GCN Circular 36011

T.-W. Chen (NCUIA), S. Yang (HNAS), S. Smartt (Oxford), H.-Y. Hsiao, W.-J. Hou, A. Sankar.K, Y.-C. Pan, M.-H. Lee, C.-H. Lai, C.-C. Ngeow, C.-S. Lin, H.-C. Lin, J.-K. Guo (all NCUIA), S. Srivastav (Oxford) and M. Fulton, T. Moore, C. Angus, A. Aamer (all QUB) report:

We observed the field of EP240331a (Pan et al. 2024, ATel #16564) using several telescopes at the Lulin Observatory, Taiwan as part of the Kinder collaboration (Chen et al., AstroNote 2021-92), including the Lulin One-meter Telescope (LOT) with r band, 40cm Super Light Telescope (SLT) with g band, and 50cm Lulin-ASIAA Telescope for Transients and Education (LATTE) with B band.  

The first LOT epoch started at 15:55 UT on April 1, 2024 (MJD = 60401.663), 0.74 days (=17.8 hours) after the discovery of the X-ray transient, EP240331a. We observed 4 pointings in order to cover its reported 20 arcmin error box. Each pointing were observed for 4 times, with 300-second exposure time each, taken under seeing conditions averaging 1".7 and at a median airmass of 1.63.

We used the Kinder pipeline (Yang et al. A&A 646, A22) to stack the images with same pointing and subtract the stacked images with the Pan-STARRS1 template images. We found a possible  variable at the location of RA = 11:17:32.8593, Dec = -20:06:28.335 in the difference images. The object was measured to have an r-band magnitude of 20.61+/-0.08 mag. We then cross checked it with several archived catalogs and found it was clearly detected in the Legacy Survey DR10 image, marked as a star with r = 20.75 mag. We measured this source in the Pan-STARRS1 image and obtained r = 21.00 +/- 0.15 mag. The variation indicates a possible variable star, but we cannot rule out AGN either.

Besides this variable, we do not detect any other significant optical counterpart candidates. In 2.5 sigma level, we obtained the following preliminary upper limits (in the AB system): 

g > 21.2  and
r > 21.9 mag.

The given limits are derived based on calibrating against Pan-STARRS1 field stars and is not corrected for the expected Galactic foreground extinction corresponding to a reddening of E_(B-V) = 0.04 mag in the direction of the burst (Schlafly & Finkbeiner 2011). Our B-band images are shallower and not calibrated so we do not report the limits here. 

Our result is consistent with MeerLICHT (Groot et al., GCN 36007
Not found
 
 
), MASTER (Lipunov et al., GCN 36009
Not found
 
 
) and GRANDMA (Freeberg et al., ATel #16567; GCN 36010
Not found
 
 
); our optical upper limit is deeper than these reports and confirm there is no significant optical counterpart within the first day after the X-ray trigger. 

We thank ASIAA for providing LATTE for the observations, and thank the Einstein Probe team for useful communication.