GRB 260320A — All Circulars

V. Panteleeva, D. Frederiks, A. Lysenko, A. Ridnaia, D. Svinkin,
A. Tsvetkova,  M. Ulanov, and T. Cline,
on behalf of the Konus-Wind team, report:

The long-duration GRB 260320A
(NuSTAR detection: Waratkar et al., GCN 44064,
Insight-HXMT/HE detection: Luo et al., GCN 44077,
Glowbug gamma-ray detection: Cheung et al., GCN 44093,
GRID detection: Li et al., GCN 44109,
IPN triangulation: Kozyrev et al., GCN 44121)
triggered Konus-Wind at T0=70801.257 s UT (19:40:01.257).

The burst light curve shows a single pulse
which starts at ~T0-0.4 s and has a total duration of ~13.1 s.
The emission is seen up to ~1 MeV.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB260320_T70801/

As observed by Konus-Wind, the burst
had the total fluence of 1.88(-0.20,+0.19)x10^-5 erg/cm2,
and the 64-ms peak flux, measured from T0+0.896 s,
of 6.92(-0.78,+0.80)x10^-6 erg/cm2/s
(both in the 20 keV - 10 MeV energy range).

The time-averaged spectrum of the burst
(measured from T0 to T0+16.640 s)
is best fit in the 20 keV - 10 MeV range
by the GRB (Band) model with the following parameters:
the low-energy photon index alpha = -1.18(-0.10,+0.10),
the high energy photon index beta = -2.44(-0.43,+0.18),
the peak energy Ep = 267(-32,+45) keV
(chi2 = 86/91 dof).

The spectrum near the maximum count rate
(measured from T0+0.256 to T0+8.448 s)
is best fit in the 20 keV - 10 MeV range
by a power law with exponential cutoff model:
dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep)
with alpha = -1.33(-0.06,+0.06)
and Ep = 359(-36,+44) keV (chi2 = 82/77 dof).
Fitting by a GRB (Band) model yields the same alpha and Ep,
and an upper limit on the high energy photon index: beta < -2.29
(chi2 = 82/76 dof).

All the quoted errors are at the 68% confidence level.
All the quoted values are preliminary.

A.S. Kozyrev, D.V. Golovin, M.L. Litvak, I.G. Mitrofanov, and A.B. Sanin
on behalf of the HEND/Mars Odyssey team,

D. Svinkin, D. Frederiks, A. Lysenko, A. Ridnaia,
and T. Cline on behalf of the Konus-Wind team,

E. Burns on behalf of the IPN,

C. Wang (IHEP), S. Xiong (IHEP), S. Zhang (IHEP),
J. Wei (NAOC),  B. Cordier (CEA) on behalf of the SVOM-GRM team,

C. Wang (IHEP), S. Xiong (IHEP), S. Zhang (IHEP)
on behalf of the Insight-HXMT team,

and

W. Boynton, C. Fellows, K. Harshman, H. Enos, R. Starr,
and A.S. Gardner on on behalf of the GRS-Odyssey GRB team,
report:

The long-duration GRB 260320A
(NuSTAR-ACS detection: Waratkar et al., GCN 44064;
Insight-HXMT/HE detection: Luo et al., GCN 44077;
Glowbug detection: Cheung et al., GCN 44093;
GRID detection: Li et al., GCN 44109)
was detected by NuSTAR (ACS), SVOM (GRM), Insight-HXMT(HE),
Konus-Wind, Glowbug, GRID-11B, GRID-10B, and Mars-Odyssey (HEND)
at about 70804 s UT (19:40:04).

We have triangulated it to a preliminary, 3 sigma error box
whose coordinates are:
---------------------------------------------
  RA(2000), deg         Dec(2000), deg
---------------------------------------------
Center:
  316.117 (21h 04m 28s) +6.388 ( +6d 23' 17")
Corners:
  318.993 (21h 15m 58s) +11.535 (+11d 32' 06")
  313.686 (20h 54m 45s)  -0.693 (+00d 41' 36")
  313.683 (20h 54m 44s)  -1.165 (-01d 09' 54")
  318.941 (21h 15m 46s) +11.286 (+11d 17' 09")
---------------------------------------------
The error box area is 1.3 sq. deg, and its maximum
dimension is 13.8 deg (the minimum one is 5.7 arcmin).
The Sun distance was 42 deg.

This localization may be improved.

A triangulation map and HEALPix FITS file are posted at
http://www.ioffe.ru/LEA/GRBs/GRB260320_T70801/IPN/
The HEALPix triangulation map is the multi-order HEALPix in units of
probability density.

