Study of the variable broadband emission of Markarian 501 during the most extreme Swift X-ray activity

¹ Inst. de Astrofísica de Canarias, 38200 La Laguna, Spain
² Universidad de La Laguna, Dpto. Astrofísica, 38206 La Laguna, Tenerife, Spain
³ Università di Udine and INFN Trieste, 33100 Udine, Italy
⁴ National Institute for Astrophysics (INAF), 00136 Rome, Italy
⁵ Croatian MAGIC Consortium: University of Rijeka, 51000 Rijeka; University of Split – FESB, 21000 Split; University of Zagreb – FER, 10000 Zagreb; University of Osijek, 31000 Osijek; Rudjer Boskovic Institute, 10000 Zagreb, Croatia
⁶ Saha Institute of Nuclear Physics, HBNI, 1/AF Bidhannagar, Salt Lake, Sector-1, Kolkata 700064, India
⁷ Centro Brasileiro de Pesquisas Físicas (CBPF), 22290-180 URCA, Rio de Janeiro, RJ, Brasil
⁸ Unidad de Partículas y Cosmología (UPARCOS), Universidad Complutense, 28040 Madrid, Spain
⁹ University of Łódź, Department of Astrophysics, 90236 Łódź, Poland
¹⁰ Deutsches Elektronen-Synchrotron (DESY), 15738 Zeuthen, Germany
¹¹ Istituto Nazionale Fisica Nucleare (INFN), 00044 Frascati, Roma, Italy
¹² Max-Planck-Institut für Physik, 80805 München, Germany
¹³ Institut de Física d’Altes Energies (IFAE), The Barcelona Institute of Science and Technology (BIST), 08193 Bellaterra, Barcelona, Spain
¹⁴ Università di Siena and INFN Pisa, 53100 Siena, Italy
¹⁵ Università di Padova and INFN, 35131 Padova, Italy
¹⁶ Università di Pisa and INFN Pisa, 56126 Pisa, Italy
¹⁷ Finnish MAGIC Consortium: Finnish Centre of Astronomy with ESO (FINCA), University of Turku, 20014 Turku, Finland; Astronomy Research Unit, University of Oulu, 90014 Oulu, Finland
¹⁸ Technische Universität Dortmund, 44221 Dortmund, Germany
¹⁹ Departament de Física and CERES-IEEC, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain
²⁰ Universitat de Barcelona, ICCUB, IEEC-UB, 08028 Barcelona, Spain
²¹ ICRANet-Armenia at NAS RA, 0019 Yerevan, Armenia
²² Japanese MAGIC Consortium: ICRR, The University of Tokyo 277-8582 Chiba, Japan; Department of Physics, Kyoto University, 606-8502 Kyoto, Japan; Tokai University, 259-1292 Kanagawa, Japan; RIKEN, 351-0198 Saitama, Japan
²³ Inst. for Nucl. Research and Nucl. Energy, Bulgarian Academy of Sciences, 1784 Sofia, Bulgaria
²⁴ Humboldt University of Berlin, Institut für Physik, 12489 Berlin, Germany
²⁵ Dipartimento di Fisica, Università di Trieste, 34127 Trieste, Italy
²⁶ Port d’Informació Científica (PIC), 08193 Bellaterra, Barcelona, Spain
²⁷ INAF-Trieste and Dept. of Physics & Astronomy, University of Bologna, Bologna, Italy
²⁸ ETH Zurich, 8093 Zurich, Switzerland
²⁹ ISDC – Department of Astronomy, University of Geneva, 16, 1290 Versoix, Switzerland
³⁰ Universität Würzburg, 97074 Würzburg, Germany
³¹ RWTH Aachen University, Aachen, Germany
³² Center for Research and Exploration in Space Science and Technology (CRESST) and NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
³³ Department of Physics, University of Maryland, Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA
³⁴ Space Science Data Center – ASI, Via del Politecnico, s.n.c., 00133 Roma, Italy
³⁵ INAF–Osservatorio Astronomico di Roma, Via di Frascati 33, 00040 Monteporzio, Italy
³⁶ Aalto University Metsähovi Radio Observatory, Metsähovintie 114, 02540 Kylmälä, Finland
³⁷ Aalto University Department of Electronics and Nanoengineering, PO BOX 15500 00076 Espoo, Finland

Received: 7 November 2018
Accepted: 25 December 2019

Abstract

Context. Markarian 501 (Mrk 501) is a very high-energy (VHE) gamma-ray blazar located at z = 0.034, which is regularly monitored by a wide range of multi-wavelength instruments, from radio to VHE gamma rays. During a period of almost two weeks in July 2014, the highest X-ray activity of Mrk 501 was observed in ∼14 years of operation of the Neil Gehrels Swift Gamma-ray Burst Observatory.

Aims. We characterize the broadband variability of Mrk 501 from radio to VHE gamma rays during the most extreme X-ray activity measured in the last 14 years, and evaluate whether it can be interpreted within theoretical scenarios widely used to explain the broadband emission from blazars.

Methods. The emission of Mrk 501 was measured at radio with Metsähovi, at optical–UV with KVA and Swift/UVOT, at X-ray with Swift/XRT and Swift/BAT, at gamma ray with Fermi-LAT, and at VHE gamma rays with the FACT and MAGIC telescopes. The multi-band variability and correlations were quantified, and the broadband spectral energy distributions (SEDs) were compared with predictions from theoretical models.

Results. The VHE emission of Mrk 501 was found to be elevated during the X-ray outburst, with a gamma-ray flux above 0.15 TeV varying from ∼0.5 to ∼2 times the Crab nebula flux. The X-ray and VHE emission both varied on timescales of 1 day and were found to be correlated. We measured a general increase in the fractional variability with energy, with the VHE variability being twice as large as the X-ray variability. The temporal evolution of the most prominent and variable segments of the SED, characterized on a day-by-day basis from 2014 July 16 to 2014 July 31, is described with a one-zone synchrotron self-Compton model with variations in the break energy of the electron energy distribution (EED), and with some adjustments in the magnetic field strength and spectral shape of the EED. These results suggest that the main flux variations during this extreme X-ray outburst are produced by the acceleration and the cooling of the high-energy electrons. A narrow feature at ∼3 TeV was observed in the VHE spectrum measured on 2014 July 19 (MJD 56857.98), which is the day with the highest X-ray flux (>0.3 keV) measured during the entire Swift mission. This feature is inconsistent with the classical analytic functions to describe the measured VHE spectra (power law, log-parabola, and log-parabola with exponential cutoff) at more than 3σ. A fit with a log-parabola plus a narrow component is preferred over the fit with a single log-parabola at more than 4σ, and a dedicated Monte Carlo simulation estimated the significance of this extra component to be larger than 3σ. Under the assumption that this VHE spectral feature is real, we show that it can be reproduced with three distinct theoretical scenarios: (a) a pileup in the EED due to stochastic acceleration; (b) a structured jet with two-SSC emitting regions, with one region dominated by an extremely narrow EED; and (c) an emission from an IC pair cascade induced by electrons accelerated in a magnetospheric vacuum gap, in addition to the SSC emission from a more conventional region along the jet of Mrk 501.