A broadband outburst of the compact steep-spectrum quasar 3C 138 in 2024-2026

arXiv:2608.05852 · astro-ph.HE, astro-ph.GA · Submitted 2026-08-06 · Read on arXiv

T. V. Mufakharov, Yu. V. Sotnikova, V. V. Vlasyuk, S. Yu. Sazonov, M. L. Khabibullina, A. G. Mikhailov, A. B. Pushkarev, T. An, Y. A. Kovalev, Y. Y. Kovalev, A. V. Popkov, M. A. Kharinov, G. S. Uskov, I. Yu. Lapshov, E. V. Filippova, A. Yu. Tkachenko, K. V. Iuzhanina, A. K. Erkenov, R. Yu. Udovitskiy, O. I. Spiridonova, I. A. Rakhimov, T. S. Andreeva, A. A. Ogloblin

Special Astrophysical Observatory of the Russian Academy of Sciences, Nizhny Arkhyz 369167, Russia · Institute for Nuclear Research, Russian Academy of Sciences, 60th October Anniversary Prospect 7a, Moscow 117312, Russia · Special Astrophysical Observatory of the Russian Academy of Sciences, Nizhny Arkhyz 369167, Russia · Space Research Institute of the Russian Academy of Sciences, Profsoyuznaya Street 84/32, Moscow 117997, Russia · Special Astrophysical Observatory of the Russian Academy of Sciences, Nizhny Arkhyz 369167, Russia · Special Astrophysical Observatory of the Russian Academy of Sciences, Nizhny Arkhyz 369167, Russia · Lebedev Physical Institute, Russian Academy of Sciences, Moscow 117997, Russia · Department of Astronomy, University of Science and Technology of China, 96 Jinzhai Road, Hefei, Anhui 230026, P. R. China · Institute for Nuclear Research, Russian Academy of Sciences, 60th October Anniversary Prospect 7a, Moscow 117312, Russia · Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, Bonn 53121, Germany · Lebedev Physical Institute, Russian Academy of Sciences, Moscow 117997, Russia · Institute of Applied Astronomy of the Russian Academy of Sciences, Kutuzova Embankment 10, St. Petersburg 191187, Russia · Space Research Institute of the Russian Academy of Sciences, Profsoyuznaya Street 84/32, Moscow 117997, Russia · Space Research Institute of the Russian Academy of Sciences, Profsoyuznaya Street 84/32, Moscow 117997, Russia · Space Research Institute of the Russian Academy of Sciences, Profsoyuznaya Street 84/32, Moscow 117997, Russia · Space Research Institute of the Russian Academy of Sciences, Profsoyuznaya Street 84/32, Moscow 117997, Russia · Special Astrophysical Observatory of the Russian Academy of Sciences, Nizhny Arkhyz 369167, Russia · Special Astrophysical Observatory of the Russian Academy of Sciences, Nizhny Arkhyz 369167, Russia · Special Astrophysical Observatory of the Russian Academy of Sciences, Nizhny Arkhyz 369167, Russia · Special Astrophysical Observatory of the Russian Academy of Sciences, Nizhny Arkhyz 369167, Russia · Institute of Applied Astronomy of the Russian Academy of Sciences, Kutuzova Embankment 10, St. Petersburg 191187, Russia · Institute of Applied Astronomy of the Russian Academy of Sciences, Kutuzova Embankment 10, St. Petersburg 191187, Russia · St. Petersburg Academic University of the Russian Academy of Sciences, Khlopina St. 8/3A, St. Petersburg 194021, Russia

astro-ph.HE, astro-ph.GA

Submitted: 2026-08-06

Comments: 20 pages, 12 figures. Submitted to MNRAS

Project page: https://johannesbuchner.github.io/UltraNest

License: http://creativecommons.org/licenses/by/4.0/

The gist: After several decades of relative quiescence, the compact steep-spectrum quasar 3C 138 entered an active phase in 2024-2026, exhibiting strong broadband flaring.

Terminology

Abstract

After several decades of relative quiescence, the compact steep-spectrum quasar 3C 138 entered an active phase in 2024-2026, exhibiting strong broadband flaring. We investigate its multiwavelength behaviour using dense multifrequency radio monitoring at 1-22 GHz with RATAN-600 and RT-32, optical R-band observations with Zeiss-1000 and AS-500/2, X-ray measurements with Swift/XRT and SRG/ART-XC, and the Fermi-LAT gamma-ray light curve. The radio brightening accelerated after 2022 and was strongest at the highest frequencies. The radio spectra hardened markedly, with the 11-22 GHz spectral index evolving from steep to flat or inverted during the active phase. The X-ray flux increased by more than a factor of three during 2025-2026, while the photon index hardened from X 1.6 to X 0.9 and softened back after the peak. Flare decomposition revealed five gamma-ray flares and a sequence of optical subflares during the later stages of the activity. The gamma-ray, X-ray, and optical maxima occur within a 13-day interval, suggesting a common activity episode, whereas the radio brightens more gradually and in a frequency-dependent manner. Under the adopted compact-zone geometries, the sparse two-state spectral energy distributions (SEDs) can be represented by one-zone synchrotron self-Compton (SSC) solutions, while the relative contribution of external Compton (EC) remains geometry dependent. The flare shifts the modelled energy partition towards relativistic electrons. These results favour a longer-lived, core-dominated activity phase, with later high-energy and optical flares superposed on the opacity-driven radio evolution of an emerging synchrotron component.

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