TITLE:   GCN CIRCULAR
NUMBER:  44259
SUBJECT: EP-WXT01709259790: Optical Upper limits with kinder observations
DATE:    26/04/11 02:08:30 GMT
FROM:    Janet Chen at National Central University <janetstars@gmail.com>

A. Aryan, T.-W. Chen, C.-S. Lin (all NCU), A. K. H. Kong (NTHU), S. Yang(HNAS), S. J. Smartt, J. Gillanders (both Oxford), A. Sankar.K, M.-H. Lee, A. Dutta, Y.-H. Lee, Y.-C. Pan, C.-C. Ngeow, C.-H. Lai, H.-C. Lin, W.-J. Hou, H.-Y. Hsiao, J.-K. Guo (all NCU), Z. N. Wang, D. C. Qiang, L. L. Fan (all HNAS), Y. J. Yang (NYUAD), H.-W. Lin (UMich), H. F. Stevance, S. Srivastav, L. Rhodes (all Oxford), M. Nicholl, M. Fulton, K. W. Smith, C. Angus, A. Aamer (all QUB), T. Moore (STScI), A. Schultz and M. Huber (both IfA, Hawaii) report:

We observed the field of the WXT trigger 01709259790 using the 40cm SLT at the Lulin observatory, as part of the Kinder collaboration (Chen et al. 2025, ApJ, 983, 86, doi:10.3847/1538-4357/adb428). The first SLT epoch of observations in r-band started at 12:10 UTC on 10th of April 2026 (MJD 61140.5068), 42 min after the EP-WXT detection.

The EP-WXT transient was later identified as the star Gaia DR3 782359908221356544 during its flaring stage (Huang et al. GCN 44256). The star has an archival SDSS magnitude of r = 16.20. In our observations, we captured the stellar flare and the star varies from r = 15.561 +/- 0.042 in our first frame to r = 15.838 +/- 0.028 in the final (12th) image.

The detailed photometric measurements using AutoPhOT are as follows:
 
Telescope | Filter | MJD (start)  | t-t0 (hr) | Exposure (s) | Magnitude| 
SLT       | r      | 61140.5068   | 0.70      | 300 * 1      | 15.561 +/- 0.042       
SLT       | r      | 61140.5104   | 0.78      | 300 * 1      | 15.582 +/- 0.048  
SLT       | r      | 61140.5141   | 0.87      | 300 * 1      | 15.614 +/- 0.049   
SLT       | r      | 61140.5177   | 0.96      | 300 * 1      | 15.718 +/- 0.043     
SLT       | r      | 61140.5215   | 1.05      | 300 * 1      | 15.740 +/- 0.045 
SLT       | r      | 61140.5251   | 1.14      | 300 * 1      | 15.750 +/- 0.038  
SLT       | r      | 61140.5289   | 1.23      | 300 * 1      | 15.762 +/- 0.038  
SLT       | r      | 61140.5326   | 1.31      | 300 * 1      | 15.786 +/- 0.041 
SLT       | r      | 61140.5363   | 1.40      | 300 * 1      | 15.793 +/- 0.043 
SLT       | r      | 61140.5400   | 1.49      | 300 * 1      | 15.805 +/- 0.035
SLT       | r      | 61140.5437   | 1.58      | 300 * 1      | 15.817 +/- 0.033  
SLT       | r      | 61140.5473   | 1.67      | 300 * 1      | 15.838 +/- 0.028   

We also utilized our Kinder Pipeline for the combined 300 sec*12 frames, we obtained an r-band limit of >20.6 mag at a median airmass of 1.13 and an average seeing of 1".5. Our limit is consistent with He et al. (GCN 44254). 

The presented magnitudes are calibrated using the field stars from the PanSTARRS catalog. The reported upper limit is not corrected for an expected galactic extinction of A_r = 0.02 mag in the direction of the transient (Schlafly & Finkbeiner 2011). The methodology, details on the Lulin observatory telescopes, and a compilation of our optical follow-up campaign for FXTs discovered within the first year of operation of the Einstein-Probe mission are presented in Aryan et al. 2025, ApJS, 281, 20, doi:10.3847/1538-4365/adfc69.