GCN Circular 36902

X. Liu, J. An (NAOC), S. Tinyanont, R. Anutarawiramkul, P. Butpan (NARIT), S.Q. Jiang, S.Y. Fu, Z.P. Zhu, D. Xu, Z. Fan, W.X. Li, N.C. Sun, Y.N. Wang (NAOC), X.Y. Wang (NJU) report on behalf of a large collaboration:

We observed the seven soft X-ray source positions detected by EP/FXT (Zhou et al., GCN 36894
Not found
 
 
) in its follow-ups of the error circle (radius ~ 0.3 deg) of the best-fit direction for the IceCube-Cascade 240714A event (Vandenbroucke et al., ATEL #16708), using the 0.7-m telescope of the Thai Robotic Telescope (TRT) network, located at Cerro Tololo Inter-American Observatory, Chile. Observations were done in two epochs, and a series of 120 s, 180 s frames in the R-band were obtained.

Firstly, no credible uncatalogued source was found at the positions of the seven EP/FXT sources. Preliminary results are as follows

Source | mid time | U.L. (5-sigma)
----------------------------------------------------------
EP J161324.4-363745  | 2024-07-16T03:58:40 | 19.7
EP J161423.8-362537  | 2024-07-18T04:50:30 | 19.6
EP J161307.1-365708  | 2024-07-18T04:59:32 | 19.6
EP J161439.0-362654  | 2024-07-18T04:50:30 | 19.6
EP J161357.7-361806  | 2024-07-18T07:00:33 | 19.0
EP J161128.2-361818  | 2024-07-18T04:41:29 | 19.3
EP J161433.2-365843  | 2024-07-18T04:32:17 | 19.5

calibrated with nearby SkyMapper stars and not corrected for Galactic extinction.

Secondly, there is no apparent brightening for unsaturated catalogued sources at the seven positions.

Thirdly, there are two very bright objects within the error circles of EP J161357.7-361806 and EP J161433.2-365843, respectively, which could be Young Stellar Objects by SIMBAD. These two objects are saturated in the TRT images. Their magnitudes from differential photometry, which should be considered as lower limits, are slightly fainter than that in SkyMapper survey. Therefore, it's hard to calculate their true magnitudes and thus hard to tell if they're brightening or not.