TITLE:   GCN CIRCULAR
NUMBER:  45387
SUBJECT: EP260812a:  AZT-22 optical upper limit
DATE:    26/08/15 14:16:38 GMT
FROM:    Xiaofeng Wang at Tsinghua University <wang_xf@mail.tsinghua.edu.cn>

D. Mirzakulov (UBAI), J. Baxtiyorov (UBAI), X.-F. Wang (THU), S. Ehgamberdiev (UBAI), A. Shaymanov (Maidanak Observatory/UBAI), and H.-W. Peng (THU) report on behalf of the UBAI and THU joint teams:

We observed the field of the Einstein Probe fast X-ray transient EP260812a (Liu et al., GCN 45362) with the AZT-22 1.5~m telescope of the Maidanak Astronomical Observatory (MAO) starting on 2026-08-12 at 15:49:06 UTC, and obtained 4x300s exposure in the B and R filters using 4kx4k CCD SNUCAM camera (Im et al., 2010). 

We did not detect any optical counterpart at positions provided either by EP-WXT (Liu et al., GCN 45362) or by Corcoran et.al. (GCN 45376) in our stacked frames. We measure the following upper limits for our stacked images:

   DATE-OBS           t-T0(hrs)   Exptime(s)  Filter Upperlimit(mag)  Site/Telescope                                                           
2026-08-12T15:49:06	4.468       4x300	 B	23.30		MAO/AZT-22
2026-08-12T16:12:58	4.866       3x300	 R	23.05		MAO/AZT-22
2026-08-12T20:29:38	9.144       4x300	 B	23.92		MAO/AZT-22
2026-08-12T20:36:41	9.261       4x300	 R	23.72		MAO/AZT-22
2026-08-13T16:29:13	29.137      4x300	 R	23.28		MAO/AZT-22

All the data have been reduced by the data processing pipeline, STDWeb (Karpov 2025). We performed images subtraction on the stacked images using the Pan-STARRS templates. The photometry was calibrated using the Gaia DR3 Synphot catalog and was not corrected for extinction.

We thanks the MAO staff for supporting the observations.