A&A 456, 977-983 (2006)
DOI: 10.1051/0004-6361:20054674

X-ray variability of  NGC 2516 stars in the XMM-Newton observations

A. Marino¹, G. Micela², I. Pillitteri¹ and G. Peres<sup>1

1</sup>  DSFA, Università di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
    e-mail: marino@astropa.unipa.it
²  INAF - Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy

(Received 9 December 2005 / Accepted 1 May 2006 )

Abstract
We present the characteristics of the X-ray variability of stars in the cluster  NGC 2516 as derived from XMM-Newton/EPIC/pn data. The X-ray variations on short (hours), medium (months), and long (years) time scales have been explored. We detected 303 distinct X-ray sources by analysing six EPIC/pn observations; 194 of them are members of the cluster. Stars of all spectral types, from the early-types to the late-M dwarfs, were detected. The Kolmogorov-Smirnov test applied to the X-ray photon time series shows that, on short time scales, only a relatively small fraction (ranging from 6% to 31% for dG and dF, respectively) of the members of  NGC 2516  are variable with a confidence level 99%; however, it is possible that the fraction is small only because of the poor statistics. The time X-ray amplitude distribution functions (XAD) of a set of dF7-dK2 stars, derived on short (hours) and medium (months) time scales, seem to suggest that medium-term variations, if present, have a much smaller amplitude than those on short time scales; a similar result is also obtained for dK3-dM stars. The amplitude variations of late-type stars in  NGC 2516 are consistent with those of the coeval Pleiades stars. Comparing these data with those of ROSAT/PSPC, collected 7-8 years earlier, and of ROSAT/HRI, just 4-5 years earlier, we find no evidence of significant  variability on the related time scales, suggesting that long-term variations due to activity cycles similar to the solar cycle are not common among young stars. Indications of spectral  variability was found in one star whose spectra at three epochs were available.

Key words: X-ray: stars -- stars: activity -- stars: early-type -- stars: late-type -- galaxy: open clusters and associations: individual: NGC 2516 -- stars: coronae

© ESO 2006

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