Convex X-ray Spectra of PKS 2155-304 and Constraints on the Minimum Electron Energy

The convex (concave upward) high-energy X-ray spectra of the blazar PKS 2155-304, observed by \emph{XMM-Newton}, is interpreted as the signature of sub-dominant inverse Compton emission. The spectra can be well fitted by a superposition of two power-law contributions which imitate the emission due to synchrotron and inverse Compton processes. The methodology adopted enables us to constrain the photon energy down to a level where inverse Compton emission begins to contribute. We show that this information supplemented with knowledge of the jet Doppler factor and magnetic field strength can be used to constrain the low-energy cutoff $\gamma_{\rm min}m_{\rm e} c^2$ of the radiating electron distribution and the kinetic power $P_{\rm j}$ of the jet. We deduce these quantities through a statistical fitting of the broadband spectral energy distribution of PKS 2155-304 assuming synchrotron and synchrotron self Compton emission mechanisms. Our results favour a minimum Lorentz factor for the non-thermal electron distribution of $\gamma_{\rm min} \gtrsim 60$, with a preference for a value around $\gamma_{\rm min} \simeq 330$. The required kinetic jet power is of the order of $P_{\rm j} \sim 3\times 10^{45}$ erg s$^{-1}$ in case of a heavy, electron-proton dominated jet, and could be up to an order of magnitude less in case of a light, electron-positron dominated jet. When put in context, our best fit parameters support the X-ray emitting part of blazar jets to be dominated by an electron-proton rather than an electron-positron composition.