SGRs and AXPs as Rotation-Powered Massive White Dwarfs

Abstract

SGR 0418$+$5729 is a “Rosetta Stone” for deciphering the energy source of Soft Gamma Ray Repeaters (SGRs) and Anomalous X-ray Pulsars (AXPs). We present a model based on canonical physics and astrophysics for SGRs and AXPs powered by massive highly magnetized rotating white dwarfs (WDs), in total analogy with pulsars powered by rotating neutron stars (NSs). We predict for SGR 0418$+$5729 a lower limit for its spin-down rate, $\dot{P}~\geq~L_{\rm X} P^3/$(4$\pi^2 I$) $=$ 1.18 $\times$ 10$^{-16}$, where $I$ is the moment of inertia of the WD. We show for SGRs and AXPs that the occurrence of the glitch and the gain of rotational energy is due to the release of gravitational energy associated to the contraction and decrease of the moment of inertia of the WDs. The steady emission and the outburst following the glitch are explained by the loss of rotational energy of the WDs, in view of the much larger moment of inertia of the WDs, as compared to that of NSs and/or quark stars. There is no need here to invoke the unorthodox concept of magnetic energy release due to the decay of overcritical magnetic fields, as assumed in the magnetar model. A new astrophysical scenario for the SGRs and AXPs associated to Supernova remnants is presented. The observational campaigns of the X-ray Japanese satellite Suzaku on AE Aquarii and the corresponding theoretical works by Japanese groups and recent results of the Hubble Space Telescope, give crucial information for our theoretical model. Follow-on missions of Hubble Telescope and VLT are highly recommended to give further observational evidence of this most fundamental issue of relativistic astrophysics: the identification of the true SGRs/AXPs energy source.