A Search for Black Hole Microlensing Signatures in Globular Cluster NGC 6656 (M22)

We report the results of a study aiming to detect signs of astrometric microlensing caused by an intermediate-mass black hole (IMBH) in the center of globular cluster M 22 (NGC 6656). We used archival data from the Hubble Space Telescope (HST) taken between 1995 and 2014, to derive long-baseline astrometric time series for stars near the center of the cluster, using state-of-the-art software to extract high-precision astrometry from images. We then modelled these time-series data, and compared microlensing model fits to simple linear proper-motion fits for each selected star. We find no evidence for astrometric microlensing in M 22, in particular for Bulge stars, which are much more likely to be lensed than cluster stars, due to the geometry of microlensing events. Although it is in principle possible to derive mass limits from such non-detections, we find that no useful mass limits can be derived for M 22 with available data, mostly due to a 10-year gap in coverage. This is a result from difficulties with deriving precise enough astrometry from Wide Field Planetary Camera 2 (WFPC2) observations, for stars that do not fall on the PC chip. However, this study shows that, for other HST instruments, we are able to reach precisions at which astrometric microlensing signals caused by IMBH in globular clusters could be detected, and that this technique is a promising tool to make a first unambiguous detection of an IMBH.