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Keywords:

  • radiation mechanisms: non-thermal;
  • galaxies: active;
  • galaxies: individual: Pictor A = PKS 0518-458;
  • galaxies: jets;
  • gamma-rays: galaxies

ABSTRACT

We report the discovery of high-energy γ-ray emission from the broad-line radio galaxy (BLRG) Pictor A with a significance of ∼ 5.8σ [test statistic (TS) = 33.4], based on three years of observations with the Fermi Large Area Telescope (LAT) detector. The three-year-averaged E > 0.2 GeV γ-ray spectrum is adequately described by a power law, with a photon index, Γ, of 2.93 ± 0.03 and a resultant integrated flux of Fγ= (5.8 ± 0.7) × 10−9 photons cm−2 s−1.

A temporal investigation of the observed γ-ray flux, which binned the flux into year-long intervals, reveals that the flux in the third year was 50 per cent higher than the three-year-averaged flux. This observation, coupled with the fact that this source was not detected in the first two years of Fermi-LAT observations, suggests variability on time-scales of a year or less.

Synchrotron self-Compton modelling of the spectral energy distribution of a prominent hotspot in Pictor A’s western radio lobe is performed. It is found that the models in which the γ-ray emission originates within the lobes predict an X-ray flux larger than that observed. Given that the X-ray emission in the radio lobe hotspots has been resolved with the current suite of X-ray detectors, we suggest that the γ-ray emission from Pictor A originates from within its jet, which is in agreement with other γ-ray-loud BLRGs. This suggestion is consistent with the evidence that the γ-ray flux is variable on time-scales of a year or less.