Disentangling the synchrotron and inverse Compton variability in the X-ray emission of the intermediate BL Lacertae object S5 0716+71

Context. The possibility to detect simultaneously in the X-ray band the synchrotron and Inverse Compton (IC) emission of intermediate BL Lac objects objects the unique opportunity to study contemporaneously the low- and high-energy tails of the electron distribution in the jets of these sources. Aims. We attempted to disentangle the X-ray spectral variability properties of both the low- and high- energy ends of the synchrotron and Inverse Compton emission of the intermediate BL Lac object S5 0716+71. Methods. We carried out spectral, temporal and cross-correlation analyses of the data from a long XMM-Newton pointing of S5 0716+71 and we compared our findings with previous results from past X-ray observations. Results. Strong variability was detected during the XMM exposure. Both the synchrotron and Inverse Compton components were found to vary on time scales of hours, implying a size of the emitting region of R less than or similar to 0.7 delta/(1 + z) light-hours. The synchrotron emission was discovered to become dominant during episodes of flaring activity, following a harder-when-brighter trend. Tight correlations were observed between variations in different energy bands. Upper limits on time lags between the soft and hard X-ray light curves are of the order of a few hundred seconds.