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Swift XRT/UVOT follow-up of blazar PKS 1118-056 after a gamma-ray flare

ATEL # 1943; C.C. Cheung, D. Donato (NASA GSFC), L.C. Reyes (KICP - University of Chicago)
on 25 Feb 2009; 22:39 UT
Password Certification: Teddy Cheung (

Subjects: Optical, X-ray, AGN

Following the Fermi-LAT detection of a GeV flare from the blazar PKS 1118-056 (z=1.297) on Feb 17th (ATEL #1932), Swift observations were obtained on Feb 18th, 20th, and 22nd. These observations confirm that the blazar is also in a high X-ray and optical state.

The blazar is well detected in each XRT observation (2.5-3.0 ks exposures). Combining the three XRT exposures, the data are well fit by a power law with a photon index Gamma=1.37 +/- 0.15 and an average 0.3-10 keV flux = 2.3e-12 erg/cm2/s (unabsorbed assuming Galactic NH = 3.5e20 cm-2). The X-ray flux decreases by ~20% over the 4 day span of the observations. The fluxes are about two times larger than the previously reported ROSAT flux (J. Siebert et al. 1998, MNRAS, 301, 261) assuming the same photon index derived from the XRT data.

The blazar is well detected in all six UVOT images (5.5e14 - 1.6e15 Hz) at all 3 epochs. During the first epoch, the extinction corrected B-band magnitude was 17.8 +/- 0.1, with the corresponding flux decreasing by ~20% by the third epoch (as in the X-rays). The source was thus about an order of magnitude brighter optically over the course of our observations than in the past (observed USNO B1.0 magnitudes I=18.2, R2=19.0, B2=21.0; D.G. Monet et al. 2003 AJ, 125, 984). In the UVOT data, we observe no significant change in the optical spectrum, which is best represented by a nu^-1.3 power law with sign of steepening at the highest frequency band (UVW2 filter).

In consideration of the recent activity of this source we encourage further multiwavelength observations. For this source the Fermi LAT contact person is C.C. Cheung (e-mail:

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

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