  We report a ~38 ks X-ray observation of McNeil's Nebula obtained with XMM on
2004 April 4. V1647 Ori, the young star in outburst illuminating McNeil's
Nebula, is detected with XMM and appears variable in X-rays. We investigate the
hardness ratio variability and time variations of the event energy distribution
with quantile analysis, and show that the large increase of the count rate from
V1647 Ori observed during the second half of the observation is not associated
with any large plasma temperature variations as for typical X-ray flares from
young low-mass stars. X-ray spectral fitting shows that the bulk (~75%) of the
intrinsic X-ray emission in the 0.5-8 keV energy band comes from a soft plasma
component (0.9 keV) reminiscent of the X-ray spectrum of the classical T Tauri
star TW Hya, for which X-ray emission is believed to be generated by an
accretion shock onto the photosphere of a low-mass star. The hard plasma
component (4.2 keV) contributes ~25% of the total X-ray emission, and can be
understood only in the framework of plasma heating sustained by magnetic
reconnection events. We find a hydrogen column density of NH=4.1E22 cm-2, which
points out a significant excess of hydrogen column density compared to the
value derived from optical/IR observations, consistent with the picture of the
rise of a wind/jet unveiled from ground optical spectroscopy. The X-ray flux
observed with XMM ranges from roughly the flux observed by Chandra on 2004
March 22 (~10 times greater than the pre-outburst X-ray flux) to a value two
times greater than that caught by Chandra on 2004 March 7 (~200 times greater
than the pre-outburst X-ray flux). We have investigated the possibility that
V1647 Ori displays a periodic variation in X-ray brightness as suggested by the
combined Chandra+XMM data set (abridged).
