IS HER X-1 REALLY STRANGE

It has recently been proposed that the compact accreting object in the well-studied X-ray binary Hercules X-1 is not a neutron star at all, but a strange star; ie one composed of bulk quark matter. This conclus ion has been arrived at by comparing a semi-empirical mass-radius (M-R ) relation for Her X-1 with theoretical M-R curves for neutron and str ange stars, and showing that the strange-star models are favoured. How ever we argue that other data in the literature, and recent observatio nal results (namely new mass and distance estimates) suggest that the hypothesis that Her X-1 is a normal neutron star is not disproved.