GRB 210610B: The Internal and External Plateau As Evidence For The Delayed Outflow Of Magnetar

Abstract

After launching a jet, outflows of magnetar were used to account for the achromatic plateau of afterglow and the early X-ray flux plateau known as "internal plateau''. The lack of detecting magnetic dipole emission together with the energy injection feature in a single observation poses confusion until the long gamma-ray burst (GRB) 210610B is detected. GRB 210610B is presented with an optical bump following an early X-ray plateau during the afterglow phase. The plateau followed by a steep decline flux overlays in the steadily decaying X-ray flux with index α X,1 ∼ 2.06, indicating an internal origin and that can be fitted by the spin-down luminosity law with the initial plateau luminosity log10 L X ∼ 48.29 erg s-1 and the characteristic spin-down timescale T ∼ 2818 s. A subsequent bump begins at ∼ 4000 s in R band with a rising index α R,1 ∼ -0.30 and peaks at ∼ 14125 s, after which a decay index α R,2 ∼ 0.87 and finally transiting to a steep decay with α R,3 ∼ 1.77 achieve the closure relation of the external shock for the normal decay phase as well as the magnetar spin-down energy injection phase, provided that the average value of the photon index Γγ = 1.80 derived from the spectral energy distributions (SEDs) between the X-ray and optical afterglow. The closure relation also works for the late X-ray flux. Akin to the traditional picture of GRB, the outflow powers the early X-ray plateau by dissipating energy internally and collides with the leading decelerating blast burst as time goes on, which could interpret the exotic feature of GRB 210610B. We carry out a Markov Chain Monte Carlo (MCMC) simulation and obtain a set of best parameters: ε B ≈ 4.2 × 10-5, ε e ≈ 0.16, E K,iso ≈ 4.2× 1053 ergs, Γ0 ≈ 851, A* ≈ 0.11, L inj,0 ≈ 2.31 × 1050 erg s-1. The artificial light curve can fit the afterglow data well. After that, we estimated the average Lorentz factor and the X-ray radiation efficiency of the later ejecta are 35 and 0.13%, respectively.