Journal Article

PKS 2123−463: a confirmed γ-ray blazar at high redshift

F. D'Ammando, A. Rau, P. Schady, J. Finke, M. Orienti, J. Greiner, D. A. Kann, R. Ojha, A. R. Foley, J. Stevens, J. M. Blanchard, P. G. Edwards, M. Kadler and J. E. J. Lovell

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 427, issue 1, pages 893-900
Published in print November 2012 | ISSN: 0035-8711
Published online November 2012 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2012.22041.x
PKS 2123−463: a confirmed γ-ray blazar at high redshift

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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 × 109 M. This is the first black hole mass estimate for this source.

Keywords: galaxies: active; galaxies: nuclei; quasars: general; quasars: individual: PKS 2123−463; gamma rays: general

Journal Article.  5898 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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