Detection of IMBHs from microlensing in globular clusters

Aims. Globular clusters have been alternatively predicted to host intermediate-mass black holes (IMBHs) or nearly impossible to form and retain them in their centres. Over the last decade enough theoretical and observational evidence have accumulated to believe that many, if not all, galactic globular clusters host IMBHs in the centres, just like galaxies do. The well-established galactic bulge mass–black hole mass correlation suggests that GCs may lie on the same line (and, as an example, globular clusters M15 and G1 fit the Mbh − σ correlation well). Methods. Most of the attempts in search of the central black holes (BHs) are not direct and present enormous observational difficulties due to the crowding of stars in the GC cores. Here we propose a method of detection of the central BH that avoids these difficulties—the microlensing of the cluster stars by the central black hole. If the core of the cluster is resolved, the direct determination of the lensing curve and lensing system parameters is possible; if unresolved, the pixel lensing technique can be applied. Results. We calculate the optical depth to central BH microlensing for a selected list of galactic globular clusters and estimate the average time duration of the events. We discuss self-lensing and some details of the observational program. Conclusions. IMBHs are the important issue in modern astronomy and we hope that using our proposal the unamibigous detection (or otherwise) will be possible.