GCN Circular 29969
Aaron Tohuvavohu (U Toronto), James DeLaunay (PSU), Jamie A. Kennea (PSU) report: As stated in the detection circular (GCN 29947) the candidate position reported was uncertain, with a near-threshold significance and an implied spectral index outside of typical astrophysical priors for GRB prompt emission. Without an independent spectral measurement of this burst, the BAT candidate image-domain arcmin localization cannot be ruled out via BAT analysis alone. However, the IPN localization (GCN 29968; which utilizes the BAT-GUANO rates-domain detection) strongly suggests that the GRB originated from outside the BAT FoV. An origin for the GRB counts from along this line of sight implies a much more reasonable (though still hard) emission spectrum. The BAT-GUANO rates-domain detection of this GRB remains robust and we are confident that the counts in the detector originate from GRB 210506A. The BAT maximum likelihood analysis (DeLaunay et al. 2021, in prep) independently shows a slight preference for an out of FoV origin. We did not report the test statistic results for this source from the maximum likelihood analysis in the detection circular due to delayed run times on the cluster. We do not identify any detector glitches, noise sources, or cosmic ray induced showers that can account for the image source. We conclude that the SNR 6.8 image source reported is likely to be rare high-significance spurious image noise caused by an unfortunate spatial correlation of true GRB counts in the detector plane. The False Alarm Probability for a BAT source at SNR 6.8 in a background subtracted image from ground analysis is low (~1-5%) and we stress that other GRBs with lower image significance GUANO localizations have been confirmed by XRT afterglow discovery.