X-ray and optical observations of M55 and NGC 6366: evidence for primordial binaries

We present Chandra X-ray Observatory ACIS-S3X-ray imaging observations and VLT/FORS2 and Hubble Space Telescope optical observations of two low-density Galactic globular clusters; NGC 6366 and M55. We detect 16 X-ray sources with 0.5-6.0 keV luminosities above L-X = 4 x 10(30) erg s(-1) within the half-mass radius of M55, of which 8 or 9 are expected to be background sources, and 5 within the half-mass radius of NGC6366, of which 4 are expected to be background sources. Optical counterparts are identified for several X-ray sources in both clusters and from these we conclude that 3 of the X-ray sources in M55 and 2 or 3 of the X-ray sources in NGC 6366 are probably related to the cluster. Combining these results with those for other clusters, we find the best fit for a predicted number of X-ray sources in a globular cluster mu(c) = 1.2 Gamma + 1.1 M-h, where G is the collision number and Mh is (half of) the cluster mass, both normalized to the values for the globular cluster M4. Some sources tentatively classified as magnetically active binaries are more luminous in X-rays than the upper limit of L-X similar or equal to 0.001 L-bol of such binaries in the solar neighbourhood. Comparison with XMM and ROSAT observations lead us to conclude that the brightest X-ray source in M55, a dwarf nova, becomes fainter in X-rays during the optical outburst, in accordance with other dwarf novae. The brightest X-ray source in NGC6366 is a point source surrounded by a slightly offset extended source. The absence of galaxies and Ha emission in our optical observations argues against a cluster of galaxies and against a planetary nebula, and we suggest that the source may be an old nova.