Ongoing High-Frequency Radio Flaring of the Blazar S4 0954+658

ATel #16491; J. Hessdoerfer (University of Wurzburg), M. Kadler (University of Wurzburg), P. Benke (MPIfR Bonn), J. Eich (University of Wurzburg), A. Gokus (Washington University St. Louis), G. F. Paraschos (MPIfR Bonn), F. Rösch (University of Wurzburg) and W. Schulga (University of Wurzburg) for the TELAMON team, S.-S. Lee (KASI/UST), S. Kang (KASI), S. Kim (KASI/UST), W. Y. Cheong (KASI/UST), and H.-W. Jeong (KASI/UST) for the MOGABA team, M. Feige, F. Hemrich, B. Horst, T. Apell, D. Reinhart, R. Steineke, C. Lorey (all Hans-Haffner-Sternwarte - Friedrich-Koenig-Gymnasium, Wuerzburg, Germany), K. Mannheim (University of Wurzburg), D. Elsaesser (TU Dortmund, Germany)
on 28 Feb 2024; 17:00 UT
Credential Certification: Matthias Kadler (matthias.kadler@astro.uni-wuerzburg.de)

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We report ongoing radio flaring activity of the blazar S4 0954+658 at high radio frequencies in the 14mm and 7mm bands detected through regular monitoring observations within the TELAMON program (see Eppel et al. 2024, A&A, in press, arXiv:2401.06296). 

In December 2023, we reported that S4 0954+658 had reached a flux density in the 7mm band of (3.95 ± 0.12) Jy, exceeding its historically reported flux density at similar frequencies (ATel#16388). With our most recent observation on February 24, 2024, we now find a flux density of (5.99 ± 0.21) Jy in the 7mm band as well as a flux density of (4.745 ± 0.093) Jy in the 14mm band, resulting in a fitted spectral index (S ∝ ν^α) from 19.25 GHz to 43.75 GHz of α∼ 0.5. The full light curve measured within the scope of the TELAMON program can be inspected on our website.

MOGABA observations on February 8 detected the source at 25, 43, 86, and 129 GHz, yielding flux densities of (4.38 ± 0.07) Jy at 25 GHz, (5.24 ± 0.06) Jy at 43 GHz, (5.38 ± 0.05) Jy at 86 GHz, and (4.60 ± 0.10) Jy at 129 GHz, with a turnover frequency of (59.93 ±1.78) GHz in a curved power-law spectrum. In a second MOGABA session of the source on February 16, we found that the source became brighter with flux densities of (5.66 ± 0.09) Jy at 86 GHz and (5.42 ± 0.19) Jy at 129 GHz.

Optical R-band observations of S4 0954+658 show a highly variable light curve, fluctuating between 16.5 mag and 13.5 mag on timescales from weeks to months between the end of 2020 and now. While its brightness was near its maximum at the end of 2023, its current brightness of ~14.5 mag does not indicate an enhanced state in the R-band.

The TELAMON program uses the Effelsberg 100-m telescope to monitor the radio spectra of very-high-energy emitting active galactic nuclei (AGN), namely TeV blazars and candidate neutrino-associated AGN, with high-cadence high-frequency observations every ~3 weeks and at multiple frequencies up to 44 GHz. 
The MOGABA program is a multi-wavelength polarization monitoring program of compact radio sources (e.g., highly polarized and variable blazars) using three (or four) 21-m radio telescopes of the Korean VLBI Network (KVN): KVN Yonsei, KVN Ulsan, KVN Tamna (and KVN PyeongChang to be added), simultaneously at 22-129 GHz in a mean cadence of 2-3 weeks. 
The optical data were acquired through a Bessel R filter (Chroma), as part of the long-term AGN monitoring program of the Naturwissenschaftliches Labor fuer Schueler am Friedrich-Koenig-Gymnasium (FKG), the Universitaet Wuerzburg, and the TU Dortmund with the 0.5 cm CDK-astrograph and a Moravian G4-16000 camera at the school and university observatory Hans-Haffner-Sternwarte in D-97265 Hettstadt-Germany (https://schuelerlabor-wuerzburg.de/en/observatory/)

We will continue monitoring the source regularly and encourage more observations especially at other wavelengths to further characterize its dynamic multiwavelength light curve during this exceptional flaring event.