Three-Peak GRBs and Their Implications for Central Engines

TL;DR

This paper proposes a unified model combining millisecond protomagnetar and collapsar scenarios to explain the three-peak lightcurve of GRB 110709B, suggesting a new way to observe black hole formation from core-collapse events.

Contribution

It introduces a novel unified model explaining multiple peaks in GRB lightcurves as different phases of a single collapsing object, supported by data analysis and theoretical estimates.

Findings

The three peaks correspond to different phases: protomagnetar, BH formation, and collapsar.

Model accurately reproduces the timing and energy of GRB 110709B.

Supports the possibility of observing black hole formation directly from GRB lightcurves.

Abstract

GRB 110709B presented a peculiar three-peak lightcurve; this burst twice triggered the BAT detector onboard Swift. The two triggers were separated by $\sim 10$ minutes. In order to explain such an event, we unify into a single description the millisecond (ms) protomagnetar and the collapsar central-engine models. We find that such a scenario could produce GRBs with three peaks. One for the ms-protomagnetar stage, a second one for the BH-formation event and a third one for the collapsar phase. We show that the three peaks for GRB 110709B originate from different phases of the same collapsing object. We estimate the energies and timescales of the different episodes of this burst using our model and compare with previous results as well as with a reanalysis we perform on the data. We show that not only the light curve, but also the photon index evolution and the delay between the prompt emission and the afterglow of the second central-engine activity phase point towards a model like the one proposed here. We find that, with reasonable assumptions, our model correctly describes the activity in GRB 110709B. We further suggest careful study of future GRBs lightcurves which may help show the validity of our model. If our model is correct, this would be the first time that the formation of a BH from a core-collapse event is observed unimpededly.