Swift follow-up of the flaring BL Lac 1ES0806+524

ATel #3208; A. Stamerra (Univ. Siena, INFN Pisa, Italy), K. Berger (IAC and Univ. La Laguna, Spain), E. Lindfors (Tuorla Observ., Finland), M. Mariotti, E. Prandini (Univ. and INFN Padova, Italy), R. Reinthal (Tuorla Observ., Finland), C. Schultz (Univ, and INFN Padova, Italy)
on 4 Mar 2011; 18:40 UT
Credential Certification: Antonio Stamerra (antonio.stamerra@pi.infn.it)

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Following the gamma-ray, VHE (E>100 GeV) flare of the blazar 1ES 0806+525 (RA:08h09m49.2s DEC:+52d18m58.2s, z=0.138) detected by MAGIC on 2011 February 25 (ATel #3192), we performed a Swift target of opportunity observation starting on February 26. Swift monitored the source in photon counting mode with 5 daily observations for a total exposure of 10 ksec.

The X-ray spectrum (0.2-10 keV) is well described by an absorbed power law model with a photon index of 2.4+/-0.1 and HI column density fixed to the Galactic value nH=4.1E20 cm^-2 (Kalberla et al. 2005). The time-averaged flux in the 2-10 keV range is (8.7+/-1.0)E-12 erg/cm2/s and shows a enhancement in the last observations on March 2, increasing to (13.0+/-0.5)E-12 erg/cm2/s, with an indication of a marginal hardening of the spectrum. The measured flux is 2-3 times higher than previous observation on February 1 and is comparable to the flux measured in March 2008, when this BL Lac object was first detected in the VHE band by VERITAS (ATel #1415, Acciari et al., 2009, ApJ, 690, 126).

Swift/UVOT data were taken with filters in the U band, changing on a daily scale ("filter of the day" mode). The source appears very bright with a magnitude 14.3(+/-0.05) both in the UV-W2 and UV-M2 bands, almost unchanged respect to February 1 (14.4+/-0.05), but ~1 mag brighter compared to the UV flux measured in March 2008. The UV band photometry is compatible within the errors with a constant flux during our observations.

Swift/XRT observations confirmed the high activity state of the source and show a clear variability. Due to the rising trend of the X-ray flux we encourage further multi-wavelength observations, particularly TeV observations. We thank the Swift science planning team, and PI for approving the Swift ToO and for their rapid scheduling of this observations.