High Temporal ResolutionXMM‐NewtonMonitoring of PKS 2155−304

The bright, strongly variable BL Lac object PKS 2155-304 was observed by XMMfor two essentially uninterrupted periods of ~11 and 16 hr on 30-31 May 2000.The strongest variations occurred in the highest energy bands. After scalingfor this effect, the three softest bands (0.1-1.7 keV) showed strongcorrelation with no measurable lag to reliable limits of $\tau \ls 0.3$ hr.However, the hardest band (~3 keV) was less well-correlated with the otherthree, especially on short time scales, showing deviations of ~10-20% in ~1 hralthough, again, no significant interband lag was detected. This result andexamination of previous ASCA and BeppoSAX cross-correlation functions suggestthat previous claims of soft lags on time scales of 0.3-4 hr could well be anartifact of periodic interruptions due to Earth-occultation every 1.6 hr.Previous determinations of the magnetic field/bulk Lorentz factor weretherefore premature, as these data provide only a lower limit of $B\gamma^{1/3} \gs 2.5$ G. The hardest band encompasses the spectral region abovethe high-energy break; its enhanced variability could be indicating that thebreak energy of the synchrotron spectrum, and therefore of the underlyingelectron energy distribution, changes independently of the lower energies.Comment: 13 pages, 3 figures, accepted by Ap