EP241021a: A Catastrophic Collapse/Merger of Compact Star Binary Leading to the Formation of a Remnant Millisecond Magnetar?

Abstract Observations of fast X-ray transients (FXRTs) with the Einstein Probe have successfully led to the discovery of some unusual extragalactic optical transients. EP241021a is a newly discovered FXRT that was featured by a significant bump at around 10 days in both optical and X-ray bands. This timescale and the exceptionally high peak bolometric luminosity up to ∼10 44 erg s −1 of the optical bump make it somewhat similar to fast blue optical transients but still distinctive from them by its relatively red color. We then suggest that the multiwavelength bump of EP241021a could represent an explosion-type transient, while the underlying power-law decaying component of the optical and X-ray emission as well as the total radio emission are produced by a moderately relativistic jet. By fitting the observed multiwavelength light curves, it is found that the explosion ejecta that produce the thermal optical emission can have a mass of ∼0.03 M ⊙ , an expanding velocity of ∼0.25 c , and an optical opacity of ∼12 cm 2 g −1 , which was continuously powered by a rapidly rotating and highly magnetized neutron star (NS; i.e., a magnetar). In addition to heating the explosion ejecta, the magnetar also provided the dominant contribution to the observed X-ray rebrightening through the nonthermal emission of its wind. These properties suggest that the explosion may result from a catastrophic collapse/merger of a compact star system, which led to the formation of a millisecond magnetar, and the possible progenitor could be an accreting white dwarf (WD) or a binary consisting of a double WD, a double NS, or a WD and an NS.