The recently reported Type II Gamma-ray Burst (GRB) 200826A challenges the collapsar models by questioning how they can generate a genuinely short duration of the event. This paper proposes that the burst can originate from the collapse of a Thorne-Z
ytkow-like Object (TZlO). The TZlO consists of a central neutron star (NS) with a dense white dwarf (WD) material envelope and a disk, which are formed as the aftermath of a WD-NS coalescence. We found the collapse of such a TZlO can naturally explain the short duration of GRB 200826A. Furthermore, the collapse can produce a magnetar as the central object, which provides additional energy injection via magnetic dipole radiation to the ejected WD materials, causing a bump-like feature in the optical band and a shallow decay of the X-ray band. The disk wind shell induced by the TZlO at a large radius also interacts with the ejected materials, which explains the ``supernova bump observed at $sim$ 28 days.
We perform a stringent search for precursor emission of short gamma-ray bursts (SGRBs) from the Fermi/GBM data and find 16 precursor events with $gtrsim4.5sigma$ significance. We find that the durations of the main SGRB emission ($T_{rm GRB}$) and th
e precursor emission ($T_{rm pre}$), as well as the waiting time ($T_{rm wt}$) in between, are roughly comparable to each other, with $T_{rm wt}approx2.8T_{rm GRB}^{1.2}$ approximately satisfied for most cases except one significant outlier. We also perform spectral analyses to the precursors and SGRBs, and find that the spectra of precursor emission can be fitted with the blackbody, non-thermal cutoff power law and/or power law models. We consider several possible models for precursor emission in SGRBs and find that the luminosity and spectral shape may be explained by the the shock breakout or the photospheric radiation of a fireball launched after the merger for thermal precursors, or magnetospheric interaction between two NSs prior to the merger for non-thermal precursors. For the fireball photospheric model, a matter-dominated jet is preferred and a constraint on the fireball Lorentz factor can be placed as $Gammasim30$. For the magnetospheric interaction model, jet launching mechanism may be constrained. In particular, those events with $T_{rm wt}/T_{rm GRB}gg1$ (e.g. GRB191221802) require the formation of a supramassive or stable neutron star after the merger, with the delay time defined by the timescale for an initially baryon-loaded jet to become magnetically dominated and relativistic.
In this paper, we revisit the scenario that an internal gradual magnetic dissipation takes place within the wind from a newborn millisecond magnetar can be responsible for gamma-ray burst production. We show that a combination of two emission compone
nts in this model, i.e., the photospheric emission from the wind and the synchrotron radiation within the magnetic reconnection region, can give a reasonable fit to the observed spectrum of the prompt emission phase of GRB 160804A. We obtain the physical parameters through a Monte Carlo procedure and deduce the initial spin period and magnetic field of the central magnetar. Furthermore, the independent afterglow fitting analysis gives a consistent result, adding great credibility to this scenario. In addition, we predict a subclass of GRBs called bursts from such a Magnetar wind Internal Gradual MAgnetic Dissipation (abbreviated as MIGMAD bursts) that have several distinctive properties.
