XMM-Newton observations of the sigma Ori cluster. I. The complex RGS spectrum of the hot star sigma Ori AB

We present XMM-Newton observations of the young (~2-5 Myr) cluster around the hot (O9.5V) star sigma Orionis AB, aimed at obtaining a high resolution RGS spectrum of the hot star as well as EPIC imaging data for the whole field. We show that the RGS spectrum of sigma Ori AB may be contaminated by weaker nearby sources which required the development of a suitable procedure to extract a clean RGS spectrum and to determine the thermal structure and wind properties of the hot star. We also report on the detection of a flare from the B2Vp star sigma Ori E and we discuss whether the flare originated from the hot star itself or rather from an unseen late-type companion. Other results of this observation include: the detection of 174 X-ray sources in the field of sigma Ori of which 75 identified as cluster members, including very low-mass stars down to the substellar limit; the discovery of rotational modulation in a late-type star near sigma Ori AB; no detectable line broadenings and shifts (<800 km s^-1) in the spectrum of sigma Ori AB together with a remarkable low value of the O VII forbidden to intercombination line ratio and unusually high coronal abundances of CNO elements.