TITLE: GCN CIRCULAR NUMBER: 39587 SUBJECT: EP250304a: Swift/UVOT detection of an optical counterpart with a rising light curve DATE: 25/03/04 12:18:09 GMT FROM: Sam Shilling at Lancaster University shilling.sam@gmail.com

S. P. R. Shilling (Lancaster U.) reports on behalf of the Swift UVOT team:

Swift/UVOT observed the field of EP250304a (Chen et al., GCN 39580) for a total of 2 ks in the U-band starting at 02:48:13 UT, 1.25 hours after the detection by Einstein Probe WXT.

An optical source consistent with the EP-FXT position (Chen et al., GCN 39580) is detected and appears to be rising in brightness. The preliminary detection magnitudes reported below are calculated using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373). The magnitude at the start of the observation is 19.29 +/- 0.09 (1.25 hours post trigger) and rises to 18.91 +/- 0.15 (~2.75 hours post trigger).

We note that an X-ray source at this position was independently observed by the Swift/XRT (Page et al., GCN 39584). We also note that, in addition to the Swift/UVOT detection, an optical counterpart at this position has been detected by the TRT (Liu et al., GCN 39583) but has not been detected by MASTER (Lipunov et al., GCN 39581). Finally, that the source has a spectroscopic redshift, observed with the VLT/X-shooter, of z=0.2 (Saccardi et al., GCN 39585).

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