The Konus-Wind time history and spectrum will be given
in a forthcoming GCN Circular. 

Longhao Li, Yuchen He, Chenyu Wang, Zirui Yang report on behalf of the GRID Collaboration:

GRID-11B reports the detection of the long-duration GRB 260320A, which was also detected by Glowbug gamma-ray(C.C. Cheung et al.,GCN#44093), Insight-HXMT/HE (Xinghao Luo et al., GCN #44077) and NuSTAR (G. Waratkar et al., GCN #44064).

The event was triggered with GRID on  2026-03-20 at 19:40:04.5 UTC. The measured burst duration (T90) in the 30-2000 keV range is approximately 9.9 +/- 1.8 seconds from GRID-11B . 

At present, only the GRID light curve is available. No spectral analysis is reported at this stage. The GRID light curve of this event can be found at https://mirrors.tuna.tsinghua.edu.cn/GRID/data/GRID-GCN/GRB260320A/GRID_260320A_ltcv.pdf

GRID is a student-led project to monitor the transient gamma-ray sky with multiple detectors onboard different nanosatellites in the era of multi-messenger astronomy. For more information about GRID, please refer to the following references: https://doi.org/10.1007/s10686-019-09636-w and https://doi.org/10.1007/s10686-021-09819-4.

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, confirms the detection of GRB 260320A, which was detected by NuSTAR (GCN 44064), SVOM/GRM (sb26032003), Insight-HXMT/HE (GCN 44077), and Konus-Wind (Trig_Time 19:40:01.257).  The burst onset is at 2026-03-20 19:40:02, with a peak at ~T0+1.5s, and a burst duration of ~7s.  Note that data throughout the detection interval 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
[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.

Xing-Hao Luo, Chen-Wei Wang, Chao Zheng, Cheng-Kui Li and Shao-Lin Xiong (IHEP) report on behalf of the Insight-HXMT team:

At 2026-03-20T19:40:04.700 (T0), Insight-HXMT/HE detected the burst GRB 260320A, which is also detected by NuSTAR (G. Waratkar et.al., GCN #44064).
	
The Insight-HXMT/HE light curve mainly consists of a single pulse with a T90 of 4.4 +1.2/-0.8 s. The 1s peak rate, measured from T0-0.7 s, is 2511 cnts/sec. Insight-HXMT/HE detected a total of 5253 counts from this burst.

The HXMT/HE light curve can be found here:
https://www.bursthub.cn//admin/static/hxmtgrb260320A.png

All measurements above are made with the CsI detectors of Insight-HXMT/HE operating in the regular mode with the energy range of about 60-900 keV (deposited energy). Only gamma-rays with energy greater than about 200 keV can penetrate the spacecraft and leave signals in the CsI detectors installed inside of the telescope.

Insight-HXMT is the first Chinese space X-ray telescope, which was funded jointly by the China National Space Administration (CNSA) and the Chinese Academy of Sciences (CAS). More information about it could be found at: http://hxmten.ihep.ac.cn/

G. Waratkar (Caltech) and B. Grefenstette (Caltech) report on behalf of the NuSTAR Search for INteresting Gamma-ray Signals (SINGS) working group:

The NuSTAR SINGS working group reports the detection of prompt emission from the long-duration GRB 260320A in both the NuSTAR CsI anti-coincidence shields. Details of the search algorithm will be described in a future paper.

The NuSTAR SINGS algorithm, triggered at 2026-03-20 19:40:03.9 UTC, shows a detection of GRB 260320A consistent with SVOM/GRM (GCN Notices).

The NuSTAR CsI shield data are recorded at 1 Hz. We detect a single bright peak lasting for ~10-s. The peak count rate is ~2000-cps with a baseline rate of ~1000-cps during this time period. We do not see any evidence in the signal above 100 keV in the CdZnTe detectors. 

The SVOM/GRM localization (GCN Notices) at RA = 38.93, Dec = 16.64 implies an offset from the NuSTAR boresight of 92-deg (i.e. from the side of the instrument) and an offset from the geocenter of 14-deg.

Lightcurves and analysis for this GRB can be found here:
https://nustarsoc.caltech.edu/NuSTAR_Public/grbs/reports/2026/260320A     

Information on NuSTAR SINGS can be found here: 
https://nustarsoc.caltech.edu/NuSTAR_Public/grbs/     

NuSTAR is a NASA Small Explorer mission led by Caltech and managed by JPL for NASA's Science Mission Directorate in Washington.