2001ApJ...554..274E

The bright, strongly variable BL Lacertae object PKS 2155-304 was observed by XMM-Newton for two essentially uninterrupted periods of ∼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 |τ|≲0.3 hr. However, the hardest band (∼3 keV) was less well correlated with the other three, especially on short timescales, showing deviations of ∼10%-20% in ∼1 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 Bγ^1/3≳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.