4.4. Magnetically active binaries

X-ray sources in globular clusters can be classified as magnetically active binaries when a stellar flare is observed in X-rays; or on the basis of the optical counterpart, when this is a known active binary, or less securely when it lies above the main sequence and/or shows weak H emission.

Two OGLE variables in NGC5139, OGLEGC15 and OGLEGC22, are identified by Cool et al. (2002) with Chandra sources (not listed by Rutledge et al. 2002, but confirmed by Gendre, private communication). A third OGLE variable in NGC5139, OGLEGC30, is detected with XMM (Gendre et al. 2003a). Yet another Chandra X-ray source, already detected with ROSAT but not detected with XMM and therefore a variable X-ray source, shows H emission, and presumably is also a magnetically active binary (Gendre et al. 2003a). Figure 12 shows Chandra X-ray sources in 47Tuc and NGC6752 that are classified on the basis of color-magnitude diagrams as magnetically active binaries; for many of these binaries in 47 Tuc the orbital lightcurve confirms their identity as coronal X-ray emitters (Edmonds et al. 2003). That care must be taken in classifying sources is shown by the example of NGC6397 CX12 (see sect. 4.2).

Interestingly, most magnetically active binaries identified with X-ray sources so far have visual magnitudes higher than or equal to the turnoff stars, implying that they are on the main sequence (BYDra's). Since the maximum X-ray luminosity of a magnetically active binary scales roughly with the surface area of the stars, this implies that the luminosities of the active binaries in globular clusters are low (typically L_x < 10³⁰erg s^-1), compared to systems with giants (RS CVn's), in the Galaxy, which can be up to a hundred times more luminous (Dempsey et al. 1993).