Complex X-ray spectral variability in Mkn 421 observed with XMM-Newton
  The bright blazar Mkn 421 has been observed four times for uninterrupted durations of ~ 9 - 13 hr during the performance verification and calibration phases of the XMM-Newton mission. The source was strongly variable in all epochs, with variability amplitudes that generally increased to higher energy bands. Although the detailed relationship between soft (0.1 - 0.75 keV) and hard (2 - 10 keV) band differed from one epoch to the next, in no case was there any evidence for a measurable interband lag, with robust upper limits of $| \tau | < 0.08 $ hr in the best-correlated light curves. This is in conflict with previous claims of both hard and soft lags of ~1 hr in this and other blazars. However, previous observations suffered a repeated 1.6 hr feature induced by the low-Earth orbital period, a feature that is not present in the uninterrupted XMM-Newton data. The new upper limit on $|\tau|$ leads to a lower limit on the magnetic field strength and Doppler factor of $ B \delta^{1/3} \gs 4.7 $ G, mildly out of line with the predictions from a variety of homogeneous synchrotron self-Compton emission models in the literature of $ B \delta^{1/3} = 0.2 - 0.8 $ G. Time-dependent spectral fitting was performed on all epochs, and no detectable spectral hysteresis was seen. We note however that the source exhibited significantly different spectral evolutionary behavior from one epoch to the next, with the strongest correlations in the first and last and an actual divergance between soft and hard X-ray bands in the third. This indicates that the range of spectral variability behavior in Mkn 421 is not fully described in these short snippets; significantly longer uninterrupted light curves are required, and can be obtained with XMM-Newton. 
S. Sembay (1), R. Edelson (1 and 2), A. Markowitz (2), R. G. Griffiths   (1) and M. J. L. Turner (1) ((1) Leicester Universi...