[vsnet-grb-info 19960] Numerous Fermi-GBM Subthreshold Notices are due to Swift J0243.6+6

[GCN Circulars]

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 at 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).