ROTATIONAL EVOLUTION OF MAGNETARS IN THE PRESENCE OF A FALLBACK DISK

Magnetars may have a strong surface dipole field. Observationally, two magnetars may have passive fallback disks. In the presence of a fallback disk, the rotational evolution of magnetars may be changed. In the self-similar fallback disk model, it is found that (1) when the disk mass is significantly lower than 10(-6). M-circle dot, the magnetar is unaffected by the fallback disk and it will be a normal magnetar. (2) When the disk mass is high but the surface dipole field of the magnetar is about or below 10(14) G, the magnetar will also be a normal magnetar. A magnetar plus a passive fallback disk system is expected. This may correspond to the observations of magnetars 4U 0142+61 and 1E 2259+586. (3) When the disk mass is high and the surface dipole field of the magnetar is as high as 4 x 10(15) G, the magnetar will evolve from the ejector phase to the propeller phase, and then enter rotational equilibrium. The magnetar will be slowed down quickly in the propeller phase. The final rotational period can be as high 2 x 10(4) s. This may correspond to the super-slow magnetar in the supernova remnant RCW 103. Therefore, the three types of magnetars can be understood in a unified way.