Search for the Optical Counterpart of Einstein Probe–discovered Fast X-Ray Transients from the Lulin Observatory

Journal article

The launch of the Einstein probe (EP) mission has revolutionized the detection and follow-up observations of fast X-ray transients (FXTs) by providing prompt and timely access to their precise localizations. In the first year of its operation, the EP mission reported the discovery of 72 high signal-to-noise FXTs. Subjected to the visibility in the sky and weather conditions, we search for the optical counterparts of 42 EP-discovered FXTs from the Lulin Observatory. We successfully detected the optical counterparts of 12 FXTs, and five of those were first discovered by us from the Lulin Observatory. We find that the optical counterparts are generally faint (r > 20 mag) and decline rapidly (>0.5 mag day−1). We also find that 12 out of 42 FXTs show direct evidence of their association with gamma-ray bursts (GRBs) through significant temporal and spatial overlapping. Furthermore, the luminosities and redshifts of FXTs with confirmed optical counterparts in our observations are fully consistent with the faintest end of the GRB population. However, the nondetection of any associated optical counterpart with a significant fraction of FXTs suggests that EP FXTs are likely a subset of the so-called “dark FXTs,” similar to “dark GRBs.” Additionally, the luminosities of two FXTs with confirmed redshifts are also consistent with jetted tidal disruption events (TDEs). However, we find that the optical luminosities of FXTs differ significantly from typical supernova shock breakout or kilonova emissions. Thus, we conclude that a significant fraction of EP-discovered FXTs are associated with events having relativistic jets; either a GRB or a jetted TDE.

Authors: Aryan, A; Chen, T; Yang, S; Gillanders, JH; Kong, AKH

• Publication status: Published
• Peer review status: Peer reviewed
• Publisher: American Astronomical Society
• Journal: The Astrophysical Journal: Supplement Series
• Volume: 281
• Issue: 1
• Article number: 20
• Publication date: 2025-11-04
• Acceptance date: 2025-08-14
• DOI: 10.3847/1538-4365/adfc69
• EISSN: 1538-4365
• ISSN: 0067-0049
• Language: English
• Keywords: Gamma-ray bursts; High energy astrophysics; X-ray transient sources; Transient sources; Optical identification