TELAMON, Metsahovi, Medicina, OVRO and RATAN-600 programs find a long-term radio flare in PKS0735+17 coincident with IceCube-211208A

ATel #15105; Matthias Kadler (JMU Wuerzburg), Petra Benke (MPIfR), Andrea Gokus (JMU Wuerzburg & FAU Erlangen-Nuremberg), Jonas Hessdoerfer (JMU Wuerzburg), Jonas Sinapius (DESY) & Philip Weber (JMU Wuerzburg), for the TELAMON Team, Merja Tornikoski (Aalto University Metsähovi Radio Observatory), Simona Righini (INAF/IRA) and Nicola Marchili (INAF/IRA), Talvikki Hovatta (Finnish Centre for Astronomy with ESO), Anthony C. Readhead (OVRO, Caltech), Sebastian Kiehlmann (IoA FORTH, OVRO), Yuri A. Kovalev (ASC Lebedev), Alexander V. Popkov (MIPT, ASC Lebedev), Yuri Y. Kovalev (ASC Lebedev, MIPT, MPIfR)
on 11 Dec 2021; 12:43 UT
Credential Certification: Matthias Kadler (matthias.kadler@astro.uni-wuerzburg.de)

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We report on contemporary radio observations of PKS0735+17 and two other radio and gamma-bright AGN possibly associated with the recently detected high-energy neutrino IceCube-211208A (GCN#31191) on Dec 8, 2021. While several compact radio sources are listed in the rfc_2021c_catalog in the immediate vicinity of the IC211208A neutrino event, we concentrate on the three radio sources associated with 4FGL Fermi gamma-ray sources (cf. ATel#15099).

We observed NVSS J073824+153839 (associated with 4FGL J0738.4+1539) on Dec 8 with the Effelsberg 100-m telescope as part of the TELAMON program at 14GHz and detected it at a faint level of about 17mJy. In the same TELAMON session, we observed TXS 0740+173 (associated with 4FGL J0743.1+1713) with broader frequency coverage and found it to show a flat spectrum between 14GHz and 36GHz at a level of about 350mJy. We note that this is near the maximum values listed in the RFC data base for this source at lower frequencies and also near its historical maximum at 15GHz based on OVRO monitoring data. PKS0735+17 is the by far brightest radio source possibly associated with IceCube-211208A. We observed it in the same session (Dec 8, only hours after the release of GCN#31191) in the TELAMON program and found a flat spectrum between 14GHz and 44GHz at a level of about 1Jy.

The high brightness of PKS0735+17 at the time of the IceCube-211208A event is confirmed by independent observations within the Metsahovi and Medicina monitoring programs. From the Metsahovi program, we report a continuous rising of the flux density at 37GHz from about 0.6Jy in January 2021 to 1.1+-0.1Jy on Dec 9, 2021. With the Medicina radio telescope, we find a steady increase of the flux density of PKS0735+17 at 8.3GHz and 24GHz over the MJD interval 59297.8-59548.1 from 0.66+-0.02 Jy to 0.98+-0.02 Jy at 8.3 GHz and from 0.61+-0.05 Jy to 0.95+-0.10 Jy at 24.1 GHz. At 15GHz, the OVRO monitoring measured PKS0735+17 at a level of 630mJy in December 2020 and at 980mJy on Nov 30, 2021. With the RATAN-600 radio telescope, we find a steady and slow increase of the flux density of the flat 2.3-22 GHz spectrum of PKS 0735+17 through the whole year of 2021 in the range 0.6-0.8 Jy. Together, the radio data show that the previously reported high flux states in the optical (ATel#15098), X-Ray (ATel#15102), and gamma-ray band (ATel#15099), occured during a long-term radio flare of PKS0735+17 that started at least several months ago.

The TELAMON program (Kadler et al. 2021, Proc. ICRC 2021) is using the Effelsberg 100-m telescope to monitor the radio spectra of active galactic nuclei (AGN) under scrutiny in astroparticle physics. The 14 metre radio telescope in Metsahovi is used to study the long-term variability of a set of sources at 22 and 37 GHz, and observe very large source samples at 37 GHz in order to study the high-frequency radio behaviour of various source populations. The Medicina 32 metre radio telescope, operated as a National Facility by the National Institute for Astrophysics (INAF), regularly monitors a set of blazars at 8.3 and 24.1 GHz with monthly cadence. The OVRO 40m Telescope at California Institute of Technology is used to regularly monitor a large number of AGN at 15 GHz . It is currently supported by private funding from the California Insitute of Technology and the Max Planck Institute for Radio Astronomy. The RATAN-600 program monitors 2-22 GHz broad-band spectra of a complete sample of blazars with VLBI flux density above 0.4 Jy with a cadence of four times per year.