XMM-Newton observations of the σ Orionis cluster. II. Spatial and spectral analysis of the full EPIC field

Abstract

We present the results of an XMM-Newton observation of the young (∼ 2{-}4 Myr) cluster around the hot star σ Orionis. In a previous paper we presented the analysis of the RGS spectrum of the central hot star; here we discuss the results of the analysis of the full EPIC field. We have detected 175 X-ray sources, 88 of which have been identified with cluster members, including very low-mass stars down to the substellar limit. We detected eleven new possible candidate members from the 2MASS catalogue. We find that late-type stars have a median log L_X/L_bol ∼ -3.3, i.e. very close to the saturation limit. We detected significant variability in ∼ 40% of late-type members or candidates, including 10 flaring sources; rotational modulation is detected in one K-type star and possibly in another 3 or 4 stars. Spectral analysis of the brightest sources shows typical quiescent temperatures in the range T_1∼ 0.3{-}0.8 keV and T_2∼ 1{-}3 keV, with subsolar abundances Z∼ 0.1{-}0.3 Z_⊙, similar to what is found in other star-forming regions and associations. We find no significant difference in the spectral properties of classical and weak-lined T Tauri stars, although classical T Tauri stars tend to be less X-ray luminous than weak-lined T Tauri stars.