The first GeV flare of the radio-loud narrow-line Seyfert 1 galaxy PKS 2004–447
A. Gokus, V. S. Paliya, S. M. Wagner, S. Buson, F. D’Ammando, P. G. Edwards, M. Kadler, M. Meyer, R. Ojha, J. Stevens, J. Wilms
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.