Abstract:
The γ-ray-loud narrow-line Seyfert 1 (γ-NLSy1) galaxy PKS 1502+036 (z = 0.409) exhibited its first γ-ray
outburst on 2015 December 20. In the energy range of 0.1–300 GeV, the highest flux measured by the Fermi-Large
Area Telescope is (3.90 ± 1.52)×10−6 ph cm s - - 2 1, which is the highest γ-ray flux ever detected from this
object. The associated spectral shape is soft (Γ0.1–300 GeV=2.57±0.17) and this corresponds to an isotropic γ-ray
luminosity of (1.2 ± 0.6)×1048 erg s−1. We generate the broadband spectral energy distribution (SED) during the
GeV flare and reproduce it using a one-zone leptonic emission model. The optical-UV spectrum can be explained
by a combination of synchrotron and accretion disk emission, whereas the X-ray-to-γ-ray SED can be satisfactorily
reproduced by inverse-Compton scattering of thermal photons that originated from the torus. The derived SED
parameters hint that the increase in the bulk Lorentz factor is a major cause of the flare and the location of the
emission region is estimated as being outside the broad-line region but still inside the torus. A comparison of the
GeV-flaring SED of PKS 1502+036 with that of two other γ-NLSy1 galaxies, namely, 1H 0323+342 (z = 0.061)
and PMN J0948+0022 (z = 0.585), and also with flat spectrum radio quasar (FSRQ) 3C 279 (z = 0.536), has led
to the conclusion that the GeV-flaring SEDs of γ-NLSy1 galaxies resemble FSRQs and a major fraction of their
bolometric luminosities are emitted at γ-ray energies.
