GCN Circular 27356

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.