We report on the detailed modelling of the X-ray spectra of three likely neutron stars .
The neutron stars , observed with XMM-Newton are found in three quiescent X-ray binaries in the globular clusters : ( catalog $ ω $ Cen ) , ( catalog M 13 ) and ( catalog NGC 2808 ) .
Whether they are accreting at very low rates or radiating energy from an accretion heated core , their X-ray spectra are expected to be those of a hydrogen atmosphere .
We use and compare publicly available hydrogen atmosphere models , with constant and varying surface gravities to constrain the masses and radii of the neutron stars .
Thanks to the high XMM-Newton throughput , and the accurate distances available for these clusters , using the latest science analysis software release and calibration of the XMM-Newton EPIC cameras , we derive the most stringent constraints on the masses and radii of the neutron stars obtained to date from these systems .
A comparison of the models indicate that previously used hydrogen atmosphere models ( assuming constant surface gravity ) tend to underestimate the mass and overestimate the radius of neutron stars .
Our data constrain the allowed equations of state to those which concern normal nucleonic matter and one possible strange quark matter model , thus constraining radii to be from 8 km and masses up to 2.4 M _ { \odot } .