EP251023a: A fast X-ray transient featuring a magnetar-powered optical internal plateau followed by a steep decay

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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 < 2.27 ms and a magnetic field strength B p < 8.33 × 10 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 < 32.15 ms and B p < 1.18 × 10 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.
Funding Information
  • NSF (AST-2308113)
  • NSF (AST-1911151)
  • Ioffe Institute (FFUG-2024-0002)
  • Spanish Ministry project (PID2023-151905OB-I00)
  • Junta de Andaluc\'ia grant (P20_010168)

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