First NuSTAR observations of the BL Lac - type blazar PKS~2155-304: constraints on the jet content and distribution of radiating particles
astro-ph.HE
/ Authors
G. M. Madejski, K. Nalewajko, K. K. Madsen, J. Chiang, M. Baloković, D. Paneque, A. K. Furniss, M. Hayashida, C. M. Urry, M. Sikora
and 28 more authors
M. Ajello, R. D. Blandford, F. A. Harrison, D. Sanchez, B. Giebels, D. Stern, D. M. Alexander, D. Barret, S. E. Boggs, F. E. Christensen, W. W. Craig, K. Forster, P. Giommi, B. Grefenstette, C. Hailey, A. Hornstrup, T. Kitaguchi, J. E. Koglin, P. H. Mao, H. Miyasaka, K. Mori, M. Perri, M. J. Pivovaroff, S. Puccetti
/ 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 gamma-ray data from the Fermi Large Area Telescope, obtained in April 2013 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(+0.2,-0.1) 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 gamma-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 the minimum electron Lorentz factor gamma_min = 1 and that there is one proton per electron, an unrealistically high total jet power L_p of roughly 10^47 erg/s 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 of at least 30, or by introducing an additional, low-energy break in the electron energy distribution at the electron Lorentz factor gamma_br1 of roughly 100. In either case, the jet composition is expected to be strongly matter-dominated.