  We present Chandra X-ray Observatory ACIS-S3 X-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 Lx=4E30 erg/s 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 NGC 6366, 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 Nc=1.2 Gamma+1.1 Mh, where Gamma
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 Lx~0.001Lbol 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
NGC 6366 is a point source surrounded by a slightly offset extended source. The
absence of galaxies and Halpha 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.
