Repeating Emission Episodes in Gamma-Ray Bursts: Millilensing or Jet Precession?

Abstract

Recently, some gamma-ray bursts (GRBs) whose light curves consist of repeating emission episodes with similar temporal profiles have attracted extensive attention. They are proposed to be candidates of millilensing events, although smoking gun evidence is lacking, since there are no redshift measurements and no angular offset detections for any of these candidates. Here we show that without invoking gravitational lensing, the repeating light-curve properties of these GRBs could also be interpreted under the jet precession model, as long as the detectable period in every precession circle is less than the precession period, and the precession period is close to the jet emission duration. By fitting the gamma-ray light curves of these GRBs, we suggest that the jet precession angle for these bursts should be relatively small (e.g., θ p < 5.°3), and the jet structure for these bursts are more likely Gaussian. The results suggest us to be careful when identifying millilensing GRBs. Multiband afterglow data and especially angular offset detections are essential to provide comprehensive justification for this identification.

Can Fallback Accretion on the Magnetar Model Power the X-Ray Flares Simultaneously Observed with Gamma Rays of Gamma-Ray Bursts?

“…This means that the dirty nature of the baryon-loading issue still exists if we invoke the magnetar propeller with fallback accretion model to interpret the X-ray flares with a high Lorentz factor. Furthermore, several physical models without a magnetar central engine have been invoked to interpret the observed X-ray flare, plateau emission, as well as the two sub-bursts of gamma-ray emission, such as late central engine reactivity (Burrows et al 2005;Fan & Wei 2005;Dai et al 2006), the relativistic jet and cocoon emissions (Ramirez-Ruiz et al 2002;Lazzati & Begelman 2010;Nakar & Piran 2017), two steps in the collapse of the progenitor star (Lipunova et al 2009), the collapse of a rapidly rotating stellar core leading to fragmentation (King et al 2005), fragmentation in the accretion disk (Perna et al 2006), the magnetic barrier around the accretor (Proga & Zhang 2006), gravitational lensing (Paynter et al 2021;Yang et al 2021;Lin et al 2022), and the jet precession model (Gao et al 2023). However, more or less, each model above cannot fully interpret all properties of the observations for the two sub-bursts of emission of GRB from the light curve, spectrum, as well as the quiescent time.…”

Gamma-Ray Burst Pulses and Lateral Jet Motion

“…Portegies Zwart et al (1999) developed a model in which a luminosity cone precesses with the same amplitude and period as the system's accretion disk could be responsible for the "complex temporal behavior of bright GRBs." Gao et al (2023) have considered that jet precession may be a better explanation for GRBs with repeating emission episodes than millilensing, but this model also hypothesized that the mechanism responsible for the emission originates in radial interactions of material in the jet. Fargion and various coauthors (e.g., Fargion & Salis 1996;Fargion 2003;Fargion & Grossi 2006) have explored GRB light curves as being due to complex, short-term precession of narrow jets overlaid on longer-timescale precessional motions.…”