GRB 260804A

T. Tamura (Kanagawa U), A. Yoshida, T. Sakamoto, S. Sugita, 
Y. Kawakubo (AGU), K. Yamaoka (Nagoya U), S. Nakahira (JAXA), Y. Asaoka (ICRR), 
S. Torii, Y. Akaike, K. Kobayashi (Waseda U), Y. Shimizu (Kanagawa U),
N. Cannady (GSFC), M. L. Cherry (LSU), S. Ricciarini (U of Florence),
P. S. Marrocchesi (U of Siena),
and the CALET collaboration:

The CALET Gamma-ray Burst Monitor (CGBM) detected GRB 260804A
at 03:07:25.16 UTC on 4 August 2026 (trigger #1469848033;
https://cgbm.calet.jp/cgbm_trigger/flight/1469848033/index.html).

No real-time CGBM GCN notice was distributed about this trigger because 
the real-time communication from the ISS was off (loss of signal).

This event was also reported by Fermi GBM (Fermi GBM team, GCN #45281
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 ;
and Glowbug (Cheung et al., GCN #45288
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 ).

The burst signal was seen by all CGBM detectors.

The burst light curve shows a multi-peaked structure starting
at T-18.7 s, peaking at T+2.4 s, and ending at T+3.3 s.
The T90 and T50 durations measured with the SGM data are 20.3 +/- 1.0 s
and 8.5 +/- 1.1 s in the 40-1000 keV band, respectively.

The ground-processed light curve is available at:
https://cgbm.calet.jp/cgbm_trigger/ground/1469848033/

The CALET data used in this analysis were provided by
the Waseda CALET Operation Center located at Waseda University.

C.C. Cheung, R. Woolf, M. Kerr, J.E. Grove (NRL), A. Goldstein (USRA), C.A. Wilson-Hodge, D. Kocevski (MSFC), and M.S. Briggs (UAH) report:

The Glowbug gamma-ray telescope [1,2,3], operating on the International Space Station, reports the detection of GRB 260804A, which was also detected by Fermi/GBM (GCN 45281
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 ). 

Using an adaptive window with a resolution of 32-ms, the burst onset is determined to be 2026-08-04 03:07:09.160 with a duration of 19.7 s and a total significance of about 23.6 sigma.  The light curve comprises a faint peak at ~T0, multi-peaked emission from ~T0+11s to +16s, and the brightest peak at ~T0+18s.  Note that data from ~T0+8s to +12s suffered from deadtime in various detectors.
 
The analysis results presented here are preliminary and use a response function that lacks a detailed characterization of the surrounding passive structure of the ISS. Glowbug is a NASA-funded technology demonstrator for sensitive, low-cost gamma-ray transient telescopes developed, built, and operated by the U.S. Naval Research Laboratory (NRL) with support from the University of Alabama in Huntsville, USRA, and NASA MSFC.  It was launched on 2023 March 15 aboard the Department of Defense Space Test Program’s STP-H9 to the ISS and operated until 2024 April when it was put in safe storage on orbit. Glowbug was removed from storage and resumed operation on 2025 September 12. 

[1] Grove, J.E. et al. 2020, Proc. Yamada Conf. LXXI, arXiv:2009.11959
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[2] Woolf, R.S. et al. 2022, Proc. SPIE, 12181, id. 121811O 
[3] Woolf, R.S. et al. 2024, Proc. SPIE, 13151, id. 1315108 

Distribution Statement A: Approved for public release.  Distribution is unlimited.

The Fermi GBM team reports the detection of a likely LONG GRB

At 03:07:09 UT on 4 Aug 2026, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 260804A (trigger 807505634.188604 / 260804130).

The on-ground calculated location, using the Fermi GBM trigger data, is RA = 93.8, Dec = 18.0 (J2000 degrees, equivalent to J2000 06h 15m, 18d 00'), with a statistical uncertainty of 3.6 degrees.

The angle from the Fermi LAT boresight is 74.0 degrees.

The skymap can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260804130/quicklook/glg_skymap_all_bn260804130.png

The HEALPix FITS file, including the estimated localization systematic, can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260804130/quicklook/glg_healpix_all_bn260804130.fit

The GBM light curve can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260804130/quicklook/glg_lc_medres34_bn260804130.gif