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

TL;DR

This work reports a systematic optical follow-up of EP-discovered FXTs with the Lulin Observatory using the LOT and SLT telescopes, identifying 12 optical counterparts among 42 targets and first-time detections for five FXTs. The counterparts are generally faint and fade quickly, with many FXTs remaining optically dark, suggesting a substantial dark FXT population alongside GRB-like and possible jetted TDE origins. Cross-wavelength comparisons show that, among counterparts, the optical luminosities and redshifts are consistent with the faint end of GRBs, while a subset of events also resemble jetted TDEs; SBOs and kilonovae are disfavored as dominant sources. The results highlight the significant role of rapid, small-to-mid telescope follow-up in unveiling FXT origins and emphasize that a substantial fraction of EP FXTs are linked to relativistic jet phenomena, with future work needed to expand multi-wavelength spectroscopy and automate rapid responses. Overall, the study strengthens the case that many EP FXTs hail from relativistic jets, either as GRBs or jetted TDEs, while also documenting a large population of dark FXTs that require timely, coordinated observations to characterize.

Abstract

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 reports 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 detect the optical counterparts of 12 FXTs, and five of those are 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 per day). 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 non-detection of any associated optical counterpart with a significant fraction of FXTs suggests that EP FXTs are likely a subset of so-called `dark FXTs', similar to `dark GRBs'. Additionally, the luminosities of {\bf 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.

Figures (10)

• Figure 1: $Top$: The sky localizations of 72 high SNR FXTs discovered by the EP-mission in the first year of its operation. Within the context of the limited number of FXTs detected, the occurrences of FXTs seem to be isotropic in nature, similar to GRBs. The FXTs with associated GRBs, FXTs with corresponding optical transients (OTs), and FXTs with respective OTs discovered from the Lulin Observatory are highlighted. $Bottom$: Angular offset distribution between the EP-WXT candidates and their associated optical counterparts, plotted as $\Delta$RA versus $\Delta$Dec in arcseconds. A point located at the center indicates perfect positional agreement between the EP-WXT detection and the corresponding OT. Black concentric circles mark angular separation radii of 1, 2, and 3 arcminutes, corresponding to the typical positional uncertainty of the EP-WXT ($\sim$3$\hbox{$^\prime$}$). Each point represents a candidate, color-coded by event group, with several labeled for clarity. Cyan contours denote the 1, 2, and 3 $\sigma$ levels of the event population density, illustrating the statistical spread in positional accuracy. This visualization highlights both the average localization precision of EP-WXT and the reliability of optical counterpart identification. The references for localizations are credited in the Appendix.
• Figure 2: Kinder observations with Lulin SLT and LOT were carried out for EP FXT follow-up. For 12 EP events, we detected optical counterparts, and for five of these, we were the first to report the discovery. In each panel, the EP event is labeled at the top. The left column shows stacked science frames obtained with the corresponding telescope and filter; the middle column presents reference images from various all-sky surveys; and the right column displays the difference frames produced by subtracting the reference images from the science frames.
• Figure 3: Top: Integrated exposure time enhances the detectable magnitude depth. In the lower left panel, the top row displays images with increasing integrated exposure time, while the bottom row shows the corresponding template-subtracted results. The marker at the center indicates the location of the optical counterpart, AT 2024gsa, of EP240414a. For a target at 21.3 mag, an integrated exposure time of 900 seconds is required to achieve a detection above 3-sigma. With a stacked exposure time of 1800 seconds, the detection depth reaches 22 mag, can even reach 23 mag in some cases with better conditions. Typically, we use long exposures (30 minutes) for detections. Bottom: Limiting magnitude as a function of seeing.
• Figure 4: $Top$: Earliest optical detections (or upper limits in case of non-detection of any optical counterpart) for the entire FXTs' sample reported in GCN Circulars. Out of 72 FXTs discovered by the EP, 23 showed corresponding optical counterparts. All reported times are in the observer frame, where $T_{\rm start}$ indicates the beginning of the observation, and $T_{\rm 0}$ refers to the trigger time of the EP-WXT detection. $Bottom$: Comparison of $T_{\rm start}-T_{\rm 0}$ of Kinder/Lulin observations with other observations reported in GCNs. 12 out of 23 FXTs with optical counterparts were also detected from the Lulin Observatory. Further, 5 out of those 12 were first discovered by us. The Kinder optical detections for EP241021a and EP250108a were delayed; thus not shown here. We had only obtained optical upper limits in their respective first epoch of observations; those upper limits have been included in this plot. The pre-EP discovery Swift-UVOT optical counterpart detection was for EP241030a, which was associated with GRB 241030A 2024GCN.37956....1K. The references for first detections or upper limits are credited in the Appendix.
• Figure 5: The comparison of the optical luminosity of FXTs in our study with a sample of 535 GRBs from 2024MNRAS.533.4023D in $g$-, $r$-, and $i$-bands (round markers). A few peculiar and bright GRBs are also shown and highlighted for comparison. The optical luminosities of FXTs are consistent with GRBs. The redshifts of EP240416a, EP241201a, and EP241202b are unknown; however, the vertical lines joining the smaller and larger markers show the range of luminosities had they occurred within the known range of redshift for FXTs in our sample (i.e., $z = 0.176$ to 4.859). The sources of optical light curve of highlighted GRBs are as follow: GRB 990123 from 1999Natur.398..400A, GRB 080319B from 2008Natur.455..183R, GRB 140102A from 2021MNRAS.505.4086G, GRB 180720B from 2021RMxAC..53..113G, GRB 190114C from 2020ApJ...892...97J, GRB 210619B from 2023NatAs...7..843O, GRB 221009A from 2023ApJ...942...34R, and GRB 230204B from 2024arXiv241218152G.