GROND+Swift/UVOT photometric redshift for the flaring blazar PKS 2123-463

ATel #3814; A. Rau, P. Schady, J. Greiner (all MPE), and D. A. Kann (TLS Tautenburg)
on 19 Dec 2011; 19:16 UT
Credential Certification: Arne Rau (arau@mpe.mpg.de)

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Following the report of the Fermi/LAT detected brightening (Orienti et al. 2011, ATel #3808) we initiated simultaneous g'r'i'zJHK GROND (Greiner et al. 2008, PASP, 120, 405) observations of the flat spectrum radio quasar PKS 2123-463 on December 18th, 2011, at 01:21 UT. We measured the following AB magnitudes:

g' = 17.74 +/- 0.03
r' = 17.55 +/- 0.0.3
i' = 17.41 +/- 0.03
z' = 17.28 +/- 0.03
J = 16.89 +/- 0.06
H = 16.57 +/- 0.06
Ks = 16.43 +/- 0.09

Magnitudes were calibrated against GROND zero points in g'r'i'z' and 2MASS field stars in JHK.

We also analyzed Swift/UVOT ToO observations executed on Dec 19th 2011, around 04:20 UT and measured the following AB magnitudes:

uvw2 = 19.37 +/- 0.12
uvm2 = 19.02 +/- 0.12
uvw1 = 18.71 +/- 0.10
u = 18.19 +/- 0.07
b = 17.89 +/- 0.08
v = 17.91 +/- 0.17

As pointed out by Orienti et al. (2011, ATel #3808) the redshift of PK 2123-463 has not been established convincingly yet. The value of z=1.67 reported by Savage & Wright (1981, MNRAS, 196, 927) based on an objective-prism spectrum was subsequently been questioned by Jackson et al. (2002, A&A, 386, 97). Following the method described in Rau et al. (2012, arXiv:1112.0025) and applied to a sample of 103 Fermi/LAT blazars, we combined the UVOT and GROND photometry of PKS 2123-463 in order to construct a 13-band spectral energy distribution. Source-intrinsic variability and the Galactic foreground of E(B-V)=0.03mag (Schlegel et al. 1998, ApJ 500, 525) were taken into account. The resulting spectral energy distribution has been fit with a set of power law models as well as with hybrid templates built from normal galaxies and AGN (Salvato et al. 2009, ApJ 690, 1250, & 2011, ApJ, 742, 61) using the Le PHARE code (Arnouts et al., 1999, MNRAS 310, 540; Ilbert et al. 2006, A&A, 457, 841).

The best-fit redshifts for both template libraries agree well. For the power law model we obtain z=1.52 +0.14/-0.24 (99% confidence) and for the hybrid models we find z=1.51 +0.07/-0.07. This is in rough agreement with the initial spectroscopic redshift of z=1.67 from Savage & Wright (1981, MNRAS, 196, 927). The better fit-statistics and tighter redshift probability distribution function for the best-fit hybrid model is in agreement with the expectation that the flat spectrum radio quasar PKS 2123-463 should be better described by a model including emission lines. Some of these emission lines, in particular CIV and MgII, are located in the optical part of the spectrum and their spectroscopic confirmation would provide further confidence in the redshift estimate.