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

TL;DR

This study presents the first NuSTAR hard X-ray observations of the blazar PKS 2155-304, combining data from XMM-Newton and Fermi to analyze jet content, particle distribution, and energy composition during a low flux state.

Contribution

It provides new insights into the jet composition and particle energy distribution of PKS 2155-304 using combined multi-wavelength observations and modeling.

Findings

The X-ray spectrum is best described by a log-parabola plus a hard power-law tail.

A jet power of about 10^47 erg/s is inferred assuming a proton-electron plasma.

Jet composition is likely matter-dominated with significant electron-positron pairs or a low-energy break in electron distribution.

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.