High temporal resolution XMM-Newton monitoring of PKS 2155-304

The bright, strongly variable BL Lacertae object PKS 2155-304 was observed by XMM-Newton for two essentially uninterrupted periods of similar to 11 and 16 hr on 2000 May 30-31. The strongest variations occurred in the highest energy bands. After scaling for this effect, the three softest bands (0.1-1.7 keV) showed strong correlation with no measurable lag to reliable limits of \tau\ less than or similar to 0.3 hr. However, the hardest band (similar to3 keV) was less well correlated with the other three, especially on short timescales, showing deviations of similar to 10%-20% in similar to1 hr, although, again, no significant interband lag was detected. This result and examination of previous ASCA and BeppoSAX cross-correlation functions suggest that previous claims of soft lags on timescales of 0.3-4 hr could well be an artifact of periodic interruptions due to Earth occultation every 1.6 hr. Previous determinations of the magnetic field/bulk Lorentz factor were therefore premature since these data provide only a lower limit of B gamma (1/3) greater than or similar to 2.5 G. The hardest band encompasses the spectral region above the high-energy break; its enhanced variability could be indicating that the break energy of the synchrotron spectrum, and therefore of the underlying electron energy distribution, changes independently of the lower energies.