Evolution of the synchrotron and inverse Compton emissions of the low energy peaked BL Lac object S5 0716+714

Abstract

This paper presents a detailed analysis of the temporal and spectral variability of the low-energy peaked BL Lac object S5 0716+714 with a long (~74 ks)X-ray observation performed by XMM-Newton on 2007 September 24-25. The source experiences recurrent flares on timescales of hours. The soft X-ray variations, up to a factor of ~4, are much stronger than the hard X-ray variations. With higher energy, the variability amplitude increases in the soft X-rays but decreases in the hard X-rays. The hard X-ray variability amplitude, however, is effectively large. For the first time, we detect a soft lag of ~1000s between the soft and hard X-ray variations. The soft lags might become larger with larger energy differences. The overall X-ray spectra exhibit a softer-when-brighter trend, whereas the soft X-ray spectra appear to show a harder-when-brighter trend. The concave X-ray spectra of the source can be interpreted as the sum of the high-energy tail of the synchrotron emission, dominating in the soft X rays, and the low-energy end of the inverse Compton (IC) emission, contributing more in the hard X-rays. The synchrotron spectra are steep (Γ~2.6), while the IC spectra are flat (Γ~1.2). The synchrotron spectra appear to harden with larger synchrotron fluxes, while the IC spectra seem to soften with larger IC fluxes. When the source brightens, the synchrotron fluxes increase but the IC fluxes decrease. The synchrotron tail exhibits larger flux variations but smaller spectral changes than the IC component does. The crossing energies between the two components and the trough energies of spectral energy distributions (SEDs) increase when the source brightens. The X-ray spectral variability demonstrates that the synchrotron and IC SED peaks of S5 0716+714 shift to higher energies when it brightens.