Radio-flaring blazar PKS 0446+11 with bright parsec-scale core as a candidate for IceCube-240105A: RATAN-600 and MOJAVE VLBA observations

ATel #16409; Y. Y. Kovalev (MPIfR), A. V. Plavin (Harvard U.), S. V. Troitsky (INR), Yu. A. Kovalev (ASC Lebedev), A. V. Popkov (MIPT, ASC Lebedev), A. B. Pushkarev (CrAO, ASC Lebedev)
on 17 Jan 2024; 15:17 UT
Credential Certification: Yuri Y. Kovalev (yykovalev@gmail.com)

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A radio-loud high redshift (z=2.153) blazar PKS 0446+11 was indicated as a likely source candidate for the recent high-energy neutrino event IceCube-240105A (GCN#35485, GCN#35498). An active state was also reported for the gamma-ray (ATel#16398), X-ray (ATel#16397), submillimeter (GCN#35499) and radio (ATel#16399) bands. For recent optical data, see ATel#16402 and ATel#16407. This blazar passes the selection criteria used in our analysis that has shown VLBI-selected radio-bright AGN as highly probable neutrino associations (Plavin et al. 2023). It is a member of complete blazar samples observed by RATAN-600 and MOJAVE VLBA programs.
Our long-term 1-22 GHz RATAN-600 monitoring of PKS 0446+11 has shown that the current flare with an inverted radio spectrum reaching 1.8 Jy at 22 GHz in January 2024 is the strongest for the last 19 years. Earlier flares of comparable strength from PKS 0446+11 include events in December 2001 and July 2004, reaching at 22 GHz 2.7 Jy (the highest level since 1997) and 1.8 Jy, respectively.
MOJAVE 15 GHz VLBA observations locate the radio flare to happen in a very compact core of this blazar, supporting neutrino production taking place within the central parsec of this object. We have observed the object most recently on 15 December 2023 (see VLBA data on previous epochs at the following link) and have detected the core at the level of 1.6 Jy. The core demonstrates an exceptionally high dominance, emitting 92% of the total parsec-scale flux density. The 15 GHz core FWHM size (fitted as a circular Gaussian) is measured as 40 microarcsecond or 0.3 parsec. There is an unresolved innermost jet component separated by 0.16 mas from the VLBI core position, with flux density of 0.1 Jy.
The observed high core brightness temperature in the frame of the host galaxy is measured as 1.5x10^13 K. The intrinsic brightness temperature is estimated to be 5x10^11 K, deboosting for the high Doppler factor of 29 (Homan at al. 2021). This turns out to be very close to the inverse Compton upper limit and one order of magnitude above the equipartition value (Kellermann & Paulini-Toth 1969, Readhead 1994), confirming particle-dominated major radio flare happening right now in this blazar.
RATAN-600 and VLBA MOJAVE programs will continue monitoring this blazar.