Soft Gamma Ray Repeaters and Anomalous X-Ray Pulsars: Magnetars or Young Quark Stars?

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. Consider a neutron star (NS) of radius R, with a magnetic field Bp at the magnetic poles, spinning with a period P. If the star’s magnetic moment is misaligned with the spin axis by an angle α, electromagnetic energy is emitted at a rate 1 : Ė = − B2p R Ω sin α 6 c3 , (1) with Ω = 2π/P the angular velocity. If the NS is observed as a pulsar, its period can be measured as a function of time. Let the NS have a moment of inertia I and let Ṗ be the rate at which its period decreases. The magnetic field required to explain the slow-down rate by magnetic dipole radiation (MDR) is: