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Update on Swift follow-up observations of the GeV flaring blazar 3C 279

ATel #7668; C. Pittori, F. Verrecchia, S. Puccetti, M. Perri (ASDC and INAF/OAR), and M. Tavani (INAF/IAPS)
on 19 Jun 2015; 11:18 UT
Credential Certification: Carlotta Pittori (carlotta.pittori@asdc.asi.it)

Subjects: Optical, X-ray, Gamma Ray, >GeV, Blazar, Transient

We report the results of the quick look analysis of 4 Swift target of opportunity observations performed on 2015, June 16-17, following the gamma-ray flare of the flat spectrum radio quasar 3C 279 detected by AGILE (ATel #7631). The brightening of the source was also observed by Fermi (ATel #7633) and by INTEGRAL (ATel #7648).

The results of the first 2.0 ksec Swift follow-up observation taken in Photon Counting mode have been already reported in ATel #7639, and are included in the following table for completeness. The following 4 Swift/XRT observations were taken in Photon Counting mode for a net exposure of about 1.0 ksec each. According to our preliminary quick look analysis the observed 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_H = 2.2 x 10^20 cm^-2, Kalberla et al. 2005, A&A, 440, 775). The photon index, and the preliminary absorbed flux values (erg cm^-2 s^-1) are:

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Obs Start (MJD) | photon_index | Absorbed Flux(0.3-10 keV)

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57188.6028 | (1.36+/-0.06) | (5.5 +/- 0.4) x 10^-11

57189.1444 | (1.32+/- 0.08) | (9.4 +/- 0.8) x 10^-11

57189.6701 | (1.4 +/- 0.1) | (3.5 +/- 0.5) x 10^-11

57190.1951 | (1.4 +/- 0.1) | (2.7 +/- 0.4) x 10^-11

57190.8750 | (1.3 +/- 0.2) | (2.0 +/- 0.5) x 10^-11

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The UVOT simultaneous observations with the optical/UV filter of the day provide the provide the preliminary observed magnitudes in the Vega system:

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MJD Filter Magnitude

57188.6180+/-0.0115 U 14.93+/-0.03

57189.1528+/-0.0056 W2 15.35+/-0.04

57189.6787+/-0.0054 W2 15.44+/-0.04

57190.2037+/-0.0054 M2 15.38+/-0.04

57190.8806+/-0.0028 M2 15.64+/-0.05

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Magnitudes are not corrected for Galactic extinction due to a reddening E(B-V)=0.025 (Schlegel et al. 1998, Schlafly et al. 2011).

As a comparison, analysis of the recent archival UVOT observation of May 23, 2015: gives U=15.89+/-0.06 and M2=16.02+/-0.07; for the observation of May 26, 2015: W2=16.11+/-0.04.

We would like to thank the Swift Team for making this observations possible, in particular the Swift PI, Neil Gehrels, and B. Sbarufatti as the Swift Observatory Duty Scientist.