Optical insights to the Einstein Probe Fast X-ray Transients from Lulin observatory: A New Class or Just Faint Long GRBs?

Amar Aryan^1*, Ting-Wan Chen¹, Sheng Yang², James H. Gillanders³, Albert K. H. Kong⁴, S. J. Smartt³, Heloise F. Stevance³, Yi-Jung Yang¹

¹Graduate Institute of Astronomy, National Central University, 300 Jhongda Road, 32001 Jhongli, Taiwan
²Henan Academy of Sciences, Zhengzhou 450046, Henan, China
³Astrophysics sub-Department, Department of Physics, University of Oxford, Keble Road, Oxford, OX1 3RH, UK
⁴Institute of Astronomy, National Tsing Hua University, Hsinchu 300044, Taiwan

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

Fast X-ray transients (FXTs) are rapid, intense bursts of soft X-ray emission in the 0.3−10 keV range, typically lasting from minutes to hours. The launch of the Einstein Probe (EP) mission has revolutionized the detection and follow-up observations of FXTs by providing prompt and timely access to their precise localizations. During the first year of the operation of EP mission, we followed 42 FXTs from Lulin observatory to search for their optical counterparts. We successfully detected the optical counterparts of 12 FXTs, and five of those were first discovered by us. We found that the optical counterparts are generally faint (r >20 mag) and decline rapidly (>0.5 mag per day). Twelve out of 42 FXTs show direct evidence of their association with Gamma-Ray Bursts (GRBs) through significant temporal and spatial overlapping. Furthermore, the optical luminosities and redshifts of FXTs with confirmed optical counterparts in our observations are exceptionally consistent with the low luminosity regime of long GRB population. A significant fraction of EP FXTs lacks optical counterpart, suggesting that the EP FXTs are likely a subset of so-called ‘dark-FXTs’, similar to ‘dark-GRBs’. These findings show a strong correlation between FXTs and low-luminosity long GRBs rather than a new population.

Keywords: Fast X-Ray transients, Gamma-Ray Bursts, Dark-FXTs, Einstein Probe , Optical