Mi Zhang, Xing Gao (Xingming Observatory), report the optical counterpart of GRB 210104A ( 17.36 r ). The transient was discovered on several 200-s survey images ( limiting mag about 18.5 ) taken by Xing Gao at Xingming Observatory #3 ( N88 ) around Jan. 04.50807, 2021 UT using a unfiltered CCD ( + 0.6-m f/8 Ritchey-Chretien telescope ). Images can be found at: http://xjltp.china-vo.org/xm49mz.html
XM49MZ C2021 01 04.50807 06 55 05.30 +64 40 34.3 17.36 r N88
| JDHEL | rmag. |
| 2459219.008 | 17.329 |
| 2459219.011 | 17.422 |
| 2459219.013 | 17.471 |
| 2459219.016 | 17.553 |
| 2459219.018 | 17.65 |
| 2459219.021 | 17.681 |
| 2459219.024 | 17.724 |
| 2459219.026 | 17.404 |
| 2459219.029 | 17.337 |
| 2459219.031 | 17.398 |
| 2459219.034 | 17.512 |
| 2459219.037 | 17.824 |
| 2459219.039 | 17.832 |
| 2459219.042 | 18.012 |
| 2459219.044 | 18.025 |
| 2459219.047 | 18.151 |
| 2459219.05 | 18.034 |
| 2459219.052 | 18.053 |
| 2459219.055 | 18.091 |
| 2459219.057 | 18.145 |
| 2459219.061 | 18.183 |
| 2459219.065 | 18.158 |
| 2459219.069 | 18.255 |
| 2459219.072 | 18.363 |
| 2459219.076 | 18.38 |
| 2459219.08 | 18.372 |
| 2459219.084 | 18.331 |
| 2459219.087 | 18.389 |
| 2459219.091 | 18.369 |
| 2459219.103 | 18.535 |
| 2459219.106 | 18.607 |
| 2459219.11 | 18.674 |
| 2459219.114 | 18.657 |
| 2459219.118 | 18.671 |
| 2459219.121 | 18.755 |
| 2459219.125 | 18.749 |
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| Discovery image taken by N88 on Jan. 04.50807 UTC, 2021 | PS Image |