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Swift follow up observations of the flaring gamma-ray blazar 4C +28.07

ATel #3676; K. V. Sokolovsky (ASC Lebedev/SAI MSU), F. D'Ammando (INAF-IRA Bologna), F. K. Schinzel (MPIfR) on behalf of the Fermi Large Area Telescope Collaboration and J. A. Kennea (PSU)
on 11 Oct 2011; 16:23 UT
Credential Certification: Kirill Sokolovsky (kirx@scan.sai.msu.ru)

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

Referred to by ATel #: 3709

Prompted by the gamma-ray flare of 4C +28.07 detected by Fermi LAT on 2011 October 3 (ATel #3670) and the following reports on the optical (ATel #3672) and radio (ATel #3674) activity, we performed two Swift target of opportunity observations on October 6 and 9 for a total exposure of 4.9 and 2.9 ksec, respectively.

Swift/XRT data were taken in Photon Counting mode. The X-ray spectrum (0.3-10 keV) 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_HI = 7.56 x10^20 cm^-2, Kalberla et al. 2005, A&A, 440, 775) and a photon index Gamma=1.53+/-0.18 on October 6, and 1.68+/-0.17 on October 9. The corresponding observed 0.3-10 keV flux is (4.0+/-0.5) x10^-12 erg cm^-2 s^-1 and (3.5+/-0.5) x10^-12 erg cm^-2 s^-1, respectively. The flux observed on 2011 October 6 is a factor of 2.5 higher than that previously observed by Swift/XRT on 2010 February 5 (1.6+/-0.5 x10^-12 erg cm^-2 s^-1 with Gamma = 1.45+/-0.24).

Swift/UVOT has found the source at U=17.99 and U=18.08 on 2011 October 6 and 9, respectively, compared to U=18.56 observed on 2010 February 5. The photometric accuracy is 0.04 mag. in all cases. There is also a hint of possible intraday variability between six individual UVOT exposures taken on October 6 (09:10 - 17:08 UT).

In summary, the Swift observations revealed an elevated level of X-ray and optical (U band) activity of 4C +28.07.

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