[vsnet-grb-info 25045] LIGO/Virgo S200219ac: Fermi GBM transient event not confidently associated with GW signal

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  27144
SUBJECT: LIGO/Virgo S200219ac: Fermi GBM transient event not confidently associated with GW signal
DATE:    20/02/19 21:57:24 GMT
FROM:    Rachel Hamburg at UAH  <rkh0007 at uah.edu>

R. Hamburg (UAH) reports on behalf of the Fermi-GBM Team:

For S200219ac and using the initial BAYESTAR skymap, Fermi-GBM was
observing 46.6% of the localization probability at event time; the southern
lobe was Earth-occulted for GBM.

There was a Fermi-GBM onboard trigger, GRB 200219B (GCN 27127),
approximately 10 min after the LIGO/Virgo detection of the GW trigger (GCN
27130). However due to the temporal offset and spatial inconsistency, we do
not consider these two events associated. An automated, blind search for
short gamma-ray bursts below the onboard triggering threshold in Fermi-GBM
also identified no counterpart candidates +/- 1 hour around the GW event
time.

The GBM targeted search, the most sensitive, coherent search for GRB-like
signals, was run from +/-30 s around merger time for S200219ac. The
targeted search identified a transient at approximately T0+11 s that is
consistent with a short GRB origin and spatially consistent with the
initial S200219ac localization map. The SNR of this event is 5.6, and the
False Alarm Rate for signals of this significance in GBM data is 3.0E-4 Hz.
However, due to the weak nature of the event, the rate of events of similar
significance in GBM, and the presence of multiple galactic transient
signals around the time of interest, we are not confident in an association.

For the interested observer, the GBM localization HEALPix file can be found
on the Gravitational Wave Treasure Map (Wyatt et al. 2020,
arXiv:2001.00588) at:
http://treasuremap.space/static/S200219ac_GBM_Event1_healpix.fit.
The skymap image can be found at:
http://treasuremap.space/static/S200219ac_GBM_Event1_skymap.png.