EP241217a: 1.6m Mephisto multi-band optical observations

We observed the field of EP241217a reported by EP-WXT (Zhou et al., GCN 38586) with the 1.6m Multi-channel Photometric Survey Telescope (Mephisto) of Yunnan University located at Lijiang Observatory. Simultaneous multi-band uvgriz band observations were initiated on 2024/12/17 between 14:07:34 to 15:48:12 UT. Multiple frames were obtained with exposure times of 180s and 300s in different bands. In our r i z band images, there is clear detection of the optical counterpart candidate (Levan et al., GCN 38587; Izzo et al., GCN 38588; Fan et al., GCN 38592; Jin et al., GCN 38607; Mohan et al., GCN 38612; Zhu et al., GCN 38613; Bochenek et al., GCN 38615) but no detection in the stacked images of u v g bands. The field was observed again in uvgriz bands on 2024/12/18 (starting from 12:43:30 UT) and three frames of 300s in each band were taken. We found a marginal detection of the transient in the i band stacked image but not in other bands. Our preliminary results for the initial frames for 2024/12/17 and 2024/12/18 (upper limits indicate 3 sigma) are listed below: UT start band Exp Mag/LimMag(AB) 2024/12/17T14:07:34 z 180s 18.73 +/- 0.13 2024/12/17T14:11:28 r 300s 21.15 +/- 0.27 2024/12/17T15:05:43 i 300s 19.34 +/- 0.06 2024/12/17T15:24:11 g 300s*6 >23.15 2024/12/17T15:24:11 u 300s*6 >22.64 2024/12/17T14:50:44 v 300s*7, 180s*1 >22.88 2024/12/18T13:00:36 u 300s*3 >22.91 2024/12/18T13:00:36 g 300s*3 >22.91 2024/12/18T12:43:30 v 300s*3 >22.80 2024/12/18T12:43:30 r 300s*3 >22.65 2024/12/18T12:43:30 z 300s*3 >20.62 2024/12/18T13:00:36 i 300s*3 21.99 +/- 0.52 ------------------------------------------------------------------------- Mephisto (Multi-channel Photometric Survey Telescope) is a 1.6-m wide-field multi-channel telescope, the first of its type in the world, capable of imaging the same field of view in three optical bands simultaneously. It provides real-time, high-quality colors of stellar objects. The on-site telescope assemblage and commissioning were carried out in September 2022. The first light in all three channels was achieved on 2023 December 21. --------------------------------------------------------------------------