[vsnet-grb-info 27866] GRB 210506A: Swift/BAT-GUANO position retraction

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  29969
SUBJECT: GRB 210506A: Swift/BAT-GUANO position retraction
DATE:    21/05/08 23:04:45 GMT
FROM:    Aaron Tohuvavohu at U Toronto  <aaron.tohu at gmail.com>

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.