TITLE:   GCN CIRCULAR
NUMBER:  22047
SUBJECT: Numerous Fermi-GBM Subthreshold Notices are due to Swift J0243.6+6
DATE:    17/10/24 00:59:56 GMT
FROM:    Michael S. Briggs at UAH and MSFC  <michael.briggs@nasa.gov>

M. S. Briggs (UAH), P. Jenke (UAH) and C. Wilson-Hodge (NASA/MSFC)
report: The Fermi Gamma-ray Burst Monitor subthreshold search for
burst-like transients
(https://gcn.gsfc.nasa.gov/admin/fermi_gbm_subthreshold_announce.txt)
is detecting the Be X-ray binary Swift J0243.6+61.�� This source was
first discovered after a Swift/BAT trigger (Cenko et al., GCN #21960)
on 2017 October 3. The GBM subthreshold search is likely detecting
high-order harmonics of this pulsar, which appear as "bursts" with a
duration of 2.0 or 2.7 s in the energy range 30 to 500 keV.�� The
first detections were on 2017 Oct 18.�� The rate increased on
2017 Oct 22, when the GBM subthreshold search generated nineteen GCN
Fermi-GBM Subthreshold Notices due to this source
(https://gcn.gsfc.nasa.gov/fermi_gbm_subthresh_archive.html). The
high rate of Subthreshold Notices continues as the pulsed flux
approaches 9 keV cm^-2 s^-1 (12-50 keV) in the GBM pulsar monitor
(https://gammaray.nsstc.nasa.gov/gbm/science/pulsars/lightcurves/swiftj0243.html) 

and the total flux in the Swift/Bat transient monitor
(https://swift.gsfc.nasa.gov/results/transients/weak/SwiftJ0243.6p6124/)
exceeds 2800 mCrab (15-50 keV).