TITLE:   GCN CIRCULAR
NUMBER:  23956
SUBJECT: Improved Fermi/GBM GRB localizations
DATE:    19/03/12 10:02:27 GMT
FROM:    Jochen Greiner at MPI  <jcg@mpe.mpg.de>

Improved Fermi/GBM GRB localizations

J. Greiner, B. Biltzinger, F. Kunzweiler, F. Berlato, J.M. Burgess 
(all MPE Garching) report:

Effective immediately, we will begin issuing GCN circulars for 
bright gamma-ray bursts triggered onboard the Gamma-Ray Burst Monitor
(GBM) on Fermi. These GCNs will include localizations generated with 
the BALROG scheme (Burgess et al. 2018, MNRAS 476, 1427). BALROG fits
the count data of the GBM detectors for spectrum and location on the 
sky simultaneously. This procedure allows us to get rid of the 
"3.7 deg core plus 10 deg tail" systematic error (Connaughton et al. 2015,
ApJS 216, 32). Our BALROG positions have a remaining systematic error
of 2 deg when arriving from directions coincident with the spacecrafts
solar panels, and 1 deg otherwise (Berlato et al. 2019, ApJ, in press;
arXiv:1902.01082).

We plan to send GBM localizations for the about 20% brightest GBM triggers
(with 2 sigma error radii < 10 deg), plus otherwise interesting GRBs. 
Information on the localizations for all GRBs, including fainter ones, 
will be logged at https://grb.mpe.mpg.de/ . The latency of these
localization fits will be about 30 min. after the onboard trigger.