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 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).