First NuSTAR observations of the BL Lac -- type blazar PKS 2155-304: constraints on the jet content and distribution of radiating particles

Current scenarios for emission mechanisms operating in relativistic jets in AGN involve synchrotron emission for the radio through UV spectrum, and inverse Compton for hard X-rays through $\gamma $-rays, but the particle content of relativistic jets - whether they are dominated by proton-electron, or e$+$/e- plasma - has not been established. Our first hard X-ray observations with NuSTAR of the BL Lac type blazar PKS 2155-304, augmented by XMM-Newton data, reveal the 0.5-60 keV spectrum as best-described by a soft power law component dominating below \textasciitilde 10 keV (photon index of \textasciitilde 3 at 2 keV), and a hard power-law tail (index \textasciitilde 2), dominating in the 20-60 keV range. The hard X-ray tail can be smoothly joined to the quasi-simultaneous Fermi/LAT $\gamma $-ray spectrum by a synchrotron self-Compton component produced by an electron distribution with index p$=$2.2. The jet content needs to (globally) obey charge neutrality; assuming that the power-law electron distribution extends down to the Lorentz factor of 1, and one proton per electron, yields an unrealistically high total jet power of 10$^{47}$ erg/s. This can be reduced by two orders of magnitude by considering a significant presence of e$+$/e- pairs with lepton-to-proton ratio of at least 30.