Swift XRT and UVOT deep observations of the high-energy peaked BL Lacertae object PKS 0548-322 close to its brightest state

Aims. We observed the high-energy peaked BL Lac object PKS 0548-322 (BZB J0550-3216) with Swift to study the temporal and spectral properties of its synchrotron emission simultaneously in the optical, ultraviolet, and X-ray energy bands. Methods. We carried out a spectral analysis of 5 Swift XRT and UVOT observations of PKS 0548-322 taken over the period April-June 2005. Results. The X-ray flux of this BL Lac source was found to be approximately constant at a level of F_{(2-10 keV)} ≃ 4 × 10^-11 erg cm^-2 s^-1 which is a factor of 2 brighter than when observed by BeppoSAX in 1999 and close to the maximum intensity reported in the Einstein Slew Survey. The very good statistics obtained in the 0.3-10 keV Swift X-ray spectrum allowed us to detect highly significant deviations from a simple power-law spectral distribution. A log-parabolic model describes the X-ray data well and gives a best-fit curvature parameter of 0.18 and a peak energy in the spectral energy distribution of about 2 keV. The UV spectral data from Swift UVOT join closely with a power-law extrapolation of the soft X-ray data points, suggesting that the same component is responsible for the observed emission in the two bands. The combination of synchrotron peak in the X-ray band and the high-intensity state confirms PKS 0548-322 as a prime target for TeV observations. X-ray monitoring and coordinated TeV campaigns are highly advisable.