Are there quark stars in nature? We review the question in the light of modern astrophysical observations. Quark stars have properties which are very similar to those of neutron stars. For example they are now known to have the same kind of cooling properties. Their masses may also be similar. On the other hand, gamma ray bursts (GRB), the brightest phenomenon observable at present in the sky, could possibly be conversion of normal or neutron matter on the surface of a quark star. The best observational evidence for the existence of quark stars seems to be some compact objects, the X-Ray burst source 4U 1820-30, the X-ray pulsar Her X-1, the star 4U 1728-34 and SAX J-1808.4-3658, this last one being the stablest and fastest rotating hard X-ray pulsar known to date. The mass of at least these four objects is high and their radius is low, placing them close to the black hole line, in the mass–radius (M–R) plot.
Super–Giant Glitches and Quark Stars: Sources of Gamma-Ray Bursts?
