We present here the results of Arecibo timing of PSR B1516+02B , a 7.95-ms pulsar in a binary system with a \sim 0.17 M _ { \odot } companion and an orbital period of 6.85 days located in the globular cluster M5 .
The eccentricity of the orbit ( e = 0.14 ) has allowed a measurement of the rate of advance of periastron : \dot { \omega } = ( 0.0136 \pm 0.0007 ) ^ { \circ } yr ^ { -1 } .
It is very likely that the periastron advance is due to the effects of general relativity ; the total mass of the binary system is ( 2.14 \pm 0.16 ) M _ { \odot } .
The small measured mass function implies , in a statistical sense , that a very large fraction of this total mass is contained in the pulsar : M _ { p } = ( 1.94 ^ { +0.17 } _ { -0.19 } ) M _ { \odot } ( 1- \sigma ) ; there is a 5 % probability that the mass of this object is below 1.59 M _ { \odot } .
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 can exclude , in a statistical sense , the “ soft ” equations of state for dense neutron matter , implying that matter at the center of a neutron star is highly incompressible .
There is also some evidence for a bimodal distribution of MSP masses , the reasons for that are not clear .