The Detection of a High-Energy Break in the X-Ray Spectrum of the BL Lacertae Object PKS 0548-32

Abstract

The results of a deep EXOSAT observation of the BL Lac object PKS 0548-32 performed in 1986 March are presented. The 0.05-10 keV X-ray spectrum is characterized by a relatively flat (power-law energy spectral index α ~ 0.9) continuum below 5 keV which steepens at higher energies. There is no evidence for any excess absorption intrinsic to the BL Lac object itself. We suggest that, with the exception of two observations made in 1977 and 1978 (when PKS 0548-32 was unusually bright), all previous measurements of the X-ray spectrum of this source are consistent with the spectral form identified by EXOSAT. Although emission from an electron-positron pair plasma with compactness parameter Lσ_T_/4πm_e_c^3^R ~ 100 can reproduce the observed EXOSAT spectrum, difficulties arise in explaining the presence of a high-energy tail in 1977. If the X-ray emission arises via the synchrotron process with the spectral break due to radiative losses, reacceleration of the emitting particles and an inhomogeneous emission region are indicated.