Context :

Aims : While observational evidence shows that most of the decline in a star ’ s X-ray activity occurs between the age of the Hyades ( \sim 8 \times 10 ^ { 8 } yrs ) and that of the Sun , very little is known about the evolution of stellar activity between these ages .
To gain information on the typical level of coronal activity at a star ’ s intermediate age , we studied the X-ray emission from stars in the 1.9 Gyr old open cluster NGC 752 .

Methods : We analysed a \sim 140 ks Chandra observation of NGC 752 and a \sim 50 ks XMM- Newton observation of the same cluster .
We detected 262 X-ray sources in the Chandra data and 145 sources in the XMM- Newton observation .
Around 90 % of the catalogued cluster members within Chandra ’ s field-of-view are detected in the X-ray .
The X-ray luminosity of all observed cluster members ( 28 stars ) and of 11 cluster member candidates was derived .

Results : Our data indicate that , at an age of 1.9 Gyr , the typical X-ray luminosity of the cluster members with M = 0.8 - 1.2 ~ { } M _ { \sun } is L _ { X } = 1.3 \times 10 ^ { 28 } erg s ^ { -1 } , so approximately a factor of 6 less intense than that observed in the younger Hyades .
Given that L _ { X } is proportional to the square of a star ’ s rotational rate , the median L _ { X } of NGC 752 is consistent , for t \ga 1 Gyr , to a decaying rate in rotational velocities v _ { rot } \propto t ^ { - \alpha } with \alpha \sim 0.75 , steeper than the Skumanich relation ( \alpha \simeq 0.5 ) and significantly steeper than observed between the Pleiades and the Hyades ( where \alpha < 0.3 ) , suggesting that a change in the rotational regimes of the stellar interiors is taking place at t \sim 1 Gyr .

Conclusions :