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

On 2019 October 25, the Fermi-Large Area Telescope observed the first gamma-ray flare from the radio-loud narrow-line Seyfert 1 (NLSy 1) galaxy PKS 2004−447 (z=0.24). 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 our multi-wavelength analysis. We study the variability across all energy bands and additionally produce γ-ray light curves with different time binnings to study the variability on short timescales during the flare. We examine the X-ray spectrum from 0.5−50 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 SEDs with a one-zone synchrotron inverse Compton radiative model. Finally, we compare our results to gamma-ray flares previously observed from other γ-loud NLSy 1 galaxies. At gamma-ray energies (0.1−300 GeV) the flare reached a total maximum flux of (2.7±0.6)×10⁻⁶~ph~cm⁻²~s⁻¹ in 3-hour binning. With a photon index of Γ0.1−300GeV=2.42±0.09 during the flare, this corresponds to an isotropic gamma-ray luminosity of (2.9±0.8)×10⁴⁷ergs⁻¹. The γ-ray, X-ray, and optical-UV light curves covering the end of September to the middle of November show significant variability, and we find indications for flux-doubling times of ∼2.2~hours at γ-ray energies. 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, similarly observed for flaring gamma-ray blazars.