The first GeV flare of the radio-loud narrow-line Seyfert 1 galaxy PKS 2004–447

¹ Remeis Observatory and Erlangen Centre for Astroparticle Physics, Universität Erlangen-Nürnberg, Sternwartstr. 7, 96049 Bamberg, Germany
e-mail: andrea.gokus@fau.de
² Lehrstuhl für Astronomie, Universität Würzburg, Emil-Fischer-Straße 31, 97074 Würzburg, Germany
³ Aryabhatta Research Institute of Observational Sciences (ARIES), Manora Peak, Nainital 263001, India
e-mail: vaidehi.s.paliya@gmail.com
⁴ Istituto di Radioastronomia – INAF, Via P. Gobetti 101, 40129 Bologna, Italy
⁵ CSIRO Astronomy and Space Science, PO Box 76 Epping, NSW 1710, Australia
⁶ Friedrich-Alexander Universität Erlangen-Nürnberg, Erlangen Centre for Astroparticle Physics, Erwin-Rommel-Str. 1, 91058 Erlangen, Germany
⁷ NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
⁸ Catholic University of America, Washington, DC 20064, USA
⁹ University of Maryland, Baltimore County, 1000 Hilltop Cir, Baltimore, MD 21250, USA
¹⁰ CSIRO Astronomy and Space Science, 1828 Yarrie Lake Road, Narrabri, NSW 2390, Australia

Received: 9 September 2020
Accepted: 20 February 2021

Abstract

Context. On 2019 October 25, the Fermi-Large Area Telescope observed the first ever γ-ray flare from the radio-loud narrow-line Seyfert 1 galaxy PKS 2004−447 (z = 0.24). Prior to this discovery, only four sources of this type had shown a flare at gigaelectronvolt energies.

Aims. We report on follow-up observations in the radio, optical-UV, and X-ray bands that were performed by ATCA, the Neil Gehrels Swift Observatory, XMM-Newton, and NuSTAR, respectively, and analyse these multi-wavelength data with a one-zone leptonic model in order to understand the physical mechanisms that were responsible for the flare.

Methods. We study the source’s variability across all energy bands and additionally produce γ-ray light curves with different time binnings to study the variability in γ-rays on short timescales during the flare. We examine the combined X-ray spectrum from 0.5 to 50 keV by describing the spectral shape with an absorbed power law. We analyse multi-wavelength datasets before, during, and after the flare and compare these with a low activity state of the source by modelling the respective spectral energy distributions (SEDs) with a one-zone synchrotron inverse Compton radiative model. Finally, we compare the variability and the SEDs to γ-ray flares previously observed from other γ-loud narrow-line Seyfert 1 galaxies.

Results. At γ-ray energies (0.1−300 GeV) the flare reached a maximum flux of (1.3 ± 0.2) × 10⁻⁶ ph cm⁻² s⁻¹ in daily binning and a total maximum flux of (2.7 ± 0.6) × 10⁻⁶ ph cm⁻² s⁻¹ when a 3 h binning was used. With a photon index of Γ_{0.1−300 GeV} = 2.42 ± 0.09 during the flare, this corresponds to an isotropic γ-ray luminosity of (2.9 ± 0.8) × 10⁴⁷ erg s⁻¹. The γ-ray, X-ray, and optical-UV light curves that cover the end of September to the middle of November show significant variability, and we find indications for flux-doubling times of ∼2.2 h at γ-ray energies. The soft X-ray excess, which is observed for most narrow-line Seyfert 1 galaxies, is not visible in this source. During the flare, the SED exhibits large Compton dominance. While the increase in the optical-UV range can be explained by enhanced synchrotron emission, the elevated γ-ray flux can be accounted for by an increase in the bulk Lorentz factor of the jet, similar to that observed for other flaring γ-ray blazars.