Can GRB 200826A Originate From the Collapse of A Thorne-Zytkow-like Object? (Planetary Science)

In the recent study, Bin-Bin Zhang and colleagues proposed that the Type II Gamma-ray Burst (GRB) 200826A can originate from the collapse of a Thorne-Zytkow-like Object (TZlO). They found that the collapse of such a TZlO can naturally explain the short duration of GRB 200826A. Their study recently appeared in Arxiv.

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.

Previously, Zhang et al. suggested a possibility that the intrinsically short duration of the Type-II GRB 200826A can be explained by a progenitor involved with a compact object such as a white dwarf, which supplies much denser materials to account for the short accretion timescale to match the observed short duration. However, isolated WDs are incapable of producing GRBs; one, therefore, has to invoke a merger process between a WD and one other compact star such as a neutron star (NS) or a black hole (BH). One of such combinations, which gains increasing interest in the field, is the WD-NS merger.

There are two fates for a WD-NS system, which is determined by the critical mass ratio between the WD and the NS. In stable mass transfer case, the system will form an ultra-compact X-ray binary. While, in unstable mass transfer case, the system will form an Throne Zytkow 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.

Now, Bin-Bin Zhang and colleagues introduced the collapse of a TZlO as the progenitor of the peculiar short-duration GRB 200826A.

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Figure 1. The formation and collapse of a TZlO from a WD-NS merger. © Bin-Bin Zhang et al.

They found that 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.

Figure 2. Multi-wavelength observation data of GRB 200826A over-plotted with our model using the best-fit parameters. Top: Data points show the optical observations. Solid line show the model predication using the best-fit parameters in Table 2. The dashed lines show the merger-nova-like components and the dotted lines show the late-time SN-bump-like component. Middle: Data points show the X-ray observaitons by Swift/XRT. Solid line show the the model predication and dashed line shows the shallow-decay component powered by the magnetar. Bottom: Radio afterglow (filled circles) overplotted with the best-fit models (solid lines). © Bin-Bin Zhang et al.

Finally, it has been found that the disk wind shell induced by the TZlO at a large radius also interact with the ejected materials, which explains the “supernova bump” observed at ∼ 28 days.

“The event rate of WD-NS mergers is ∼ 0.5 – 1 × 10⁴ Gpc¯3yr¯1, or as lower as ∼ 0.7 – 7 × 10³ Gpc¯3yr¯1 in the near cosmos. A non-negligible fraction of those mergers can undergo the UMT process and produce TZlOs. Whether or not all of those TZlOs can further collapse and generate GRB 200826A-like events remains an open question. Nevertheless, discoveries of similar events in the future or the existing GRB samples can shed some light on this new class of GRB events and the fate of the WD-NS mergers.”, they concluded.

Reference: Zong-Kai Peng, Zi-Ke Liu, Bin-Bin Zhang, “GRB 200826A: Collapse of a Thorne-Zytkow-like Object as the Aftermath of a WD-NS Coalescence”, Arxiv, pp. 1-10, 2021.

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