First NuSTAR Observations of the BL Lac-type Blazar PKS 2155-304 Constraints on the Jet Content and Distribution of Radiating Particles

We report the first hard X-ray observations with NuSTAR of the BL Lac-type blazar PKS 2155-304, augmented with soft X-ray data from XMM-Newton and gamma-ray data from the Fermi Large Area Telescope, obtained in 2013 April when the source was in a very low flux state. A joint NuSTAR and XMM spectrum, covering the energy range 0.5-60 keV, is best described by a model consisting of a log-parabola component with curvature beta ={0.3}<SUB>-0.1</SUB><SUP>+0.2</SUP> and a (local) photon index 3.04 0.15 at photon energy of 2 keV, and a hard power-law tail with photon index 2.2 0.4. The hard X-ray tail can be smoothly joined to the quasi-simultaneous gamma-ray spectrum by a synchrotron self-Compton component produced by an electron distribution with index p = 2.2. Assuming that the power-law electron distribution extends down to gamma <SUB>min</SUB> = 1 and that there is one proton per electron, an unrealistically high total jet power of L <SUB>p</SUB> ˜ 10<SUP>47</SUP> erg s<SUP>-1</SUP> is inferred. This can be reduced by two orders of magnitude either by considering a significant presence of electron-positron pairs with lepton-to-proton ratio {n}<SUB>{{e</SUB>}+{{e}}-}/{n}<SUB>{{p</SUB>}}˜ 30, or by introducing an additional, low-energy break in the electron energy distribution at the electron Lorentz factor gamma <SUB>br1</SUB> ˜ 100. In either case, the jet composition is expected to be strongly matter-dominated.