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A&A 446, 501-513 (2006)
DOI: 10.1051/0004-6361:20053605

XMM-Newton observations of the $\sigma$ Orionis cluster

II. Spatial and spectral analysis of the full EPIC field
E. Franciosini1, R. Pallavicini1 and J. Sanz-Forcada2

1  INAF - Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
    e-mail: francio@astropa.unipa.it
2  Astrophysics Division - Research and Science Support Department of ESA, ESTEC, Postbus 299, 2200 AG Noordwijk, The Netherlands

(Received 9 June 2005 / Accepted 15 September 2005)

Abstract
We present the results of an XMM-Newton observation of the young ( ${\sim} 2{-}4$ Myr) cluster around the hot star $\sigma$ 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 $\log L_\mathrm{X}/L_\mathrm{bol} \sim -3.3$, i.e. very close to the saturation limit. We detected significant variability in ${\sim} 40$% 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 $T_1\sim 0.3{-}0.8$ keV and $T_2\sim 1{-}3$ keV, with subsolar abundances $Z\sim 0.1{-}0.3 \,Z_\odot$, 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.


Key words: open clusters and associations: individual: $\sigma$ Orionis -- stars: activity -- stars: coronae -- stars: pre-main sequence -- stars: late-type -- X-rays: stars

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