GRBs from Collapse of Thorne-\.Zytkow-like Objects as the Aftermath of WD-NS Coalescence

Abstract

The Type II gamma-ray burst (GRB) 200826A challenges collapsar models by questioning how they can generate a genuinely short-duration event. The other Type I GRB 211211A confused us with a kilonova signature observed in the afterglow of a \textit{long} burst. Here, we propose a comprehensive model in which both bursts are the results of the collapse of Thorne-\.Zytkow-like Objects (TZlOs). The TZlO consists of a central neutron star (NS), with a dense white dwarf (WD) material envelope, which is formed as the aftermath of a WD-NS coalescence. We find that the characteristics of the resultant GRBs depend on whether the TZlO collapses immediately following the WD-NS merger or not. Additionally, the observational properties of the consequent GRBs manifest variations contingent upon whether the collapse of the TZlO results in a magnetar or a black hole. We also show that our model is consistent with the observations of GRB 211211A and GRB 200826A. Specifically, the optical excess in GRB 211211A can be attributed to an engine-fed kilonova, while the \textit{supernova bump} in GRB 200826A is likely due to the collision between the ejecta and the disk wind shell.