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

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

Madejski, G. M. and Nalewajko, K. and Madsen, K. K. and Chiang, J. and Baloković, M. and Paneque, D. and Furniss, A. K. and Hayashida, M. and Urry, C. M. and Sikora, M. and Ajello, M. and Blandford, R. D. and Harrison, F. A. and Sanchez, D. and Giebels, B. and Stern, D. and Alexander, D. M. and Barret, D. and Boggs, S. E. and Christensen, F. E. and Craig, W. W. and Forster, K. and Giommi, P. and Grefenstette, B. and Hailey, C. and Hornstrup, A. and Kitaguchi, T. and Koglin, J. E. and Mao, P. H. and Miyasaka, H. and Mori, K. and Perri, M. and Pivovaroff, M. J. and Puccetti, S. and Rana, V. and Westergaard, N. J. and Zhang, W. W. and Zoglauer, A. (2016) 'First NuSTAR observations of the BL Lac-type blazar PKS 2155-304 : constraints on the jet content and distribution of radiating particles.', Astrophysical journal., 831 (2). p. 142.

## 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 $\beta ={0.3}_{-0.1}^{+0.2}$ 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 ${n}_{{\rm{e}}+{\rm{e}}-}/{n}_{{\rm{p}}}\sim 30$, 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.