Strange Quark Matter in Neutron Stars? - New Results from Chandra and XMM

It has been predicted that quark and hybrid stars, containing strange quark matter in their core, have a significantly smaller radius than ordinary neutron stars. Preliminary X-ray observations of isolated neutron stars indicated a surprisingly small radius consistent with quark matter in the interior of the star. However, a new analysis of the data led to a radius corresponding to an ordinary neutron star. In the present talk we will discuss theoretical calculations of the mass-radius relation for quark and hybrid stars, taking into account medium effects in quark matter, and report on recent X-ray observations by Chandra and XMM and their latest interpretation.