The awakening of the γ-ray narrow-line Seyfert 1 galaxy PKS 1502+036

Abstract

After a long low-activity period, a γ-ray flare from the narrow-line Seyfert 1 PKS 1502+036 (z = 0.4089) was detected by the Large Area Telescope (LAT) on board Fermi in 2015. On 2015 December 20 the source reached a daily peak flux, in the 0.1-300 GeV band, of (93 ± 19)×10 −8 ph cm −2 s −1 , attaining a flux of (237 ± 71)×10 −8 ph cm −2 s −1 on 3-hr time-scales, which corresponds to an isotropic luminosity of (7.3 ± 2.1)×10 47 erg s −1 . The γ-ray flare was not accompanied by significant spectral changes. We report on multi-wavelength radio-to-γ-ray observations of PKS 1502+036 during 2008 August-2016 March by Fermi-LAT, Swift, XMM-Newton, Catalina Real-Time Transient Survey, and the Owens Valley Radio Observatory (OVRO). An increase in activity was observed on 2015 December 22 by Swift in optical, UV, and X-rays. The OVRO 15 GHz light curve reached the highest flux density observed from this source on 2016 January 12, indicating a delay of about three weeks between the γ-ray and 15 GHz emission peaks. This suggests that the γ-ray emitting region is located beyond the broad line region. We compared the spectral energy distribution (SED) of an average activity state with that of the flaring state. The two SED, with the highenergy bump modelled as an external Compton component with seed photons from a dust torus, could be fitted by changing the electron distribution parameters as well as the magnetic field. The fit of the disc emission during the average state constrains the black hole mass to values lower than 10 8 M ⊙ . The SED, high-energy emission mechanisms, and γ-ray properties of the source resemble those of a flat spectrum radio quasar.

A Panchromatic View of Relativistic Jets in Narrow-Line Seyfert 1 Galaxies

“…On the contrary, a relatively hard X-ray spectrum was detected in the Swift/XRT observations of SBS 0846+513 [12,21], PMN J0948+0022 [22,23,29], 1H 0323+342 [24], PKS 1502+036 [25,30], and PKS 2004−447 [31,32]. This suggests a significant contribution of inverse Compton radiation from a relativistic jet in X-rays, similar to what is found for FSRQ (e.g., [33]).…”

“…Variability was observed during all γ-ray flares on a daily time-scale; in case of PKS 1502+036 sub-daily variability was observed [25]. Moreover, SBS 0846+513 and PMN J0948+0022 showed a γ-ray flaring activity combined with a moderate spectral evolution, a behaviour that was already observed in bright FSRQ and low-synchrotron-peaked BL Lacs [26].…”

“…Moreover, several strong γ-ray flares were observed from SBS 0846+513, PMN J0948+0022, PKS 1502+036, and 1H 0323+342, reaching a peak apparent isotropic γ-ray luminosity of ∼10 48 erg · s −1 [21][22][23][24][25], comparable to that of bright FSRQ [20]. Variability was observed during all γ-ray flares on a daily time-scale; in case of PKS 1502+036 sub-daily variability was observed [25].…”

“…The optical and UV part of the spectrum of FSRQ is usually dominated by the thermal emission of the accretion disc, while in BL Lac objects the disc is radiatively inefficient, with the disc emission overwhelmed by the non-thermal jet emission (e.g., [45]). The accretion disc emission is visible in the low activity state of the SED of PMN J0948+0022 [23], 1H 0323+342 [5] and PKS 1502+036 [25]. On the contrary, no significant evidence of thermal emission from the accretion disc has been observed in SBS 0846+513 [21] and PKS 2004−447 [31].…”

“…A better fit was obtained by using a broken power-law model, suggesting the presence of two emission components in X-rays, but the uncertainties related to the spectral parameters are quite large [25].…”