Swift XRT/UVOT follow-up of blazar PKS B0906+015 after a gamma-ray flare

ATel #2582; K. V. Sokolovsky (MPIfR/ASC Lebedev), F. K. Schinzel (MPIfR), F. D'Ammando (INAF - IASF Palermo), M. Kadler (Remeis-Observatory/ECAP/CRESST/USRA), C. S. Chang (MPIfR) on behalf of the Fermi Large Area Telescope Collaboration; M. A. Stark (PSU) and E. Ros (MPIfR/Uni. Valencia)
on 23 Apr 2010; 12:03 UT
Credential Certification: Frank Schinzel (schinzel@mpifr-bonn.mpg.de)

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Following the Fermi-LAT detection of a GeV flare from the flat spectrum radio quasar PKS B0906+015 on 2010 April 08 (ATel #2543), Swift Target of Opportunity observations were performed on 2010 April 18. The Swift X-ray telescope (XRT) was operated in photon counting mode during a 3 ksec exposure, the UV/Optical Telescope (UVOT) was using the U filter. An absorbed power law fit to the observed XRT spectrum of the source gives a photon index of 1.38 +/-0.12, and a (0.3-10) keV flux of (6.2 +/-0.8)x10^-12 ergs/cm^2/s. The neutral hydrogen column density along the line of sight was fixed at the Galactic value N_HI = 0.283 x10^21 cm^-2 (Kalberla et al. 2005, A&A, 440, 775).

Previous observations of the source were obtained in March 2008 and March 2010 in the framework of the MOJAVE-Swift program aimed to provide first-epoch data sets for potential gamma-ray blazars. Comparison of the latest XRT observations with those conducted on 2010 March 09-10 show an X-ray flux increase by a factor of 3, as well as signs of spectral variability: photon index = 1.65 +/- 0.17, (0.3-10) keV flux = (2.2 +/-0.3) x 10^-12 erg/cm^2/s. The 2008 March 08 observations indicate an intermediate flux level of (4.1 +/-0.4) x 10^-12 ergs/cm^2/s with a photon index of 1.45 +/-0.12.

Surprisingly, simultaneous U band photometry indicates only marginal variability between March and April 2010:
JD U err
2455260.12 16.63 0.11
2455260.98 16.61 0.17
2455264.95 16.67 0.06
2455266.01 16.75 0.07
2455304.82 16.53 0.02

In consideration of the recent activity of this source we encourage further multi-wavelength observations. For this source the Fermi LAT contact person is K. Sokolovsky (ksokolov[at]mpifr-bonn.mpg.de).

We thank the Swift team for their rapid scheduling of these observations.