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

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 0.5-50 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-hour binning was used. With a photon index of Γ 0.1−300GeV = 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 fluxdoubling times of ∼ 2.2 hours 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.Conclusions.

Hunting for the nature of the enigmatic narrow-line Seyfert 1 galaxy PKS 2004-447

“…inated by a power-law component coming from the relativistic jet, while the thermal Comptonization model coming from the disk corona accounts for only 2% of the total flux (Gallo et al 2006;Kreikenbohm et al 2016;Berton et al 2019a). It is worth mentioning that Gokus et al (2021) did not detect any component associated with the accretion disk in their analysis of PKS 2004-447 X-ray spectrum observed after the flare, and this is in good agreement with our conclusions. If the jet flux is strongly enhanced, while the corona contribution does not change significantly, the latter will likely become too weak to be detected.…”

“…This result is in agreement with what was found by (Gallo et al 2006), who derived A(V) = 1.9 ± 1.5 from the AAT spectrum 3 . As they already pointed out, this extinction is significantly higher than what can be estimated from the X-ray spectra, which instead show negligible absorption, as confirmed by more recent observations (Kreikenbohm et al 2016;Berton et al 2019a;Gokus et al 2021). They also suggested that a possible explanation for this is a very different gas/dust ratio than what is seen in the Milky Way, and the jet may play a role in this by transferring material from the nucleus into the NLR.…”

“…Its X-ray spectrum, in particular, is well described by a single power law, likely due to the non-thermal emission of the relativistic jet (Kreikenbohm et al 2016). A soft X-ray excess was detected in XMM-Newton observations (Gallo et al 2006;Foschini et al 2009), although its presence is not confirmed in all observations and is correlated to a high flux level (Kreikenbohm et al 2016;Gokus et al 2021). It seems to be the high-energy tail of the synchrotron emission (Foschini et al 2009;Foschini 2020).…”

“…where r is the radius of the emitting region, and τ the observed time scale. Using the 6-hour binned γ-ray light curve derived by Gokus et al (2021), the variability between adjacent points indicated that the doubling time is of the order of ∼ 2 − 4 hours, consistent with the values observed at hard X-rays by Berton et al (2019a). If we assume that the jet structure is self-similar, and that its semi-opening angle is 0.1 radians, the size of the emitting region depends on the distance of the dissipation region from the central source.…”

“…Here we present new spectroscopic data for the southernmost (to date) γ-NLS1, PKS 2004-447. This NLS1 was identified as a γ-ray source soon after the launch of the Fermi Satellite (Abdo et al 2009c), and at the end of 2019 it underwent its first γ-ray flare ever recorded (Gokus 2019;Gokus et al 2021). The goal of this work is to study its long-term behavior and the nature of this flare by means of optical spectroscopy.…”