GRB 260429A

SVOM/GRM team: Zheng-Hang Yu, Chen-Wei Wang, Chao Zheng, Yue Huang, Shi-Jie Zheng, Shao-Lin Xiong, Shuang-Nan Zhang (IHEP)

SVOM/ECLAIRs team: Sebastien Guillot, Olivier GODET (IRAP), Stéphane Schanne (CEA)

Report on behalf of the SVOM team:

SVOM/GRM was triggered in-flight by a burst GRB 260429A (SVOM trigger reference: sb26042902) at 2026-04-29T23:24:02.100 UTC (T0), which is also detected by Fermi/GBM (Fermi GBM team, GCN #44425
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 ).

With the event-by-event data downloaded through the X-band ground station, the GRM light curve shows that this burst consists of a single peak a T90 of 0.12 +0.12/-0.03 s in the 15-5000 keV band. 

The SVOM/GRM light curve can be found here:
https://www.bursthub.cn//admin/static/svgrb260429A.png

In addition, the position of this burst, as determined by Fermi/GBM (RA= 283.9 deg, DEC= 18.1 deg, ERR=2.6 deg, GCN#44425
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 ), is located at about 99 degrees from the SVOM optical axis, which is outside the ECLAIRs field of view. Nevertheless, ECLAIRs detected the GRB through the shield above 50 keV. 

With this localization, the time-averaged spectrum from T0-0.2 to T0+0.5 s is best fitted by a power law function with an exponential high-energy cutoff. The power law index is -1.19 +0.12/-0.14 and the cutoff energy, parameterized as Epeak, is 961 +672/-308 keV. The event fluence (10-1000 keV) in this time interval is (2.57 +/-0.16)E-06 erg/cm^2. 

The localization of GRB 260429A in the 'Amati' relation diagram is shown at: https://www.bursthub.cn//admin/static/svgrb260429A_amati.png

The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA, China), National Center for Space Studies (CNES, France) and the Chinese Academy of Sciences (CAS, China), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. GRM is developed by the Institute of High Energy Physics (IHEP) of CAS.

The SVOM point of contact for this burst is: Zheng-Hang Yu(IHEP)(zhyu@ihep.ac.cn)

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/"

V.Lipunov, E.Gorbovskoy, A.Kuznetsov, K.Zhirkov, I.Panchenko, N.Tiurina, P.Balanutsa, V.Topolev, D.Vlasenko, 
G.Antipov,  A.Sankovich, Yu.Tselik, Ya.Kechin, V.Senik, A.Chasovnikov, K.Labsina, I. Gorbunov (Lomonosov MSU),
O.Gress, N.Budnev (ISU),
C.Francile,  F. Podesta, R.Podesta, E. Gonzalez  (Observatorio Astronomico Felix Aguilar (OAFA),
A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory),
A.Sosnovskij (CrAO),
A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity),
D.Buckley (SAAO),
R.Rebolo (The Instituto de Astrofisica de Canarias),
L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez,
A.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory) 

MASTER-OAFA robotic telescope  [1]  located in Argentina (OAFA observatory of San Juan National University) started inspect of the Fermi GRB 260429A ( Fermi GBM team, GCN 44425
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 ) errorbox  24026 sec after notice time and 24067 sec after trigger time at 2026-04-30 06:05:09 UT, with upper limit up to  17.9 mag. The observations began at zenith distance = 65 deg. The sun  altitude  is -63.1 deg. 

The galactic latitude b =  7 deg., longitude l = 50 deg.


Real time updated cover map and OT discovered available here: 
https://master.sai.msu.ru/site/master2/observ.php?id=3281490

We obtain a following upper limits.  

Tmid-T0  |      Date Time      |          Site       |             Coord (J2000)          |Filt.| Expt. | Limit| Comment
_________|_____________________|_____________________|____________________________________|_____|_______|_______|________

   24098 | 2026-04-30 06:05:09 |         MASTER-OAFA | (18h 56m 08.86s , +18d 11m 55.3s) |   C |    60 | 17.6 |        
   24177 | 2026-04-30 06:06:28 |         MASTER-OAFA | (18h 56m 09.20s , +18d 12m 58.1s) |   C |    60 | 17.6 |        
   24255 | 2026-04-30 06:07:47 |         MASTER-OAFA | (18h 56m 14.55s , +18d 12m 00.2s) |   C |    60 | 17.5 |        
   24363 | 2026-04-30 06:09:04 |         MASTER-OAFA | (18h 56m 09.34s , +18d 11m 03.2s) |   C |   120 | 17.7 |        
   24498 | 2026-04-30 06:11:20 |         MASTER-OAFA | (18h 56m 15.66s , +18d 11m 31.9s) |   C |   120 | 17.8 |        
   24636 | 2026-04-30 06:13:37 |         MASTER-OAFA | (18h 56m 11.82s , +18d 12m 47.8s) |   C |   120 | 17.9 |        
Filter C is a clear (unfiltred) band. 


The observation and reduction will continue. 
The message may be cited.

[1] - V.M. Lipunov, V.G. Kornilov, E.S. Gorbovskoy, N.A. Tiurina & A.S.Kuznetsov, 2023,  Astronomical Robotic Networks and Operative Multichanel Astrophysics, Lomonosov MSU PRESS, 591pp.
http : // www.pereplet.ru/lipunov/625.html

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

At 23:24:02 UT on 29 Apr 2026, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 260429A (trigger 799197847.01658 / 260429975).

The on-ground calculated location, using the Fermi GBM trigger data, is RA = 283.9, Dec = 18.1 (J2000 degrees, equivalent to J2000 18h 55m, 18d 06'), with a statistical uncertainty of 2.6 degrees.

The angle from the Fermi LAT boresight is 76.0 degrees.

The skymap can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260429975/quicklook/glg_skymap_all_bn260429975.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/bn260429975/quicklook/glg_healpix_all_bn260429975.fit

The GBM light curve can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260429975/quicklook/glg_lc_medres34_bn260429975.gif