National Aeronautics and Space Administration


Date of this Version



Mon. Not. R. Astron. Soc. 427, 893–900 (2012); doi:10.1111/j.1365-2966.2012.22041.x


The flat spectrum radio quasar (FSRQ) PKS 2123−463 was associated in the first Fermi- Large Area Telescope (LAT) source catalogue with the γ -ray source 1FGL J2126.1−4603, but when considering the full first two years of Fermi observations, no γ -ray source at a position consistent with this FSRQ was detected, and thus PKS 2123−463 was not reported in the second Fermi-LAT source catalogue. On 2011 December 14 a γ -ray source positionally consistent with PKS 2123−463 was detected in flaring activity by Fermi-LAT. This activity triggered radio-to-X-ray observations by the Swift, Gamma-ray Optical/Near-Infrared Detector (GROND), Australia Telescope Compact Array (ATCA), Ceduna and Seven Dishes Karoo Array Telescope (KAT-7) observatories. Results of the localization of the γ -ray source over 41 months of Fermi-LAT operation are reported here in conjunction with the results of the analysis of radio, optical, ultraviolet (UV) and X-ray data collected soon after the γ -ray flare. The strict spatial association with the lower energy counterpart together with a simultaneous increase of the activity in optical, UV, X-ray and γ -ray bands led to a firm identification of the γ -ray source with PKS 2123−463. A new photometric redshift has been estimated as z = 1.46 ± 0.05 using GROND and Swift Ultraviolet/Optical Telescope (UVOT) observations, in rough agreement with the disputed spectroscopic redshift of z = 1.67.We fit the broad-band spectral energy distribution with a synchrotron/external Compton model. We find that a thermal disc component is necessary to explain the optical/UV emission detected by Swift/UVOT. This disc has a luminosity of∼1.8×1046 erg s−1, and a fit to the disc emission assuming a Schwarzschild (i.e. non-rotating) black hole gives a mass of ∼2 × 109Mʘ. This is the first black hole mass estimate for this source.