Imprints of Electron–positron Winds on the Multiwavelength Afterglows of Gamma-Ray Bursts

Abstract

Optical rebrightenings in the afterglows of some gamma-ray bursts (GRBs) are unexpected within the framework of the simple external shock model. While it has been suggested that the central engines of some GRBs are newly born magnetars, we aim to relate the behaviors of magnetars to the optical rebrightenings. A newly born magnetar will lose its rotational energy in the form of Poynting-flux, which may be converted into a wind of electron-positron pairs through some magnetic dissipation processes. As proposed by Dai, this wind will catch up with the GRB outflow and a long-lasting reverse shock (RS) would form. By applying this scenario to GRB afterglows, we find that the RS propagating back into the electron-positron wind can lead to an observable optical rebrightening and a simultaneous X-ray plateau (or X-ray shallow decay). In our study, we select four GRBs (i.e., GRB 080413B, GRB 090426, GRB 091029, and GRB 100814A), of which the optical afterglows are well observed and show clear rebrightenings. We find that they can be well interpreted. In our scenario, the spin-down timescale of the magnetar should be slightly smaller than the peak time of the rebrightening, which can provide a clue to the characteristics of the magnetar.

Late Afterglow Bump/Plateau around the Jet Break: Signature of a free-to-shocked wind Environment in Gamma-ray Burst

“…De Pasquale et al (2015) presented a broadband observations of GRB 100814A and they attribute the late optical rebrightening to a long-lived external-reverse shock and external-forward shock. Besides, Geng et al (2016) used an ultrarelativistic e + e − wind injection model to explain the late rebrightening. It should be noted that the optical/X-ray afterglows all turn into the steep decay after the peak time of the optical bump.…”

“…Our fitting is performed based on the Markov Chain Monte Carlo (MCMC) method to produce posterior predictions for the model parameters. MCMC method is widely used in finding a best set of parameters for a specified model, e.g., GRB 080413B (Geng et al 2016); GRBs 100418A, 100901A, 120326A, and 120404A (Laskar et al 2015). In our work, fitting the afterglows of GRBs 120326A, 120404A, and 100814A with the MCMC method is to test whether or not the late simultaneous bumps followed by a post-jet-break-phase can be explained with an off-axis observed external-forward shock in a free-to-shocked wind environment.…”

“…Guidorzi et al (2014) attribtuted the optical/X-ray bumps of GRB 120404A to a decelerating jet viewed close to the jet edge, combined with some early re-energization of the shock. For GRB 100814A, Geng et al (2016) proposed an ultrarelativistic electron-positron pair wind model to explain the late optical bumps at t obs ⋍ 10 4 s. Yu et al (2015) invoked a magnetar with spin evolution to explain the afterglow emission. Nardini et al (2014) attributed the feature to the late-time activity of the central engine in the observed afterglow emission.…”

Evolution Patterns of the Peak Energy in the GRB Prompt Emission

“…is the Doppler factor, and θ is the angle between the line of sight and the local radial direction. When we take the EATS effect into account, the observed flux density now writes as (Geng et al 2016)…”

Diverse Features of the Multiwavelength Afterglows of Gamma-Ray Bursts: Natural or Special?

“…We thus proposed that steep optical rebrightenings are caused by the fall-back processes of central accreting BHs. By contrast, the energy flow from a magnetar may be in the form of a continuous e + e − wind [41]. The e + e − wind model was initially proposed to explain the X-ray plateau [42].…”