TITLE: GCN CIRCULAR NUMBER: 27356 SUBJECT: GRB 200307A: Updated localization from Fermi/GBM and Swift/BAT-GUANO DATE: 20/03/10 20:06:13 GMT FROM: Adam Goldstein at Fermi-GBM, USRA Adam Goldstein (USRA), Aaron Tohuvavohu (U Toronto), Jimmy DeLaunay (Penn State), and Jamie Kennea (Penn State) report on behalf of Fermi/GBM and Swift/BAT: The Fermi/GBM Flight-Position notice, distributed at T0+7 seconds, from the Fermi/GBM detected GRB 200307A (GCN. 27344) triggered the Swift Mission Operations Center operated Gamma-ray Urgent Archiver for Novel Opportunities (GUANO; Tohuvavohu et al. 2020, in prep). Upon trigger by the Fermi notice, GUANO sent a command to the Swift Burst Alert Telescope (BAT) to save 200 seconds of BAT event-mode data from [-50,+150] seconds around the time of GRB 200307A. All the requested event mode data was delivered to the ground. In a ground analysis of the event data, we detect GRB 200307A, determine the signal in BAT is of astrophysical origin, and can confidently rule out an origin from inside the contemporaneous BAT field-of-view. At T0, the Earth's position and size relative to Swift were: Earth RA: 30.0439 Earth Dec: 5.2549 Earth Radius: 67.139 deg The Earth, with respect to Swift, occulted over 60% of the Fermi/GBM localization region and can be ruled out for the GRB localization. Fermi GBM provides an updated map by removing the Earth-occulted regions and can be found at the following links: Skymap image: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2020/bn200307894/quicklook/glg_skymap_all_bn200307894.png HEALPix: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2020/bn200307894/quicklook/glg_healpix_all_bn200307894.fit The 50% confidence region has decreased from 885 square degrees to 290 square degrees and the 90% confidence region has decreased from 2784 square degrees to 978 square degrees.