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Precision astrometry of pulsars and other compact radio sources in the globular cluster M15

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 $(μ_α,\,μ_δ)_{\text{M15A}}=(-0.54\pm0.14,\,-4.33\pm0.25)\,$mas$\,$yr$^{-1}$, $(μ_α,\,μ_δ)_{\text{M15C}}=(-0.75\pm0.09,\,-3.52\pm0.13)\,$mas$\,$yr$^{-1}$, and $(μ_α,\,μ_δ)_{\text{AC211}}=(-0.46\pm0.08,\,-4.31\pm0.20)\,$mas$\,$yr$^{-1}$, respectively. Based on these measurements we estimate the global proper motion of M15 to be $(μ_α,\,μ_δ)=(-0.58\pm0.18,\,-4.05\pm0.34)\,$mas$\,$yr$^{-1}$. 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$_{\bullet}<500$ M$_{\odot}$.