Deep X-ray survey of the young open cluster NGC 2516 with XMM-Newton

¹ Università degli Studi di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy e-mail: pilli@astropa.unipa.it
² INAF – Osservatorio Astronomico di Palermo Piazza del Parlamento 1, 90134 Palermo, Italy e-mail: [giusi;damiani;sciorti]@astropa.unipa.it

Received: 6 August 2005
Accepted: 6 January 2006

Abstract

Aims.We report a deep X-ray survey of the young (~140 Myr), rich open cluster NGC 2516 obtained with the EPIC camera on board the XMM-Newton satellite.

Methods.By combining data from six observations, a high sensitivity, greater than a factor of 5 with respect to recent Chandra observations, has been achieved. Kaplan-Meier estimators of the cumulative X-ray luminosity distribution are built, statistically corrected for non members contaminants and compared to those of the nearly coeval Pleiades. The EPIC spectra of the X-ray brightest stars are fitted using optically thin model plasma with one or two thermal components.

Results.We detected 431 X-ray sources and 234 of them have as optical counterparts cluster stars spanning the entire NGC 2516 Main Sequence. On the basis of X-ray emission and optical photometry, we indicate 20 new candidate members of the cluster; at the same time we find 49 X-ray sources without known optical or infrared counterpart. The X-ray luminosities of cluster stars span the range (erg s^-1) = 28.4–30.8. The representative temperatures span the 0.3–0.6 keV (3.5–8 MK) range for the cool component and 1.0–2.0 keV (12–23 MK) for the hot one; similar values are found in other young open clusters like the Pleiades, IC 2391, and Blanco 1. While no significant differences are found in X-ray spectra, NGC 2516 solar type stars are definitely less luminous in X-rays than the nearly coeval Pleiades. The comparison with a previous ROSAT survey reveals the lack of variability amplitudes larger than a factor of 2 in solar type stars in a  yr time scale of the cluster and thus activity cycles like in the Sun are probably absent or have a different period and amplitude in young stars.