Astronomy and Astrophysics – Astrophysics
Scientific paper
2001-02-27
Astronomy and Astrophysics
Astrophysics
13 pages, 3 figures, accepted by ApJ
Scientific paper
10.1086/321332
The bright, strongly variable BL Lac object PKS 2155-304 was observed by XMM for two essentially uninterrupted periods of ~11 and 16 hr on 30-31 May 2000. 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 $\tau \ls 0.3$ hr. However, the hardest band (~3 keV) was less well-correlated with the other three, especially on short time scales, 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 time scales 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, as these data provide only a lower limit of $B \gamma^{1/3} \gs 2.5$ 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.
Edelson Rick
Griffiths Gareth
Markowitz Alex
Sembay Steve
Turner Martin J. L.
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