2012A&A...542A..44K

Intermediate mass black holes (IMBHs) with expected masses M_o≃ 10⁴ M_☉ are thought to bridge the gap between stellar mass black holes (M_o≃3 - 100 M_☉) and supermassive black holes found at the centre of galaxies (M_o> 10⁶ M_☉). Until today, no IMBH has been confirmed observationally. The most promising objects to host an IMBH as their central mass are globular clusters. Here, we present high sensitivity multi-epoch 1.6 GHz very long baseline interferometry observations of the globular cluster M15 that has been suggested to host an IMBH. Assuming the IMBH to be accreting matter from its surrounding we expect to detect it as a point source moving with the global motion of the cluster. However, we do not detect any such object within a radius of 6000 AU of the cluster centre in any of the five observations spread over more than one year. This rules out any variability of the putative IMBH on the time scale of one to two months. To get the most stringent upper limit for the flux density of the putative IMBH we concatenate the data of all five epochs. In this data we measure a 3σ upper flux limit of 10 µJy for a central source. We employ the fundamental plane of black hole activity to estimate the mass of the central IMBH candidate. Based on previous X-ray observations of M15 our measurements indicate a 3σ upper mass limit of ≃500M_☉.