XMM-Newton observation of the young open cluster Blanco 1^*

II. X-ray time variability and flares

¹ Dip. di Astronomia, Università di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy e-mail: pilli@astropa.unipa.it,reale@astropa.unipa.it
² INAF - Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy e-mail: giusi@astropa.unipa.it

Received: 23 April 2004
Accepted: 14 September 2004

Abstract

We study the X-ray variability of the young open cluster Blanco 1 observed with the EPIC camera on board the XMM-Newton X-ray observatory. The time coverage of EPIC observations has allowed us to address short time scale (hours) variability, while the comparison with previous ROSAT observations has allowed us to investigate the variability on time scale of six years. On the time scale of hours, dM stars of the cluster are more variable than solar-mass stars. The main features of X-ray light curves in dM stars appear to be essentially flare-like events with a typical duration of the order of a few ks, while dF–dG stars show smooth variations. Two intense flares were observed in the ZS76 cluster member. From the analysis of the light curves and of the time resolved X-ray spectroscopy we infer that they occurred in arcades, made of several similar coronal loops (~10¹⁰ cm long), ignited at ~15 ks time delay. Minor heat pulses may occur in the first flare. The similarity of the two flares and of the flaring inferred structures may suggest that the flares are physically connected to each other. Amplitude variations on the time scale of 6 years are smaller by a factor 2, whereas a factor of 10 is observed on the Sun in the same band. This implies a smaller amplitude of any analog of the 11-yr cycle in these young active coronae.