TITLE: GCN CIRCULAR NUMBER: 30206 SUBJECT: GRB 210610B: Zwicky Transient Facility afterglow detection DATE: 21/06/11 18:11:12 GMT FROM: Daniel Perley at Liverpool JMU D. A. Perley (LJMU), Y. Yao (Caltech), A. Y. Q. Ho (UC Berkeley), M. Bulla (Stockholm/OKC), I. Andreoni (Caltech), M. Coughlin (U. Minnesota), and E. Kool (Stockholm/OKC) report: The Zwicky Transient Facility (ZTF; ATel #11266) observed the location of GRB 210610B (Page et al., GCN 30170) during the night of 2021-06-11 UT as part of the regular operations of the ZTF high-cadence partnership survey. Four separate observations of the field (ZTF field ID 533) were obtained between 2021-06-11 05:34:44 and 2021-06-11 08:38:29, two each in g-band and r-band. The associated optical transient (e.g., Page et al., GCN 30170; Kumar et al., GCN 30174) was automatically identified by the ZTF image subtraction pipeline and assigned the identifier ZTF21abfmpwn. The source was independently flagged as a fast transient candidate by both the ZTF fast-transient filter pipeline developed by A. Ho and Y. Yao (Perley et al. 2021, arXiv:2103.01968) and by the ZTFReST pipeline (Andreoni et al. 2021, arXiv:2104.06352), on the basis of its rapid evolution over the course of the night and coincidence with a galaxy in Pan-STARRS and Legacy Survey reference imaging. We provide the following photometry: MJD t_GRB(d) filter magnitude 59376.2324 0.4050 g 18.49 +/- 0.10 59376.2751 0.4477 r 18.29 +/- 0.07 59376.3206 0.4932 r 18.44 +/- 0.07 59376.3600 0.5326 g 18.81 +/- 0.07 Photometry is as provided by the ZTF alert packets and is reference-subtracted. Magnitudes are AB and are not corrected for Galactic extinction. ZTF is supported by the National Science Foundation under Grant No. AST-2034437 and a collaboration including Caltech, IPAC, the Weizmann Institute for Science, the Oskar Klein Center at Stockholm University, the University of Maryland, Deutsches Elektronen-Synchrotron and Humboldt University, the TANGO Consortium of Taiwan, the University of Wisconsin at Milwaukee, Trinity College Dublin, Lawrence Livermore National Laboratories, and IN2P3, France. Operations are conducted by COO, IPAC, and UW. DisclaimerNone