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Published November 10, 2016 | Published + Submitted
Journal Article Open

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

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 ~ 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.

Additional Information

© 2016 American Astronomical Society. Received 2016 August 2; revised 2016 August 25; accepted 2016 August 29; published 2016 November 3. The Fermi-LAT Collaboration acknowledges support for LAT development, operation and data analysis from NASA and DOE (United States), CEA/Irfu and IN2P3/CNRS (France), ASI and INFN (Italy), MEXT, KEK, and JAXA (Japan), and the K.A. Wallenberg Foundation, the Swedish Research Council and the National Space Board (Sweden). Science analysis support in the operations phase from INAF (Italy) and CNES (France) is also gratefully acknowledged. This work was partially supported under the NASA contract No. NNG08FD60C, and made use of observations from the NuSTAR mission, a project led by California Institute of Technology, managed by the Jet Propulsion Laboratory, and funded by NASA. We thank the NuSTAR Operations, Software, and Calibration teams for support in the execution and analysis of these observations. This research has made use of the NuSTAR Data Analysis Software (NuSTARDAS) jointly developed by the ASI Science Data Center (ASDC, Italy) and the California Institute of Technology (USA). K.N. was supported by NASA through Einstein Postdoctoral Fellowship grant number PF3-140130 awarded by the Chandra X-ray Center, and by the Polish National Science Centre grant 2015/18/E/ST9/00580. M.B. acknowledges support from NASA under the NASA Earth and Space Science Fellowship Program, grant NNX14AQ07H.

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Published - Madejski_2016_ApJ_831_142.pdf

Submitted - 1609.02203v1.pdf

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August 22, 2023
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