2013 at 8 GHz show a very compact jet with the parsec-scale core dominance 0.9, core size 0.4 mas, and core flux density 170 mJy.
We have triggered observations of 1801+253 at the RATAN-600 telescope immediately after the neutrino detection. Observations from August 12 till November 7, 2021, show a radio flare which has started about the time of neutrino detection: the flux density at 22 GHz has raised from 97+/-10 mJy to 179+/-30 mJy. Broad-band spectral evolution between 5 and 22 GHz agrees with the expectations from a typical synchrotron flare in the VLBI core, with flux density variations at lower frequency delayed relative to 22 GHz. Namely, in August 2021 the quasar had a steep radio spectrum at low frequencies with 5-8 GHz spectral index alpha=-0.84+/-0.09 (S~nu^+alpha), and a flat spectrum at 8-22 GHz with alpha=-0.12+/-0.02. The flare resulted in an inverted spectrum with September alpha=+0.46+/-0.08 and November alpha=+0.31+/-0.07 at 8-22 GHz.
The blazar is a strongly variable gamma-ray source listed in the Catalog of Long-term Transient Sources in the First 10 yr of Fermi-LAT Data as 1FLT J1803+2523. It was present in the 3FGL catalog as 3FGL J1804.1+2532 but absent in 4FGL. Fermi LAT followup GCN of IceCube-210811A mentioned 4FGL sources only and did not discuss the 1FLT or 3FGL targets though they are well within the 90% statistical IceCube uncertainties.
This blazar is the brightest within the IceCube error region on the basis of available VLBI catalogs, has shown a radio flare coincident with the neutrino event, and is a highly variable gamma-ray source. This makes it a highly probable neutrino association. We have triggered VLBA in early November 2021 which will monitor this blazar at 15, 24 and 43 GHz monthly for the next half a year. RATAN-600 monthly monitoring will continue. We encourage multi-band observations of the blazar.