Abstract
EP251023a is an extragalactic fast X-ray transient (eFXT) that was only detected by the Einstein Probe (EP) and that lacks a gamma-ray counterpart. The prompt emission consisted of a main emission with a duration
T
90
= 292 ± 19 s, followed by a long-lasting tail emission that persisted until the observation ended at
T
0
+ 1571 s. With the upper limit of Konus–Wind, we derived a conservative upper limit on the isotropic gamma-ray energy
E
γ
,iso
of 5.7 × 10
52
erg for the main emission phase. We identified a redshift of
z
= 2.232 ± 0.001 based on the strong absorption features in the Keck spectrum, which also indicate a relatively low host-galaxy HI column density. Based on the broadband spectral energy distribution, the late-time light curves show an achromatic plateau, followed by an extremely steep decay with a slope of 3.99 after a break at about 49 ks. This is consistent with a rapidly spinning millisecond magnetar engine. Under the isotropic wind scenario, we obtained an initial period
P
0
B
p
14
G for the magnetar. If a jet collimation with a typical opening angle of 0.1 rad is considered, these constraints are relaxed to
P
0
B
p
16
G. Together with GRB 070707, EP251023a might represent a rare class of optical magnetar-powered internal plateaus with little external-shock contamination. This is different from previous examples that were primarily detected in X-rays. Future discoveries of similar events will help us to clarify the relation between magnetar-powered internal emission observed in the optical band and the emission that is only detected in X-rays.