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

¹ Max Planck Institut für Radioastronomie (MPIfR), Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: franz@astro.uni-bonn.de
² Argelander Institut für Astronomie (AIfA), University of Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
³ Department of Earth and Space Sciences, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
⁴ Joint Institute for VLBI in Europe, Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands

Received: 12 December 2013
Accepted: 27 March 2014

Abstract

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^-1, (μ_α,μ_δ)_M15C = (−0.75 ± 0.09, −3.52 ± 0.13) mas yr^-1, and (μ_α,μ_δ)_AC211 = (−0.46 ± 0.08, − 4.31 ± 0.20) mas yr^-1, respectively. Based on these measurements we estimate the global proper motion of M15 to be (μ_α,μ_δ) = (−0.58 ± 0.18, −4.05 ± 0.34) 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_•< 500 M_⊙.