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The large gamma-ray flare of the flat-spectrum radio quasar ... › publication › fulltext › The-large-... › publication › fulltext › The-large-...by R Angioni · ‎2019 · ‎Cited by 3 · ‎Related articlestarget for future ground-based Cherenkov observatories s

Astronomy & Astrophysics

c R. Angioni et al. 2019

The large gamma-ray flare of the flat-spectrum radio quasar PKS 0346−27? R. Angioni1,2 , R. Nesci3 , J. D. Finke4 , S. Buson2 , and S. Ciprini5,6 1 2 3 4 5 6

Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany e-mail: [email protected] Institut für Theoretische Physik und Astrophysik, Universität Würzburg, Emil-Fischer-Str. 31, 97074 Würzburg, Germany INAF/IAPS, Via Fosso del Cavaliere 100, 00133 Roma, Italy US Naval Research Laboratory, Code 7653, 4555 Overlook Ave. SW, Washington, DC 20375-5352, USA Space Science Data Center – Agenzia Spaziale Italiana, Via del Politecnico, snc, 00133 Roma, Italy Istituto Nazionale di Fisica Nucleare, Sezione di Perugia, 06123 Perugia, Italy

Received 13 March 2019 / Accepted 19 June 2019 ABSTRACT Aims. In this paper, we characterize the first γ-ray flaring episode of the flat-spectrum radio quasar PKS 0346−27 (z = 0.991), as revealed by Fermi-LAT monitoring data, and the concurrent multi-wavelength variability observed from radio through X-rays. Methods. We studied the long- and short-term flux and spectral variability from PKS 0346−27 by producing γ-ray light curves with different time binning. We complement the Fermi-LAT data with multi-wavelength observations from the Atacama Large Millimeter Array (radio mm-band), the Rapid Eye Mount telescope (near-infrared) and Swift (optical-UV and X-rays). This quasi-simultaneous multi-wavelength coverage allowed us to construct time-resolved spectral energy distributions (SEDs) of PKS 0346−27 and compare the broadband spectral properties of the source between different activity states using a one-zone leptonic emission model. Results. PKS 0346−27 entered an elevated γ-ray activity state starting from the beginning of 2018. The high-state continued throughout the year, displaying the highest fluxes in May 2018. We find evidence of short-time scale variability down to approximately 1.5 h, which constrains the γ-ray emission region to be compact. The extended flaring period was characterized by a persistently harder spectrum with respect to the quiescent state, indicating changes in the broadband spectral properties of the source. This was confirmed by the multi-wavelength observations, which show a shift in the position of the two SED peaks by approximately two orders of magnitude in energy and peak flux value. As a result, the non-thermal jet emission completely outshines the thermal contribution from the dust torus and accretion disk during the high state. The broadband SED of PKS 0346−27 transitions from a typical LowSynchrotron-Peaked (LSP) to the Intermediate-Synchrotron-Peaked (ISP) class, a behavior previously observed in other flaring γ-ray sources. Our one-zone leptonic emission model of the high-state SEDs constrains the γ-ray emission region to have a lower magnetic field, larger radius, and higher maximum electron Lorentz factors with respect to the quiescent SED. Finally, we note that the bright and hard γ-ray spectrum observed during the peak of flaring activity in May 2018 implies that PKS 0346−27 could be a promising target for future ground-based Cherenkov observatories such as the Cherenkov Telescope Array (CTA). The CTA could detect such a flare in the low-energy tail of its energy range during a high state such as the one observed in May 2018. Key words. galaxies: active – galaxies: jets – quasars: individual: PKS 0346−27 – gamma rays: galaxies

1. Introduction Radio-loud active galactic nuclei (AGN) are the most common astrophysical source class in the γ-ray sky. Their relativistic jets produced by the central black hole emit bright nonthermal radiation across a wide range of wavelengths and energies, from radio to TeV. The vast majority of γ-ray detected AGN are blazars, that is sources where the relativistic jet is aligned at a small angle with the observer’s line of sight, leading to strong relativistic Doppler boosting and beaming effects. Blazars have historically been divided into two subclasses: flatspectrum radio quasars (FSRQs), showing strong broad emission lines in their optical spectra, and BL Lacertae objects (BL Lacs), which typically have weak or absent optical spectral features. There is also evidence that flat-spectrum radio-loud ? Tables of the measured fluxes are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/627/ A140

narrow-line Seyfert 1 (NLS1) galaxies host low-power blazartype jets aligned with our line of sight (Berton et al. 2018). The broadband spectral energy distributions (SED) of blazars show a typical double-peaked structure. The low-energy peak is produced by synchrotron emission from the relativistic electrons in the jet. The emission processes giving rise to the highenergy emission peak are somewhat less clear. The simplest models invoke inverse Compton (IC) emission from the same electrons producing the low-energy synchrotron emission (the so called leptonic emission models), while more advanced models involve relati