A star-by-star correspondence between X-ray activity and rotation in the young open cluster NGC 2516 with eROSITA

¹ Leibniz Institute for Astrophysics Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
² Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
³ Space Science Institute, 4750 Walnut St., Boulder, CO 80301, USA
⁴ Department of Astronomy & Space Sciences, Faculty of Science, University of Ege, 35100 Bornova, Izmir, Turkey

^★ Corresponding author: dario.fritzewski@kuleuven.be

Received: 14 March 2025
Accepted: 10 August 2025

Abstract

Context. The coronal soft X-ray emission of cool stars, especially when taken in combination with their measured rotation periods, offers insights into their levels of magnetic activity and related transitions.

Aims. We study the X-ray properties of low-mass (FGKM-type) members of the open cluster NGC 2516 to explicate their detailed dependencies on mass and rotation.

Methods. We analysed the pointed SRG/eROSITA satellite observations of NGC 2516 obtained during the calibration and performance verification phase of the mission. We found 1561 X-ray sources within the field of view and related 1007 of them to their optical stellar counterparts, including 655 members of NGC 2516 (433 of which have measured rotation periods). We combined these detections with auxiliary optical data to facilitate their interpretation. Furthermore, we extracted X-ray spectra for all sources and fit two-component APEC models to them. To aid the analysis, we grouped stars with similar mass and rotational properties together, which allowed us to investigate the influence of rotation on various X-ray properties.

Results. The colour-activity diagram (CAD) of NGC 2516 displays a general increase in the fractional X-ray luminosity with spectral type change from F through G and K to M-type. However, the behaviour of K-type stars, representing the ones that best sample the fast-to-slow rotational transition, is more complex, with both increased and decreased X-ray emission relative to G-type stars for fast and slow rotators, respectively. The rotation-activity diagram is analogous, with an identifiable desaturated group of X-ray emitters that corresponds to stars in the rotational gap between the fast and slow rotator sequences. We prefer to describe the normalised X-ray emission for all cluster stars as declining logarithmically with Rossby number over those using broken power laws. Coronal temperatures appear to be largely independent of mass or rotation. The coronal abundances are significantly sub-solar for most stars, as shown in prior works, but near-solar for both the least-active stars in our sample and a few of the very active stars as well.

Conclusions. The wealth of X-ray detections and rotation periods for stars in NGC 2516 enables a detailed view of the connection between rotation and X-ray activity in a homogeneous and coeval cluster sample of young stars.