Repeating flaring activity of the blazar AO 0235+164

¹ Instituto de Astrofísica de Andalucía, CSIC, Glorieta de la Astronomía s/n, 18080 Granada, Spain
e-mail: jescudero@iaa.es
² Department of Astronomy, University of Geneva, ch. d’Ecogia 16, 1290 Versoix, Switzerland
³ Institute for Astrophysical Research, Boston University, 725 Commonwealth Avenue, Boston, MA 02215, USA
⁴ Institute of Astrophysics, Foundation for Research and Technology – Hellas, Voutes, 70013 Heraklion, Greece
⁵ Department of Physics, University of Crete, 71003 Heraklion, Greece
⁶ Institut de Radioastronomie Millimétrique, Avenida Divina Pastora 7, Local 20, 18012 Granada, Spain
⁷ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁸ Department of Astronomy and Atmospheric Sciences, Kyungpook National University, Daegu 702-701, Republic of Korea
⁹ INAF – Istituto di Radioastronomia, Via Gobetti 101, 40129 Bologna, Italy
¹⁰ INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate (LC), Italy
¹¹ Crimean Astrophysical Observatory RAS, P/O Nauchny 298409, Russia
¹² Saint Petersburg State University, 7/9 Universitetskaya nab., St. Petersburg 199034, Russia
¹³ Special Astrophysical Observatory, Russian Academy of Sciences, 369167 Nizhnii Arkhyz, Russia
¹⁴ Pulkovo Observatory, St.Petersburg 196140, Russia

Received: 12 May 2023
Accepted: 24 October 2023

Abstract

Context. Blazar AO 0235+164, located at a redshift of z = 0.94, has undergone several sharp multi-spectral-range flaring episodes over recent decades. In particular, the episodes that peaked in 2008 and 2015, which were subject to extensive multi-wavelength coverage, exhibited an interesting behavior.

Aims. We study the actual origin of these two observed flares by constraining the properties of the observed photo-polarimetric variability as well as of the broadband spectral energy distribution and the observed time-evolution behavior of the source. We use ultra-high-resolution total-flux and polarimetric very-long-baseline interferometry (VLBI) imaging.

Methods. The analysis of VLBI images allowed us to constrain kinematic and geometrical parameters of the 7 mm jet. We used the discrete correlation function to compute the statistical correlation and the delays between emission at different spectral ranges. The multi-epoch modeling of the spectral energy distributions allowed us to propose specific models of the emission; in particular, with the aim to model the unusual spectral features observed in this source in the X-ray region of the spectrum during strong multi spectral-range flares.

Results. We find that these X-ray spectral features can be explained by an emission component originating in a separate particle distribution than the one responsible for the two standard blazar bumps. This is in agreement with the results of our correlation analysis, where we did not find a strong correlation between the X-ray and the remaining spectral ranges. We find that both external Compton-dominated and synchrotron self-Compton-dominated models are able to explain the observed spectral energy distributions. However, the synchrotron self-Compton models are strongly favored by the delays and geometrical parameters inferred from the observations.