High-energy properties of the high-redshift flat spectrum radio quasar PKS 2149−306

Abstract

We investigate the γ-ray and X-ray properties of the flat spectrum radio quasar PKS 2149−306 at redshift z = 2.345. A strong γ-ray flare from this source was detected by the Large Area Telescope on board the Fermi Gamma-ray Space Telescope satellite in 2013 January, reaching on January 20 a daily peak flux of (301 ± 36) × 10^(−8) ph cm^(−2) s^(−1) in the 0.1–100 GeV energy range. This flux corresponds to an apparent isotropic luminosity of (1.5 ± 0.2) × 10^(50) erg s^(−1), comparable to the highest values observed by a blazar so far. During the flare the increase of flux was accompanied by a significant change of the spectral properties. Moreover significant flux variations on a 6-h time-scale were observed, compatible with the light crossing time of the event horizon of the central black hole. The broad-band X-ray spectra of PKS 2149−306 observed by Swift-XRT and NuSTAR are well described by a broken power-law model, with a very hard spectrum (Γ_1 ∼ 1) below the break energy, at E_(break) = 2.5–3.0 keV, and Γ_2 ∼ 1.4–1.5 above the break energy. The steepening of the spectrum below ∼3 keV may indicate that the soft X-ray emission is produced by the low-energy relativistic electrons. This is in agreement with the small variability amplitude and the lack of spectral changes in that part of the X-ray spectrum observed between the two NuSTAR and Swift joint observations. As for the other high-redshift FSRQ detected by both Fermi-LAT and Swift-BAT, the photon index of PKS 2149−306 in hard X-ray is 1.6 or lower and the average γ-ray luminosity higher than 2 × 10^(48) erg s^(−1).