The first observations conducted as part of the Chandra ACIS survey of ( catalog M~33 ) ( ChASeM33 ) sampled the eclipsing X-ray binary ( catalog M~33~X $ - $ 7 ) over a large part of the 3.45 d orbital period and have resolved eclipse ingress and egress for the first time .
The occurrence of the X-ray eclipse allows us to determine an improved ephemeris of mid-eclipse and binary period as HJD ( 245 3639.119 \pm 0.005 ) \pm N \times ( 3.453014 \pm 0.000020 ) and constrain the eclipse half angle to 26.5 \arcdeg \pm 1.1 \arcdeg .
There are indications for a shortening of the orbital period .
The X-ray spectrum is best described by a disk blackbody spectrum typical for black hole X-ray binaries in the Galaxy .
We find a flat power density spectrum and no significant regular pulsations were found in the frequency range of 10 ^ { -4 } –0.15 Hz .
HST WFPC2 images resolve the optical counterpart , which can be identified as an O6III star with the help of extinction and colour corrections derived from the X-ray absorption .
Based on the optical light curve , the mass of the compact object in the system most likely exceeds 9 M _ { \sun } .
This mass , the shape of the X-ray spectrum and the short term X-ray time variability identify ( catalog M~33~X $ - $ 7 ) as the first eclipsing black hole high mass X-ray binary .