A Massive Neutron Star in the Globular Cluster M5
Dec, 2007Citations per year
Abstract: (arXiv)
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 center 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.References(27)
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