2014A&A...565A..43K

The globular cluster (GC) M15 (NGC 7078) is host to at least eight pulsars and two low mass X-ray binaries (LMXBs), one of which is also visible in the radio regime. Here we present the results of a multi-epoch global very long baseline interferometry (VLBI) campaign aiming at i) measuring the proper motion of the known compact radio sources; ii) finding and classifying thus far undetected compact radio sources in the GC; and iii) detecting a signature of the putative intermediate mass black hole (IMBH) proposed to reside at the core of M15. We measure the sky motion in right ascension (µ_α) and declination (µ_δ) of the pulsars M15A and M15C and of the LMXB AC211 to be (µ_α,µ_δ)_M15A=(-0.54±0.14,-4.33±0.25)mas/yr, (µ_α,µ_δ)_M15C=(-0.75±0.09,-3.52±0.13)mas/yr, and (µ_α,µ_δ)_AC211=(-0.46±0.08,-4.31±0.20)mas/yr, respectively. Based on these measurements we estimate the global proper motion of M15 to be (µ_α,µ_δ)=(-0.58±0.18,-4.05±0.34)mas/yr. We detect two previously known but unclassified compact sources within our field of view. Our observations indicate that one them is of extragalactic origin while the other one is a foreground source, quite likely an LMXB. The double neutron star system M15C became fainter during the observations, disappeared for one year and is now observable again - an effect possibly caused by geodetic precession. The LMXB AC211 shows a double lobed structure in one of the observations indicative of an outburst during this campaign. With the inclusion of the last two of a total of seven observations we confirm the upper mass limit for a putative IMBH to be M_o<500M_☉.