TITLE: GCN CIRCULAR NUMBER: 42378 SUBJECT: EP251021b: BOOTES-7 optical upper limit DATE: 25/10/22 09:05:45 GMT FROM: I. Perez-Garcia at Institute of Astrophysics of Andalusia ipg@iaa.es

I. Perez-Garcia, S.-Y. Wu, E. Fernandez-Garcia, M. D. Caballero-Garcia, R. Sanchez-Ramirez, S. Guziy and A. J. Castro-Tirado (IAA-CSIC, Granada), C. Perez del Pulgar, A. Castellon, A. Reina (Univ. de Malaga),Y.-D. Hu (GXU), L. Hernandez-Garcia (Univ. de Valparaiso), M. Gritsevich (Univ. of Helsinki), D. R. Xiong (Yunnan Observatories of CAS), B.-B. Zhang (Nanjing Univ.) and A. Maury (Space, San Pedro de Atacama), on behalf of a larger collaboration, report:

Following the detection of the fast X-ray transient EP251021b by the Einstein Probe (Dai et al. GCN 42377), the 0.6m BOOTES-7 robotic telescope at San Pedro de Atacama (Chile) observed the fast X-ray transient location starting on Oct 22, 03:57 UT (~13.08 h after trigger) in different optical bands. No new optical source is detected on the first co-added clear-filter images (mid exposure time 2025-10-22 04:07:57) within the EP/FXT 20 arcsec radius error circle down to 20.0 mag.

We thank the staff at San Pedro de Atacama Celestial Explorations observatory for their excellent support.

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