Speaker
Description
Short gamma-ray bursts (sGRBs) are associated with compact binary mergers, as established by the joint detection of GW 170817 and GRB 170817A, although the nature of the post-merger remnant remains uncertain. A long-lived millisecond magnetar provides a possible central engine for the extended X-ray emission observed in a fraction of sGRBs. We investigate nine sGRBs detected between 2016 and 2025, selected on the basis of extended emission in the Swift/BAT and XRT light curves, measured redshifts, and a luminosity cut-off of ($10^{52}$) erg/s. We model their extended emission using fallback accretion onto a magnetar coupled to the propeller mechanism. Joint BAT–XRT fits are examined using three energy prescriptions: the XRT (0.3–10 keV) band, BAT (15–50 keV) band, and their native energy bands. The parameters derived from these fits are consistent with predictions for magnetar properties and fallback accretion models with an exception of GRB~231117A. Two events favour the XRT band, three favour the BAT band, while the remaining bursts are better described in their native bands. Using the inferred initial disc masses, we further constrain the progenitor binary parameter space, linking the neutron-star mass ratio and total binary mass through numerical-relativity prescriptions for the remnant disc. We demonstrate that sGRBs with extended emission follow the universal relation akin to other accreting compact objects transitioning from accretion to propeller, with significantly lower radiative efficiencies than the X-ray binaries. Finally, we highlight that future late-time radio follow-ups of kilonova afterglows from these events with MeerKAT and the upcoming SKA, will be crucial for obtaining direct evidence of magnetar central engines in compact binary mergers.