GCN Circular 44430

L. Scotton (UAH) reports on behalf of the Fermi Gamma-ray Burst Monitor Team:

"At 23:24:02.02 UT on 29 April 2026, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 260429A (trigger 799197847/260429975).

The on-ground calculated location, using the Fermi GBM trigger data,
is RA = 283.92, Dec = 18.10 (J2000 degrees, equivalent to
J2000 18h 55m, +18d 6'), with a statistical uncertainty of 2.61 degrees.
(radius, 1-sigma containment, statistical only; there is additionally a
systematic error which we have characterized as a mixture of two Gaussians,
one with a radius of 1.8 degrees (52% contribution) and one with a radius
of 4.1 degrees (47% contribution) [A. Goldstein et al. 2020, ApJ, 895, 1]).

The angle from the Fermi LAT boresight is 76 degrees.

The GBM light curve consists of a single emission episode with a duration (T90)
of about 0.16 s (50-300 keV). The time-averaged spectrum
from T0-0.1 to T0+0.4 s is best fit by
a power law function with an exponential high-energy cutoff.
The power law index is -1 +/- 0.04 and the cutoff energy,
parameterized as Epeak, is 900 +/- 20 keV.

The event fluence (10-1000 keV) in this time interval is
(2.41 +/- 0.07)E-06 erg/cm^2. The 64-ms peak photon flux measured
starting from T0+0.0 s in the 10-1000 keV band is 69 +/- 2 ph/s/cm^2.

A Band function fits the spectrum equally well
with Epeak= 600 +/- 20 keV, alpha = -0.92 +/- 0.03 and beta = -2 +/- 0.09.


The spectral analysis results presented above are preliminary;
final results will be published in the GBM GRB Catalog:
https://heasarc.gsfc.nasa.gov/W3Browse/fermi/fermigbrst.html

For Fermi GBM data and info, please visit the official Fermi GBM Support Page:
https://fermi.gsfc.nasa.gov/ssc/data/access/gbm/"