First NuSTAR Observations of the BL Lac-type Blazar PKS 2155-304: Constraints on the Jet Content and Distribution of Radiating Particles
Harrison, F. A.; Hornstrup, A.; Hailey, C.; Stern, D.; Zhang, W. W.; Alexander, D. M.; Ajello, M.; Blandford, R. D.; Chiang, J.; Madejski, G. M.; Paneque, D.; Giebels, B.; Giommi, P.; Hayashida, M.; Sikora, M.; Westergaard, N. J.; Barret, D.; Boggs, S. E.; Christensen, F. E.; Craig, W. W.; Madsen, K. K.; Perri, M.; Puccetti, S.; Urry, C. M.; Mori, K.; Baloković, M.; Kitaguchi, T.; Sanchez, D.; Rana, V.; Forster, K.; Pivovaroff, M. J.; Mao, P. H.; Miyasaka, H.; Zoglauer, A.; Nalewajko, K.; Furniss, A. K.; Grefenstette, B.; Koglin, J. E.
United States, Poland, Germany, Japan, France, United Kingdom, Denmark, Italy
Abstract
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 L p ∼ 1047 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.