Spectral energy distribution of PKS 1424+240. The lines show SSC-model fits assuming different redshifts. The inset shows a zoom of the SED on the VERITAS data in a nu F_nu representation. The Fermi data are presented together with their corresponding power-law fit and one standard deviation uncertainty. The upper limits correspond to 95% confidence level.

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

Full text version

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.

Figures from paper (click to get full size image):

Figure 1: Distribution of theta-2 for VERITAS events selected with soft cuts. The points with error bars denote the on-source events. The background is shown by the shaded histogram. The dashed vertical line shows the applied theta-2-cut. The expected distribution for a point source is given by the dotted line.

Figure 2: Light curves of PKS 1424+240 in VHE gamma rays (VERITAS), HE gamma rays (Fermi-LAT), X-rays (Swift XRT), UV (Swift UVOT), and optical (Swift UVOT, MDM). The X-ray, UV, and optical light curves cover the time period indicated in the upper two light curves by the shaded region. The horizontal bars in the VHE and HE light curves give the range over which the flux has been integrated. The HE upper limit is at the 95% confidence level.

Figure 3: Time-averaged differential photon spectrum of PKS 1424+240 measured by VERITAS between 2009 February 19 and June 21. The triangles are from the soft-cuts analysis and the squares from the medium-cuts analysis. The flux point at 260 GeV is reconstructed in both analysis. The solid lines shows the fit with a power law. The shaded area shows the systematic uncertainty of the fit, which is dominated by a 20% uncertainty on the energy scale.

Figure 4 (shown above): Spectral energy distribution of PKS 1424+240. The lines show SSC-model fits assuming different redshifts. The inset shows a zoom of the SED on the VERITAS data in a nu F_nu representation. The Fermi data are presented together with their corresponding power-law fit and one standard deviation uncertainty. The upper limits correspond to 95% confidence level.