TITLE:   GCN CIRCULAR
NUMBER:  37501
SUBJECT: EP240913a: NOT optical upper limits
DATE:    24/09/14 11:40:33 GMT
FROM:    Dong Xu at NAOC/CAS <dxu@nao.cas.cn>

Z.P. Zhu, S.Y. Fu, S.Q. Jiang, J. An, X. Liu, D. Xu (NAOC), J.P.U. Fynbo (DAWN/NBI), P. Hakala (NOT) report on behalf of a large collaboration:

We observed the field of EP240913a detected by Einstein Probe (EP, Li et al., GCN 37492), using the ALFOSC instrument mounted on the 2.56-m Nordic Optical Telescope (NOT), with 3 x 300 s photometry in the SDSS-r band and 5 x 200 s in the SDSS-z band. Observations started at 01:13:39 UT on 2024-09-13, i.e., 13.6 hr after the EP trigger.

No new optical source is detected within the error circle of the proposed X-ray counterpart (Jiang et al., GCN 37497), down to the following 3-sigma upper limits:

T_mid (UT) | T_mid (hr) | filter | Exposue(s)| U.L. (AB)
2024-09-14T01:21:35 | 13.701 | r | 3x300     | 24.0
2024-09-14T01:39:43 | 14.003 | z | 5x200     | 23.0

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