TITLE:   GCN CIRCULAR
NUMBER:  43196
SUBJECT: EP251221a: NOT optical upper limit
DATE:    25/12/23 01:19:56 GMT
FROM:    L. B. He at NAOC <helb@bao.ac.cn>

L. B. He, J. An, X. Liu, D. Xu (NAOC), D. B. Malesani (DAWN/NBI and Radboud), J. A. Chacon (PUC), F. E. Bauer (SSI and UTA), P. G. Jonker (Radboud), A. Kadela (NOT and NBI), report on behalf of a larger collaboration:

We observed the field of EP251221a (Dai et al., GCN 43178) using the Nordic Optical Telescope (NOT) equipped with the ALFOSC camera. Observations were obtained in the r and z band, starting on 2025-12-22 at 03:00:12 UTC (5.35 hr after the WXT trigger). 

No new optical source is detected within the EP/WXT error circle. And we obtained the following upper limits:

| Band | t_mid - T0 (hr) | Exposure time | 5-sigma upper limit |
|--|--|--|--|
| r | 5.47 | 1x300s+1x180s | 23.5 |
| r | 6.25 | 3x300s | 23.8 |
| z | 7.21 | 5x200s | 22.3 |

calibrated with nearby Pan-STARRS stars and not corrected for Galactic extinction.