Search for the Optical Counterpart of Einstein Probe Discovered Fast X-ray Transients from Lulin Observatory

Amar Aryan^1*, T. -W. Chen¹, S. Yang²

¹Graduate Institute of Astronomy, National Central University, Jhongli, Taoyuan, Taiwan
²Henan Academy of Sciences, Zhengzhou 450046, Henan, China

* Presenter:Amar Aryan, email:amar@astro.ncu.edu.tw

Fast X-ray transients (FXTs) are short-lived bursts in soft X-rays, lasting from a few seconds to hours. Their origins remain mysterious, but potential explanations link them to supernova shock breakouts, tidal disruption events (TDEs), and gamma-ray bursts (GRBs). FXTs have been known for over a decade, with around 40 detected by various X-ray observatories; however, their multi-wavelength counterparts are rare. The Einstein Probe (EP), launched in January 2024, has dramatically boosted the detection rate, identifying over 30 high signal-to-noise FXTs in only the first six months of its operation. The groundbreaking discovery of multi-wavelength counterparts, such as for EP240315a and EP240414a, offers new insights into FXTs’ mechanisms. In this work, we utilize the 40cm-SLT and 1m-LOT telescopes at the Lulin observatory to search for optical counterparts of FXTs detected by the EP. In our sample of 19 FXTs observed, five displayed strong evidence of association with GRBs. The alignment of their optical luminosity and redshift with a sample of 535 GRBs suggests a strong link between the two phenomena. Additionally, EP240315a and EP240801a also show consistency with jetted-TDE events. Our study concludes that a fraction of FXTs detected by EP is likely connected to GRBs or associated with relativistic jets.

Keywords: Fast X-ray Transients, Gamma-Ray Burts, Tidal Disruption events, Lulin Observatory, Optical Counterpart