Madejski, G. M.G. M.MadejskiNalewajko, K.K.NalewajkoMadsen, K. K.K. K.MadsenChiang, J.J.ChiangBaloković, M.M.BalokovićPaneque, D.D.PanequeFurniss, A. K.A. K.FurnissHayashida, M.M.HayashidaUrry, C. M.C. M.UrrySikora, M.M.SikoraAjello, M.M.AjelloBlandford, R. D.R. D.BlandfordHarrison, F. A.F. A.HarrisonSanchez, D.D.SanchezGiebels, B.B.GiebelsStern, D.D.SternAlexander, D. M.D. M.AlexanderBarret, D.D.BarretBoggs, S. E.S. E.BoggsChristensen, F. E.F. E.ChristensenCraig, W. W.W. W.CraigForster, K.K.ForsterGiommi, P.P.GiommiGrefenstette, B.B.GrefenstetteHailey, C.C.HaileyHornstrup, A.A.HornstrupKitaguchi, T.T.KitaguchiKoglin, J. E.J. E.KoglinMao, P. H.P. H.MaoMiyasaka, H.H.MiyasakaMori, K.K.MoriPERRI, MatteoMatteoPERRIPivovaroff, M. J.M. J.PivovaroffPuccetti, S.S.PuccettiRana, V.V.RanaWestergaard, N. J.N. J.WestergaardZhang, W. W.W. W.ZhangZoglauer, A.A.Zoglauer2021-02-032021-02-0320160004-637Xhttp://hdl.handle.net/20.500.12386/30209We 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 γ-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 β = 0.3(-0.1,+0.2) 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 γ-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 γ_min = 1 and that there is one proton per electron, an unrealistically high total jet power of L_p ∼ 10^47 erg s^-1 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_e+_e-/n_p ∼ 30, or by introducing an additional, low-energy break in the electron energy distribution at the electron Lorentz factor γ_br1 ∼ 100. In either case, the jet composition is expected to be strongly matter-dominated.STAMPAenArticle10.3847/0004-637X/831/2/1422-s2.0-84994486633000400474300006https://iopscience.iop.org/article/10.3847/0004-637X/831/2/1422016ApJ...831..142MFIS/05 - ASTRONOMIA E ASTROFISICAERC sectors::Physical Sciences and Engineering::PE9 Universe sciences: astro-physics/chemistry/biology; solar systems; stellar, galactic and extragalactic astronomy, planetary systems, cosmology, space science, instrumentation::PE9_10 High energy and particle astronomy – X-rays, cosmic rays, gamma rays, neutrinos