2008A&A...491..961L

The objective of this study is to determine the general X-ray properties of the young stars in the external regions of the σ Orionis cluster (τ∼3Ma, d∼385pc) and determine constraints on the X-ray emission of brown dwarfs. We carried out a careful analysis of public data obtained from an unexplored region to the west of the centre of the cluster with the three EPIC cameras onboard the XMM-Newton mission. We looked for new X-ray young stars among the 41 identified X-ray sources in the area with maximum likelihood parameters L>15 by cross-correlation with the USNO-B1, DENIS, and 2MASS databases. Based on colour-colour, colour-magnitude, and hardness ratio diagrams, and previous spectroscopic, astrometric, and infrared-flux excess information, we classified the optical/near-infrared counterparts of the X-ray sources into: young stars (15), field stars (4), galaxies (19), and sources of unknown nature (3). Most of the X-ray detections, including those of nine young stars, are new. We derived the X-ray properties (e.g. temperatures, metallicities, column densities) of the twelve young stars with the largest signal-to-noise ratios. The X-ray parameters determined here are in good agreement with those found in the cluster centre, where the stellar density is higher. There is no relation between infrared excess and column density in the X-ray measurements in our data. We detected flaring events in two young stars of the sample. One of them showed a very large (∼30) relative increase in flux. Both stars showed high coronal temperatures during the observation. Finally, we determined upper limits to the flux of the young stars and brown dwarfs not detected by our search algorithm.