PKS 1424+240 - First detection of a new BL Lac object at an unknown redshift
Reference: V. A. Acciari et al. (The VERITAS Collaboration with the Fermi LAT team and Swift and MDM collaborators), The Astrophysical Journal Letters, 708:L100, 2010
ArXiv version: ArXiV:0912.0730
Contact person: Nepomuk Otte
We report the first detection of very high energy (VHE) gamma-ray emission above 140 GeV from PKS 1424+240, a BL Lac object with an unknown redshift. The photon spectrum above 140 GeV measured by VERITAS is well described by a steep power law with a photon index of 3.8 ± 0.5 (stat ) ± 0.3 (syst) and a flux level that is about 5% that of the Crab Nebula gamma-ray flux. The VHE flux is steady over the observation period between MJD 54881 and 55003 (from 2009 February 19 to June 21). Flux variability is also not observed in contemporaneous high energy observations with the Fermi Large Area Telescope (LAT). Contemporaneous X-ray and optical data were also obtained from the Swift XRT and MDM observatory, respectively.
This is the first time that observations with the Fermi LAT have led to the discovery of a new source in the adjacent VHE band. The combination of VERITAS and Fermi data indicate a break in the spectrum at several tens of GeV. The break can be explained by a one-zone SSC model assuming a wide range of redshifts or could result from EBL absorption if the redshift is about 0.5 and the intrinsic photon index is 1.7, from which a redshift upper limit of 0.66 is inferred. The modeling favors a lower redshift but cannot exclude that PKS 1424+240 is among the most distant sources detected in the VHE regime. PKS 1424+240 is the third extragalactic source detected in the VHE regime with an unknown or uncertain redshift. It is evident that increased efforts are needed to determine the redshifts of VHE detected blazars. A redshift measurement will allow a better understanding of the source-intrinsic mechanisms and the absorption effects which go along with the gamma-ray propagation.
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