TITLE:   GCN CIRCULAR
NUMBER:  39657
SUBJECT: IceCube-250309A: Upper limits from Fermi-GBM Observations
DATE:    25/03/10 23:19:19 GMT
FROM:    Lorenzo Scotton at UAH <lscottongcn@outlook.com>

L. Scotton (UAH) reports on behalf of the Fermi-GBM Team:

For the IceCube high-energy neutrino candidate event IceCube-250309A
(GCN 39631), at the event time Fermi-GBM was observing the reported
neutrino location at:

RA: 211.07 (+0.31 -0.30 deg  90% PSF containment) J2000
Dec: -10.73 (+0.26 -0.30 deg 90% PSF containment) J2000

Fermi-GBM detected GRB 250309B (GCNs 39635, 39642) around 
146s after the time of the neutrino candidate. However, 
the ZTF and GROWTH collaborations reported the detection 
of a potential optical counterpart ZTF25aaitvjt | AT2025dws 
(R. Stein et al, GCN 39639), suggesting that GRB 250309B 
and IceCube-250309A are unrelated. The IPN triangulation 
(Kozyrev A. S. et al, GCN 39652) also suggests that 
GRB 250309B and IceCube-250309A are unrelated.    

An automated, blind search for short gamma-ray bursts below 
the onboard triggering threshold in Fermi-GBM also identified 
no counterpart candidates. The GBM targeted search, the most 
sensitive, coherent search for GRB-like signals, was run 
from +/-30 s around the neutrino candidate time. From this search, 
no significant signal was found related to IceCube-250309A.

Using the representative soft, normal, and hard GRB-like templates 
described in arXiv:2308.13666, we set the following 3 sigma flux 
upper limits over 10-1000 keV (in units of 10^-7 erg/s/cm^2):

Timescale   Soft     Normal   Hard
-------------------------------------------
0.128 s:    1.0      1.8      3.7
1.024 s:    0.35     0.56     1.3
8.192 s:    0.11     0.11     0.24

These results are preliminary.