SGRs and AXPs - Magnetars or Young Quark Stars ?

Abstract

Recent measurements of the spin-down rates of soft gamma ray repeaters (SGRs) and anomalous X-ray pulsars (AXPs) have been interpreted as evidence that these objects are ``magnetars'': neutron stars spinning down by magnetic dipole radiation, but with a magnetic field two orders of magnitude larger than that of ordinary neutron stars. We discuss the evidence disfavouring this interpretation. We argue that, instead, the observations support the hypothesis that SGRs and AXPs are neutron stars that have suffered a transition into a denser form of nuclear matter to become, presumably, strange stars or quark stars. Their gravitational energy release while they are cooling and contracting can power both their quiescent X-ray emission and their star quakes which produce `soft' gamma ray bursts.