Publication
The first GeV flare of the radio-loud narrow-line Seyfert 1 galaxy PKS 2004–447
Publisher:
EDP Sciences
Date:
05-2021
DOI:
10.1051/0004-6361/202039378
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) × 10 47 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.