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GCN Circular 34748

Subject
Fermi GBM Statement for Trigger 716527670/230916144
Date
2023-09-20T18:05:31Z (10 months ago)
From
Oliver J Roberts at USRA/NASA <oliver.roberts@nasa.gov>
Via
Web form
O.J. Roberts (USRA/NASA-MSFC), S. Bala (USRA), C. Meegan (UAH)
report on behalf of the Fermi GBM Team:

"At 03:27:45.98 UT on 16 September 2023, the Fermi Gamma-ray Burst Monitor (GBM)
triggered on trigger 716527670/230916144 as a GRB with 64% confidence). This trigger 
was later followed up by GOTO (Vail et al., GCN 34730), identifying a candidate afterglow 
(AT 2023sva) and suggested it was the counterpart.

Careful inspection of the GBM trigger by the Fermi-GBM Team identified a weak 
C1-class solar flare occurring contemporaneously at the trigger time, which dominates 
the spectrum below 50 keV. There is approximately several hundred counts in the 
50-300 keV band over 12s, which the Team was able to localize. The on-ground calculated 
location using the Fermi GBM trigger data over this energy range is 
RA = 143.0, Dec = 70.5 (J2000 degrees, equivalent to J2000 9h 32m, +/- 70d 30'), with a 
statistical uncertainty of 8.0 degrees (radius, 1-sigma containment, statistical only; there 
is additionally a systematic error which we have characterized as a core-plus-tail model, 
with 90% of GRBs having a 3.7 deg error and a small tail suffering a larger than 10 deg 
systematic error. [Connaughton et al. 2015, ApJS, 216, 32] ). 

We note that this localization is 71 degrees from the Sun, and is quite far from the 
RA and Dec of the afterglow candidate reported by Vail et al. in GCN 34730. We therefore 
believe these two events to be unrelated and suggest that AT2023sva is an orphan afterglow. 
We also note that this weak burst is contaminated by the solar flare and that any meaningful 
data regarding the nature of this transient is likely unrecoverable.

The skymap can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2023/bn230916144/quicklook/glg_skymap_all_bn230916144.png

The HEALPix FITS file, including the estimated localization systematic, can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2023/bn230916144/quicklook/glg_healpix_all_bn230916144.fit

The GBM light curve can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2023/bn230916144/quicklook/glg_lc_medres34_bn230916144.gif"
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