GCN Circular 36815

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

We observed the field of the X-ray transient, EP240703a, detected by EP/WXT (Wang et al., GCN 36807
Not found
 
 
), and also by Konus-Wind (Frederiks et al., GCN 36809
Not found
 
 
), thus confirming that it is likely GRB 240703A, using the Thai Response Telescope (TRT) SBO node and the JinShan 100C & 50A telescopes.

Image subtraction of the stacked 5 x 360 s R-band TRT image against the PanSTARRS archival r-band image reveals multiple optical afterglow candidates, among which an uncatalogued optical transient (OT) is localized at coordinates

R.A. (J2000) = 18:15:03.89
Dec. (J2000) = -09:42:02.96

with an uncertainty of ~ 0.5 arcsec. The OT has r ~ 20.4 +/- 0.1 mag at a median time of ~ 13.9 hr post-trigger, calibrated with nearby PanSTARRS stars and not corrected for Galactic extinction of A_r ~ 3 mag.

We then obtained 10 x 300 s frames in the Sloan r-band and 10 x 300 s in the Sloan z-band at the JinShan 100C telescope. The OT is not detected in the stacked r-band image down to r ~ 22.2 mag at a median time of 16.07 hr post-trigger, also not detected in z-band down to z ~  20.6 mag at a median time of 16.91 hr post-trigger, both calibrated with the nearby PanSTARRS field.

The presence and the decay of the OT make it an optical afterglow candidate of EP240703a/GRB 240703A.