Flux and spectral variability of the blazar PKS 2155-304 with
  XMM-Newton: Evidence of Particle Acceleration and Synchrotron Cooling
  We have analyzed XMM-Newton observations of the high energy peaked blazar,
PKS 2155-304, made on 24 May 2002 in the 0.3 - 10 keV X-ray band. These
observations display a mini-flare, a nearly constant flux period and a strong
flux increase. We performed a time-resolved spectral study of the data, by
dividing the data into eight segments. We fitted the data with a power-law and
a broken power-law model, and in some of the segments we found a noticeable
spectral flattening of the source's spectrum below 10 keV. We also performed
time-resolved cross-correlation analyses and detected significant hard and soft
lags (for the first time in a single observation of this source) during the
first and last parts of the observation, respectively. Our analysis of the
spectra, the variations of photon-index with flux as well as the correlation
and lags between the harder and softer X-ray bands indicate that both the
particle acceleration and synchrotron cooling processes make an important
contribution to the emission from this blazar. The hard lags indicate a
variable acceleration process. We also estimated the magnetic field value using
the soft lags. The value of the magnetic field is consistent with the values
derived from the broad-band SED modeling of this source.
