* Astronomy

Title: A Massive Neutron Star in the Globular Cluster M5
Authors: Paulo C. C. Freire, Alex Wolszczan, Maureen van den Berg, Jason W. T. Hessels

We report the results of 18 years of Arecibo timing of two pulsars in the globular cluster NGC 5904 (M5), PSR B1516+02A and PSR B1516+02B. This has allowed the measurement of the proper motions of these pulsars and of the cluster. PSR B1516+02B is a 7.95-ms pulsar in a binary system with a ~0.2 solar-mass companion and an orbital period of 6.86 days. In deep HST images, no optical counterpart is detected at the position of the pulsar, implying the companion is either a white dwarf or a low-mass MS star. The eccentricity of the orbit (e = 0.14) has allowed a measurement of the rate of advance of periastron: 0.0136 +/ 0.0007 degrees per year. It is very likely that the periastron advance is due to the effects of general relativity; the total mass of the binary system is then 2.14 ± 0.16 solar masses. The small measured mass function implies, in a statistical sense, that a very large fraction of this total mass is contained in the pulsar: 1.94 +0.17/-0.19 solar masses (1 sigma); there is a 5 % probability that the mass of this object is smaller than 1.59 solar masses and a 1.3% probability that it is between 1.2 and 1.44 solar masses. With the possible exception of PSR J1748-2021B, this is the largest neutron star mass measured to date. When combined with similar measurements made previously for Terzan 5 I and J, we conclude that there is a 99 % probability that at least one of these MSPs is more massive than 1.72 solar masses. Confirmation of these mass measurements would exclude most of the ``soft'' equations of state for dense neutron matter, implying that matter at the centre of a neutron star is highly incompressible. Furthermore, we see evidence for a bi-modal MSP mass distribution, but the reasons for this are not clear.

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