2008ApJ...682..789Z

This paper reports the first discovery of possible inverse Compton (IC) X-ray emission below 10 keV in the typical high-energy peaked BL Lac object (HBL) PKS 2155-304. Two XMM-Newton observations performed in 2006 reveal that the 0.6-10 keV X-ray spectra of the source harden (ΔΓ∼0.1-0.3) at break energies of ∼4 keV. The concave X-ray spectra of the source could be easily interpreted as a mixture of a steep component (i.e., the high-energy tail of the synchrotron emission) and a flat one (i.e., the low-energy side of the IC emission). However, the steep spectra (Γ∼2.3) in the hard X-rays indicate that the synchrotron emission still dominates over the IC one, while the latter is effectively present and perceived as flattening the synchrotron spectrum in this energy range. The quasi-simultaneous optical/UV/X-ray spectral energy distributions (SEDs) obtained with XMM-Newton suggest that the concave X-ray spectra of the source could be the result of downward shift of the synchrotron peak frequency to the optical band that causes the IC emission to become more important in the hard (∼4-10 keV) X-ray band with respect to other cases in which the synchrotron emission peaks in the UV to soft X-ray range. This discovery provides a new clue for a smooth transition between HBLs and the low-energy peaked BL Lac objects (LBLs).