XMM − Newton observations of Markarian 421 ?

The BL Lac object Mrk 421 was observed on May 25, 2000 during the XMM−Newton Cal/PV phase. The high throughput of the X-ray telescopes and the spectral capabilities of the instruments allow an uninterrupted temporal and spectral study of the source with unprecedented time resolution. Mrk 421 was found at a relatively high state with a 2−6 keV flux of (1.3−1.9)×10−10 erg cm−2 s−1. The observed intensity variations by more than a factor of three at highest X-ray energies are accompanied by complex spectral variations with only a small time lag (τ = 265 −102 seconds) between the hard and soft photons. The (0.2−10) keV spectrum can be well fitted by a broken power law and no absorption structures are found in the source spectrum at the high spectral resolution of the transmission gratings.