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 raising upon the magnetar plateau would be expected. In this work, such peculiar cases are connected to the accretion process of the magnetars, and an implication for magnetar-disc structure is given. We investigate the repeated accretion process with multi-flare GRB 050730, and give a discussion for the accretion-induced variation of the magnetic field in GRB 111209A. Two or more flares exhibit in the GRB 050730, 060607A and 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 × 10−4M⊙ s−1, 4.23×10−4 M⊙ s−1, and 4.33×10−4M⊙ s−1 , and the corresponding average sizes of the magnetosphere are 5.01 × 106 cm, 6.45 × 106 cm, and 1.09 × 107 cm, respectively. A statistic analysis that contains eight GRBs within 12 flares shows that the total mass loading in single flare is ∼ 2 × 10−5M⊙. In the lost mass of a disk, there are about 0.1% used to feed a collimated jet.
Keywords accretion, accretion disk — stars: magnetars — gamma-ray burst: individual (GRB 050730, GRB 111209A)
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