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

TL;DR

This paper models GRB 101225A as an off-axis gamma-ray burst powered by a newborn magnetar with precession, explaining its multiwavelength lightcurves and QPOs, and compares it to other off-axis GRBs like 170817A.

Contribution

It introduces a magnetar dipole radiation model with precession to explain the lightcurves and QPOs of an off-axis GRB, providing new insights into magnetar-driven GRB emissions.

Findings

Optical afterglow fits a Gaussian jet structure with a 1.67° half-opening angle.

QPO signatures at 488s and 250-300s are tentatively detected.

Magnetar spindown is dominated by GW emission, with a low Lorentz factor of 8.

Abstract

The unusual multiwavelength lightcurves of GRB 101225A are revisited by assuming that it is from an off-axis GRB powered by a newborn magnetar. We show that its 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 $1.67^{\rm o}$. The derived initial Lorentz factor ($\Gamma_0$) is 120, and the viewing angle to the jet axis is $\theta_v=3.7^{\rm o}$. Tentative QPO signatures of $P=488$ seconds and $P=250\sim 300$ seconds are found with a confidence level of 90\% by analysing its X-ray flares observed in the time interval of $[4900,\ 7500]$ seconds. 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 the GW emission. The bulk Lorentz factor of the DR ejecta is limited to 8, being much lower than $\Gamma_0$. 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. Discussion on detectability of its gravitational wave emission is also presented.