A Massive Neutron Star in the Globular Cluster M5

We report the results of 19 years of Arecibo timing for two pulsars in the globular cluster NGC 5904 (M5), PSR B1516+02A (M5A) and PSR B1516+02B (M5B). This has resulted in the measurement of the proper motions of these pulsars and, by extension, that of the cluster itself. M5B is a 7.95 ms pulsar in a binary system with a >0.13 M sub([image]) companion and an orbital period of 6.86 days. In deep HST images, no optical counterpart is detected within [image]2.5 capital sigma of the position of the pulsar, implying that the companion is either a white dwarf or a low-mass main-sequence star. The eccentricity of the orbit has allowed a measurement of the rate of advance of periastron: [image]. We argue that it is very likely that this periastron advance is due to the effects of general relativity, the total mass of the binary system then being [image]. The small measured mass function implies, in a statistical sense, that a very large fraction of this total mass is contained in the pulsar: [image] (1 capital sigma ); there is a 5% probability that the mass of this object is