TITLE: GCN CIRCULAR NUMBER: 41470 SUBJECT: EP250821a: Rapidly fading optical source detected in Las Cumbres followup DATE: 25/08/21 16:53:21 GMT FROM: Wenxiong Li at NAOC liwenxiong1992@gmail.com

Wenxiong Li (NAOC), Iair Arcavi (TAU), Ido Keinan (TAU), David Sand (U of Arizona) We observed the position of EP250821a (Hu, Liang and Jin, GCN 41459; Liang, Hu and Jin, GCN 41467) with a Las Cumbres 1m telescope at Siding Spring Observatory 1.25 hours after the Einstein Probe WXT trigger and again 6.5 hours after the trigger. We took 2x300s exposures in each visit in the broad optical w band. We find a rapidly fading source at RA=289.8853 Dec=-43.3813 (slightly offset from the position of the Swift counterpart; Dichara et al. GCN 41469) within the EP/FXT error circle and measure the following preliminary photometry calibrated to the r band: Epoch 1: MJD 60908.362 Mag 18.11 +- 0.03 Epoch 2: MJD 60908.535 Mag 19.32 +- 0.02 We find a faint red likely extended source at this position in Legacy Survey data (https://www.legacysurvey.org/viewer). Addition followup is encouraged.

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