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Swift follow-up of the high redshift flaring blazar PKS 2123-463

ATel #3815; F. D'Ammando (INAF-IRA Bologna), M. Orienti (Univ. Bologna, INAF-IRA Bologna) on behalf of the Fermi Large Area Telescope Collaboration, and C. J. Mountford (U Leicester)
on 21 Dec 2011; 00:54 UT
Credential Certification: Filippo D'Ammando (filippo.dammando@iasf-roma.inaf.it)

Subjects: Optical, Ultra-Violet, X-ray, AGN, Blazar, Quasar

Following the gamma-ray flare of the high redshift flat spectrum radio quasar PKS 2123-463 (also known as 1FGL J2126.1-4603, Abdo et al. 2010, ApJS, 188, 405, and 2FGL J2125.0-4632, Abdo et al. 2011, submitted to ApJS, arXiv:1108.1435) detected by Fermi LAT on December 14 (ATel #3808), two Swift target of opportunity observations were performed on December 15 and 19.

Swift/XRT data were taken in Photon Counting mode for a total exposure of about 4.9 ksec and 5.4 ksec for December 15 and 19, respectively. The X-ray spectrum (0.3-10 keV) observed in December 15 can be fit by an absorbed power law model with a HI column density consistent with the Galactic value in the direction of the source (n_H = 2.34 x 10^20 cm^-2, Kalberla et al. 2005, A&A, 440, 775) and a photon index of 1.86+/-0.15. The corresponding observed 0.3-10 keV flux is (1.79+/-0.25) x10^-12 erg cm^-2 s^-1. This flux is 50% higher than that observed by Swift/XRT on 2011 July 3, (1.20+/-0.38) x10^-12 erg cm^-2 s^-1, with a photon index of 1.67+/-0.27. The source was observed by XRT in December 19 at a similar flux level of December 15, (1.83+/-0.21) x10^-12 erg cm^-2 s^-1.

Simultaneous Swift/UVOT observations in December 15, 2011 found PKS 2123-463 about 0.3 mag brighter in U-band, 0.45 mag in M2-band, and 1.0 mag in W2-band compared to the UVOT observations performed on July 3, 2011. The measured AB magnitudes are:

Band 2011-07-03 2011-12-15
V 17.93+/-0.24 17.63+/-0.26
B 17.77+/-0.13 -
U 18.26+/-0.09 17.98+/-0.20
W1 18.58+/-0.08 18.40+/-0.15
M2 19.48+/-0.13 19.03+/-0.22
W2 20.17+/-0.25 19.14+/-0.21

The source continued to be bright in UV also in December 19, at M2 = 19.02+/-0.12 and W2 = 19.37+/-0.12, as reported in ATel #3814.

Recently, a photometric redshift of z = 1.52+0.14/-0.24 or z = 1.51+/-0.07 (99% confidence level) was estimated by Rau et al. combining the UVOT and GROND photometry of PKS 2123-463 (ATel #3814), in rough agreement with the initial spectroscopic redshift of z = 1.67 from Savage & Wright (1981, MNRAS, 196, 927).

Further multiwavelength observations are encouraged. For this source the Fermi LAT contact person is M. Orienti (orienti@ira.inaf.it).

We would like to thank the Swift Team for making these observations possible.