X-ray Flares Raising upon Magnetar Plateau as an Implication of a Surrounding Disk of Newborn Magnetized Neutron Star

Abstract

The X-ray flares have usually been ascribed to long-lasting activities of the central engine of gamma-ray bursts (GRBs), e.g., fallback accretion. The GRB X-ray plateaus, however, favor a millisecond magnetar central engine. The fallback accretion can be significantly suppressed due to the propeller effect of a magnetar. Therefore, if the propeller regime cannot resist the mass flow onto the surface of the magnetar efficiently, the X-ray flares raise upon the magnetar plateau would be hinted. In this work, such peculiar cases are connected to the accretion process of a magnetar, and an implication for magnetar-disc structure is given. We investigate the repeating accretion process with multi-flare GRB 050730, and give a discussion for the accreting induced variation of the magnetic field in GRB 111209A. Two or more flares exhibit in the GRB 050730, GRB 060607A, and GRB 140304A; by adopting magnetar mass M = 1.4 M ⊙ and radius R = 12 km, the average mass flow rates of the corresponding surrounding disk are 3.53 × 10 −4 M ⊙ s −1 , 4.23 × 10 −4 M ⊙ s −1 , and 4.33 × 10 −4 M ⊙ s −1 , and the corresponding average sizes of the magnetosphere are 5.01 × 10 6 cm, 6.45 × 10 6 cm, and 1.09 × 10 7 cm, respectively. A statistic analysis that contains 8 GRBs within 12 flares shows that the total mass loading in single flare is ∼ 2 × 10 −5 M ⊙ . In the lost mass of a disk, there are about 0.1% used to feed a collimated jet.