The First Gev Flare Of The Radio-Loud Narrow-Line Seyfert 1 Galaxy Pks 2004-447

Context. On 2019 October 25, the Fermi-Large Area Telescope observed the first ever gamma -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 gamma -ray light curves with different time binnings to study the variability in gamma -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 gamma -ray flares previously observed from other gamma -loud narrow-line Seyfert 1 galaxies.Results. At gamma -ray energies (0.1-300 GeV) the flare reached a maximum flux of (1.3 +/- 0.2) x 10(-6) ph cm(-2) s(-1) in daily binning and a total maximum flux of (2.7 +/- 0.6) x 10(-6) ph cm(-2) s(-1) when a 3 h binning was used. With a photon index of Gamma (0.1-300 GeV)=2.42 +/- 0.09 during the flare, this corresponds to an isotropic gamma -ray luminosity of (2.9 +/- 0.8) x 10(47) erg s(-1). The gamma -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 similar to 2.2 h at gamma -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 gamma -ray flux can be accounted for by an increase in the bulk Lorentz factor of the jet, similar to that observed for other flaring gamma -ray blazars.