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

© ESO 2020. Artículo firmado por 185 autores. The authors thank the anonymous referee for providing a constructive list of remarks that allowed us to clarify and improve some of the results reported in the manuscript. We would like to thank the Instituto de Astrofísica de Canarias for the excellent working conditions at the Observatorio del Roque de los Muchachos in La Palma. The financial support of the German BMBF and MPG, the Italian INFN and INAF, the Swiss National Fund SNF, the ERDF under the Spanish MINECO (FPA2015-69818-P, FPA2012-36668, FPA2015-68378-P, FPA2015-69210-C6-2-R, FPA2015-69210-C6-4-R, FPA2015-69210-C6-6-R, AYA2015-71042-P, AYA2016-76012-C3-1-P, ESP2015-71662-C2-2-P, FPA2017-90566-REDC), and the Japanese JSPS and MEXT is gratefully acknowledged. This work was also supported by the Spanish Centro de Excelencia "Severo Ochoa" SEV-2016-0588 and SEV-2015-0548, and Unidad de Excelencia "María de Maeztu" MDM-2014-0369, by the Croatian Science Foundation (HrZZ) Project IP-2016-06-9782 and the University of Rijeka Project 13.12.1.3.02, by the DFG Collaborative Research Centers SFB823/C4 and SFB876/C3, the Polish National Research Centre grant UMO-2016/22/M/ST9/00382 and by the Brazilian MCTIC, CNPq and FAPERJ. The Fermi-LAT Collaboration acknowledges generous ongoing support from a number of agencies and institutes that have supported both the development and the operation of the LAT as well as scientific data analysis. These include the National Aeronautics and Space Administration and the Department of Energy in the United States, the Commissariat a l'Energie Atomique and the Centre National de la Recherche Scientifique/Institut National de Physique Nucleaire et de Physique des Particules in France, the Agenzia Spaziale Italiana and the Istituto Nazionale di Fisica Nucleare in Italy, the Ministry of Education, Culture, Sports, Science and Technology (MEXT), High Energy Accelerator Research Organization (KEK) and Japan Aerospace Exploration Agency (JAXA) in Japan, and the K. A. Wallenberg Foundation, the Swedish Research Council and the Swedish National Space Board in Sweden. Additional support for science analysis during the operations phase is gratefully acknowledged from the Instituto Nazionale di Astrofisica in Italy and the Centre National d'Etudes Spatiales in France. The important contributions from ETH Zurich grants ETH-10.08-2 and ETH-27.12-1 as well as the funding by the Swiss SNF and the German BMBF (Verbundforschung Astro- und Astroteilchenphysik) and HAP (Helmoltz Alliance for Astroparticle Physics) are gratefully acknowledged. Part of this work is supported by Deutsche Forschungsgemeinschaft (DFG) within the Collaborative Research Center SFB 876, project C3. We thank the Instituto de Astrofísica de Canarias for allowing us to operate the telescope at the Observatorio del Roque de los Muchachos in La Palma, the Max-Planck-Institut fur Physik for providing us with the mount of the former HEGRA CT3 telescope, and the MAGIC Collaboration for their support. JBG acknowledges the support of the Viera y Clavijo program funded by ACIISI and ULL. C. W. is grateful for the support by the project "Promotion inklusive" of the Universitat zu Koln and the German Bundesministerium fur Arbeit und Soziales. This publication makes use of data obtained at Metsahovi Radio Observatory, operated by Aalto University, Finland.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 similar to 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 Metsahovi, 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 similar to 0.5 to similar 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 similar to 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 3 sigma. A fit with a log-parabola plus a narrow component is preferred over the fit with a single log-parabola at more than 4 sigma, and a dedicated Monte Carlo simulation estimated the significance of this extra component to be larger than 3 sigma. 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.Depto. de Estructura de la Materia, Física Térmica y ElectrónicaFac. de Ciencias FísicasTRUEMinisterio de Economía y Competitividad (MINECO)/FEDERCentro de Excelencia "Severo Ochoa"Unidad de Excelencia "María de Maeztu"German BMBF Federal Ministry of Education & Research (BMBF)German MPGItalian INFN Istituto Nazionale di Fisica Nucleare (INFN)Italian INAF Istituto Nazionale Astrofisica (INAF)Swiss National Fund SNF Swiss National Science Foundation (SNSF)Japanese JSPSJapanese MEXTCroatian Science Foundation (HrZZ)University of Rijeka ProjectDFG Collaborative Research Centers German Research Foundation (DFG)Polish National Research CentreBrazilian MCTICBrazilian CNPq National Council for Scientific and Technological DevelopmentBrazilian FAPERJETH ZurichGerman BMBF (Verbundforschung Astro- und Astroteilchenphysik)German HAP (Helmoltz Alliance for Astroparticle Physics)Deutsche Forschungsgemeinschaft (DFG) within the Collaborative Research Center German Research Foundation (DFG)Viera y Clavijo program - ACIISIULLProject "Promotion inklusive" of the Universitat zu KolnGerman Bundesministerium fur Arbeit und Sozialespu