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

¹ ASI Science Data Center, via Galileo Galilei, 00044 Frascati, Italy e-mail: perri@asdc.asi.it
² Dipartimento di Fisica, Università La Sapienza, Piazzale A. Moro 2, 00185 Roma, Italy
³ Agenzia Spaziale Italiana, Unità Osservazione dell'Universo, Viale Liegi 26, 00198 Roma, Italy
⁴ INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica, Sezione di Palermo, via La Malfa 153, 90146 Palermo, Italy
⁵ INAF – Osservatorio Astronomico di Brera, via Bianchi 46, 23807 Merate, Italy
⁶ Università degli Studi di Milano-Bicocca, Dipartimento di Fisica, Piazza delle Scienze 3, 20126 Milano, Italy
⁷ Department of Astronomy & Astrophysics, Pennsylvania State University, University Park, PA 16802, USA
⁸ NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA

Received: 18 July 2006
Accepted: 2 October 2006

Abstract

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   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.