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Swift follow-up of the blazar 4C+21.35 (PKS 1222+21) after a bright gamma-ray flare

ATel #2698; F. Verrecchia, M. Capalbi, C. Pittori (ASDC), S. Vercellone (INAF/IASF-Pa), M. Tavani (INAF/IASF-Rm, and Univ. Tor Vergata), P. Giommi (ASDC), and F. D'Ammando (INAF/IASF-Pa), D. Donato (NASA/GSFC), G. Iafrate (INAF/OATs Trieste), Y. Tanaka (ISAS/JAXA) on behalf of the AGILE and Fermi Collaborations
on 26 Jun 2010; 16:58 UT
Distributed as an Instant Email Notice Request For Observations
Credential Certification: Francesco Verrecchia (francesco.verrecchia@asdc.asi.it)

Subjects: Optical, Ultra-Violet, X-ray, Gamma Ray, >GeV, AGN, Blazar, Quasar

Referred to by ATel #: 2708, 6207

Following the AGILE/GRID (ATel #2686) and Fermi/LAT (ATel #2687) detection of a gamma-ray flare on June 19, 2010, from a source positionally consistent with the Flat Spectrum Radio Quasar 4C +21.35 (also known as PKS 1222+21 and 1FGL J1224.7+2121; R.A.=12h24m54.5s, Dec=+21d22m46.4s, J2000) at z=0.43, six Swift Target of Opportunity observations were performed between 20 and 23 June.

The Swift/XRT data were taken in Photon Counting mode for a total exposure time of 13.5 ks, while the Swift/UVOT was operated with all filters. The mean XRT spectrum in the range 0.3-10 keV can be described with an absorbed power law model with a column density fixed to the galactic value (N_H=2.1e20 cm^-2, Kalberla et al. 2005, A&A, 440, 775) and a photon index 1.63 +/- 0.06. The unabsorbed 0.3-10 keV mean flux was (5.6 +/- 0.2)e-12 ergs/cm^2/s. The X-ray flux does not show significant variability among the observations.

The Swift/UVOT detected the source at the following mean magnitudes:

 Filter  Total Expos.     Mag         
  V        1184.1      15.25+/-0.05   
  B        1208.6      15.38+/-0.05   
  U        1262.7      14.40+/-0.04   
  UVW1     2660.8      14.12+/-0.02   
  UVM2     4483.7      13.93+/-0.03   
  UVW2     4745.0      13.93+/-0.05   

The optical/UV bands photometry are compatible within 2 sigma with a constant flux.

Further multiwavelength observations of the source are encouraged. We thank the Swift team for their rapid scheduling of these observations.