The bright, strongly variable BL Lacertae object PKS 2155-304 was observed
by XMM-Newton for two essentially uninterrupted periods of similar to 11 an
d 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 limit
s of \tau\ less than or similar to 0.3 hr. However, the hardest band (simil
ar to3 keV) was less well correlated with the other three, especially on sh
ort timescales, showing deviations of similar to 10%-20% in similar to1 hr,
although, again, no significant interband lag was detected. This result an
d examination of previous ASCA and BeppoSAX cross-correlation functions sug
gest 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 wer
e therefore premature since these data provide only a lower limit of B gamm
a (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 there
fore of the underlying electron energy distribution, changes independently
of the lower energies.