TITLE: GCN CIRCULAR NUMBER: 41637 SUBJECT: EP250831a: NOT optical observations and tentative afterglow candidate DATE: 25/09/01 19:46:02 GMT FROM: Gregory Corcoran at University College Dublin <gregory.corcoran@ucdconnect.ie> S.Y. Fu (HUST), J. Sanchez-Sierras (Radboud), J. An, L.B. He, S.Q. Jiang, X. Liu, Z.P. Zhu, D. Xu (NAOC), J.P.U. Fynbo (DAWN/NBI), G. Corcoran (UCD), P. G. Jonker (Radboud), A. J. Levan (Radboud and Warwick), D. B. Malesani (DAWN/NBI and Radboud), A. Martin-Carrillo (UCD), J. A. Quirola-Vasquez (Radboud), A. Henderson (NOT) report: We observed the field of EP250831a (Yang et al., GCN 41619) using the Nordic Optical Telescope (NOT) located in La Palma (Canary Islands, Spain). We obtained 6 x 300 s r-band and 9 x 200 s z-band images. Within the EP-FXT error circle (Yang et al., GCN 41625), a new optical source is marginally visible in the stacked r-band image, at coordinates R.A. (J2000) = 02:51:55.49 Dec. (J2000) = -36:11:14.9 with an uncertainty of ~ 0.5 arcsec. We measure an AB magnitude r ~ 23.8 mag at an epoch of 18.23 hr after the EP trigger, calibrated against the Legacy Survey DR10 and not corrected for Galactic extinction. This source is not detected in the stacked z-band image, with a limiting magnitude of z > 22.0 mag (AB). There is a source in the Legacy Survey about 1.7” SE of our candidate, with a tabulated magnitude r = 23.83. We caution about the low S/N of our detection so we only report this object as a tentative optical counterpart to EP250831a. Further observations are planned in order to secure a template imaging and assess fading. View this GCN Circular online at https://gcn.nasa.gov/circulars/41637. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...