First Nustar Observations of the Bl Lac-Type Blazar Pks 2155-304: Constraints on the Jet Content and Distribution of Radiating Particles

G. M. Madejski, K. Nalewajko, K. K. Madsen, J. Chiang, M. Baloković, D. Paneque, A. K. Furniss, M. Hayashida, C. M. Urry, M. Sikora, M. Ajello, R. D. Blandford, F. A. Harrison, D. Sanchez, B. Giebels, D. Stern, D. M. Alexander, D. Barret, S. E. Boggs, Finn Erland ChristensenW. W. Craig, K. Forster, P. Giommi, B. Grefenstette, C. Hailey, Allan Hornstrup, T. Kitaguchi, J. E. Koglin, P. H. Mao, H. Miyasaka, K. Mori, M. Perri, M. J. Pivovaroff, S. Puccetti, V. Rana, Niels Jørgen Stenfeldt Westergaard, W. W. Zhang, A. Zoglauer

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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 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 Lp ~ 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 ne + e-/np ~ 30, or by introducing an additional, low-energy break in the electron energy distribution at the electron Lorentz factor γbr1 ~ 100. In either case, the jet composition is expected to be strongly matter-dominated.
Original languageEnglish
Article number142
JournalThe Astrophysical Journal
Issue number2
Number of pages8
Publication statusPublished - 2016


  • BL Lacertae objects: individual (PKS 2155-304)
  • Galaxies: active
  • Galaxies: jets
  • Gamma rays: galaxies
  • Radiation mechanisms: non-thermal

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