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One of the most important questions in the study of compact objects is the nature of pulsars, including whether they are composed of β-stable nuclear matter or strange quark matter. Observations of the newly discovered millisecond X-ray pulsar SAX J1808.4-3658 with the Rossi X-Ray Timing Explorer place firm constraint on the radius of the compact star. Comparing the massradius relation of SAX J1808.4-3658 with the theoretical mass -radius relation for neutron stars and for strange stars, we find that a strange star model is more consistent with SAX J1808.4-3658, and suggest that it is a likely strange star candidate.

Abstract.We explore the scenario where the core of a neutron star (having experienced a transition to an up and down quark phase) shrinks into the equilibrated quark object after reaching strange quark matter saturation density (where a composition of up, down and strange quarks is the favored state of matter). The overlaying (envelope) material free-falls following the core contraction releasing upto 10 53 ergs in energy as radiation, partly as a result of the conversion of envelope material to quarks. This phenomena, we named Quark-Nova, leads to a wide variety of ejectae ranging form the Newtonian, "dirty" to the ultra-relativistic fireball. The mass range of the corresponding compact remnant (the quark star) ranges from less than 0.3 M up to a solar mass. We discuss the connection between Quark-Novae and Gamma ray bursts and suggest the recently studied GRB011211 event as a plausible Quark-Nova candidate.

There are three categories of stars whose masses have been found accurately in recent times:(1) two for which Shapiro delay is used which is possible due to GR light bending as the partner is heavy : PSR J1614 − 2230 and PSR J1903+0327 (2) six eclipsing stars for which numerical Roche Lobe geometry is used and (3) 3 stars for which spectroscopic methods are used and in fact for these three the mass and radii both are estimated. Motivated by large color (N c ) expansion using a modified Richardson potential, along with density dependent quark masses thereby allowing chiral symmetry restoration, we get compact strange stars fitting all the observed masses.