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

ATel #16388; J. Hessdoerfer (University of Wurzburg), M. Kadler (University of Wurzburg), P. Benke (University of Wurzburg & MPIfR Bonn), L. Debbrecht (MPIfR Bonn), J. Eich (University of Wurzburg), F. Eppel (University of Wurzburg), A. Gokus (Washington University St. Louis), S. Haemmerich (ECAP/Remeis-Observatory Bamberg), G. Paraschos (MPIfR Bonn), F. Roesch (University of Wurzburg), W. Schulga (University of Wurzburg) for the TELAMON team
on 21 Dec 2023; 16:49 UT
Credential Certification: Matthias Kadler (matthias.kadler@astro.uni-wuerzburg.de)

Tweet

We report radio flaring activity of the blazar S4 0954+658 at high radio frequencies in the 7mm band detected through regular monitoring observations within the TELAMON program (Link).
On October 1, 2023, S4 0954+658 showed a flux density of (1.519 ± 0.082) Jy at 7mm, roughly corresponding to the long-term median brightness known from this source in this wavelength regime. Taking into account the flux density in the 14mm band (1.251 ± 0.016) Jy, the spectrum was already slightly inverted at this epoch. In the following weeks, the spectrum hardened considerably, predominantly due to a rapid and strong increase of the 7mm-band emission.
Our most recent observation on December 17, 2023, detected a flux density in the 7mm band of (3.95 ± 0.12) Jy, exceeding the historical flux density reported for this source at similar frequencies. In the last three consecutive epochs (Nov 13, Nov 29, and Dec 17), the source showed a strongly inverted spectrum with spectral indices α > 0.9 between 19 GHz and 44 GHz (S ∝ ν^α).

This radio flare coincides with an ongoing extremely high state in the optical (ATel#16380).

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.

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.