1996A&A...312..818D

The ROSAT PSPC detector was used to obtain a deep pointing with its center on the young open cluster NGC 2516 and total integration time of 9284 sec. Altogether 64 X-ray sources were detected showing strong concentration towards the optical cluster centre. This suggests that most of the sources are actually associated with NGC 2516. 42 of these sources could be identified with optical stars in the field of the cluster; after correction for a mean offset between optical and nominal ROSAT positions of about 13", optical star and X-ray source positions are found to agree, on the average, within about 9", while maximum observed positional differences amount to 27". X-ray sources identified with optical cluster stars include HR 3147 (=HD 66194; B2.5IVe), the visually brightest B star in the cluster, three close visual late B-type binaries (h 4027 A,B, h 4031 A,B, and I 1104 A,B) with projected separations ranging between about 4x10¹⁶cm and 6x10¹⁶cm, as well as several (six) chemically peculiar Bp/Ap(Si) and Ap(SrCrEu) stars. According to their known spectral or photometric properties, optical counterparts for most identified cluster X-ray sources are main-sequence stars between spectral types B9 and K0. No indication for a pulsed component of the X-ray flux pointing to a compact companion could be detected from X-ray photon arrival times for the prominent blue straggler star HR 3147, while one of the visual binary counterparts of X-ray cluster sources (I 1104 A/B) possibly exhibits a pulsed component. The fact that about half of the chemically peculiar Bp/Ap stars known in the cluster, all of them of the (magnetic) CP2 variety, are found to be counterparts to ROSAT X-ray sources, can be taken to suggest that stellar magnetic fields play a role in generating X-ray fluxes from early-type main-sequence stars. Luminosities of X-ray sources detected in the cluster are rather uniform, ranging between L_x(0.1-2.5keV)=0.4x10³⁰ and 4.5x10³⁰erg/s. 80 percent of the sources have relatively hard spectra, X-ray photons being recorded only in PSPC high-energy channels above 0.5keV. For only very few (six) bright X-ray sources in the field of NGC 2516, at least 30% of observed X-ray quanta fall into the low-energy channels between 0.16 and 0.5keV; four of these soft-spectrum sources could be identified with optical late-type stars in front of the cluster, including the nearby solar-type star HR 3138 (=HD 65907, MK: G0V).