GCN Circular 22048
Subject
Swift trigger 781740: Further analysis from Swift and Fermi/GBM
Date
2017-10-24T19:34:05Z (7 years ago)
From
Amy Lien at GSFC <amy.y.lien@nasa.gov>
P.A. Evans (U. Leicester). A.Y. Lien (GSFC), K.L. Page (U. Leicester)
S.D. Barthelmy (GSFC), M.S. Briggs (UAH), E. Burns (GSFC),
A. Goldstein (USRA), C.M. Hui (MSFC), R. Hamburg (UAH),
J. Racusin (GSFC), P. Veres (UAH)
report on behalf of the Swift and Fermi-GBM teams:
We have conducted further analysis of the subthreshold Swift-BAT
trigger 781740 on M31 (Evans et al., GCN Circ. 22044). Using the full
dataset, the BAT mask-weighted light curve shows a weak and soft pulse
from ~T0 to ~T+1s, which is consistent with the spike seen in the raw
light curve. However, because of the marginal image significance
(6.1 sigma), it is also possible that this weak pulse arises from noise
fluctuation.
Although the original BAT trigger was temporally coincident with a
subthreshold Fermi-GBM transient detection, the high rate of such
detections due to Swift J0243.6+61 (Briggs et al., GCN Circ. 22047)
renders this coincidence not significant. However, given this possible
coincidence, the potential emission seen by BAT, the fact that the source
could be in nearby M31, and that the detection is of marginal significance,
it could be due to either an SGR, a sGRB, or noise.
No new sources are seen in the Swift-XRT data compared to historical
observations with Swift-XRT. Many sources (of the 47 detected in our new
XRT observations) are variable; two appear to be significantly above their
mean rate from the 1SXPS catalogue (Evans et al., 2014) but neither show
any sign of ongoing evolution.
Based on these analyses we believe that Swift trigger 781740 does not
correspond to an astrophysical source, and was simply a subthreshold noise event.