GRB 101225A as Orphan Dipole Radiation of a Newborn Magnetar with Precession Rotation in an Off-axis Gamma-ray Burst

Abstract

Abstract The unusual multiwavelength lightcurves of GRB 101225A are revisited by assuming that they are from an off-axis GRB powered by a newborn magnetar. We show that GRB 101225A’s optical afterglow lightcurve is fitted with the forward shock model by parameterizing its jet structure as a Gaussian function with a half-opening angle of the jet core as 1.°67. The derived initial Lorentz factor (Γ0) is 120, and the viewing angle to the jet axis is θ v = 3.°7. Tentative QPO signatures of P = 488 s and P = 250 ∼ 300 s are found with a confidence level of 90% by analyzing its X-ray flares observed in the time interval of [4900, 7500] s. Its global gamma-ray/X-ray lightcurve and the QPO signatures are represented with the magnetar dipole radiation (DR) model by considering the magnetar precession motion, assuming that the magnetar spindown is dominated by GW emission. The bulk Lorentz factor of the DR ejecta is limited to 8, being much lower than Γ0. Comparing GRB 101225A with the extremely off-axis GRB 170817A, we suspect that the nature of the two-component jet in GRB 170817A is a combination of a co-axial GRB jet and a DR ejecta. GRB 101225A would be among the brightest ones of the CDF-S XT2-like X-ray transient population driven by newborn magnetars. A discussion of the detectability of its gravitational wave emission is also presented.