GRB 260401A — All Circulars

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

SVOM/ECLAIRs team: Hui Yang (IRAP)
Report on behalf of the SVOM team:

SVOM/GRM was triggered in-flight by GRB 260401A (SVOM trigger reference: sb26040101) at 2026-04-01T10:53:33.600 UTC (T0), which is also detected by Fermi/GBM (Fermi GBM Team, GCN #44174), MAXI/GSC (Usuki et. al., GCN #44176), CALET/GBM (Cannady et al., GCN #44177),  NuSTAR (Waratkar et al., GCN #44178), and GECAM-B (Ren et al., GCN #44187).

With the event-by-event data downloaded through the X-band ground station, the GRM light curve shows that this burst consists of multiple pulses with a T90 of 24 +2/-2 s in the 15-5000 keV band.

The SVOM/GRM light curve can be found here:
https://www.bursthub.cn//admin/static/svgrb260401A.png

In addition, the position of this burst, as determined by Fermi/GBM (RA = 219.5, Dec = -17.4, GCN #44174), is located at about 74 degrees from the SVOM optical axis, which is outside the ECLAIRs field of view, but this burst still clearly detected by ECLAIRs through the shield. 

With this localization, the time-averaged spectrum from T0-10 to T0+10 s is best fitted by a power law function with an exponential high-energy cutoff. The power law index is -1.64 +0.06/-0.06 and the cutoff energy, parameterized as Epeak, is 321 +141/-77 keV. The event fluence (10-1000 keV) in this time interval is (1.44 +0.06/-0.07)E-05 erg/cm^2.

The 1s peak spectrum, measured from T0-0.1 to T0+0.9 s, if fitted by a power law function with an exponential high-energy cutoff, the power law index is -1.26 +0.07/-0.07 and the cutoff energy, parameterized as Epeak, is 391 +84/-59 keV. The flux (10-1000 keV) in this time interval is (3.28 +0.14/-0.15)E-06 erg/cm^2/s. 

The localization of GRB 260401A in the 'Amati' relation diagram is shown at: 
https://www.bursthub.cn//admin/static/svgrb260401A_amati.png
The localization of GRB 260401A in the 'Yonetoku' relation diagram is shown at: 
https://www.bursthub.cn//admin/static/svgrb260401A_yonetoku.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: Yang-Zhao Ren (IHEP)(renyz@ihep.ac.cn)

Yang-Zhao Ren, Chen-Wei Wang, Shao-Lin Xiong (IHEP) report on behalf of GECAM team:

GECAM-B was triggered on-ground by GRB 260401A, at 2026-04-01T10:53:29.200 UTC (denoted as T0), which is also detected by Fermi/GBM (Fermi GBM Team, GCN #44174), MAXI/GSC (Usuki et. al., GCN #44176), CALET/GBM (Cannady et al., GCN #44177), and NuSTAR (Waratkar et al., GCN #44178). 

According to the GECAM-B light curves in about 70-6000 keV, this burst mainly consists of multiple pulses with a duration (T90) of 28.8 +0.1/-0.2 s.

The GECAM-B light curve can be found here:
https://www.bursthub.cn//admin/static/gecambgrb260401A.png

The time-averaged spectrum from T0 to T0 + 15 s is best fitted by a power law function. The power law index is -1.51 +0.08/-0.07. The event fluence (10-1000 keV) in this time interval is (2.20 +0.11/-0.10) E-05 erg/cm^2.

Gravitational wave high-energy Electromagnetic Counterpart All-sky Monitor (GECAM) mission originally consists of two micro-satellites (GECAM-A and GECAM-B) launched in Dec. 2020. As the third member of GECAM constellation, GECAM-C was launched onboard SATech-01 experimental satellite in July 2022. GECAM mission is funded by the Chinese Academy of Sciences (CAS).

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 260401A ( Fermi GBM team, GCN 44174) errorbox  63483 sec after notice time and 63518 sec after trigger time at 2026-04-02 04:32:04 UT, with upper limit up to  19.0 mag. The observations began at zenith distance = 29 deg. The sun  altitude  is -63.7 deg. 

The galactic latitude b = 38 deg., longitude l = 337 deg.


Real time updated cover map and OT discovered available here: 
https://master.sai.msu.ru/site/master2/observ.php?id=3225119

We obtain a following upper limits.  

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

   33732 | 2026-04-01 20:15:08 |             MASTER- | (14h 42m 26.86s , -15d 41m 21.2s) |   C |    60 | 12.9 |        
   63549 | 2026-04-02 04:32:04 |         MASTER-OAFA | (14h 25m 14.66s , -15d 38m 17.7s) |   C |    60 | 18.4 |        
   63609 | 2026-04-02 04:32:04 |         MASTER-OAFA | (14h 25m 14.66s , -15d 38m 17.8s) |   C |   180 | 19.0 |  Coadd 
   63624 | 2026-04-02 04:33:19 |         MASTER-OAFA | (14h 25m 11.96s , -15d 36m 34.5s) |   C |    60 | 18.3 |        
   63700 | 2026-04-02 04:34:36 |         MASTER-OAFA | (14h 25m 12.06s , -15d 37m 47.2s) |   C |    60 | 18.4 |        
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

D. Fernandez (UAH), M. Dafčíková (MUNI) and C. Meegan (UAH) report on behalf of
the Fermi Gamma-ray Burst Monitor Team:

"At 10:53:26.00 UT on 01 April 2026, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 260401A (trigger 796733611/260401454),
which was also detected by MAXI (T. Usuki et al. 2026, GCN 44176).
The Fermi GBM on-ground location (GCN 44174) is consistent with the MAXI position.

The angle from the Fermi LAT boresight is 60 degrees.

The GBM light curve consists of a bright emission episode with multiple peaks with a duration (T90)
of about 27 s (50-300 keV). The time-averaged spectrum
from T0-0.8 to T0+32.1 s is best fit by
a power law function with an exponential high-energy cutoff.
The power law index is -1.78 +/- 0.01 and the cutoff energy,
parameterized as Epeak, is 239 +/- 1 keV.

The event fluence (10-1000 keV) in this time interval is
(1.80 +/- 0.02)E-05 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+7.5 s in the 10-1000 keV band is 21.1 +/- 0.4 ph/s/cm^2.

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/"

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 260401A 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 2026-04-01 10:53:26.0 UTC, shows a detection of GRB 260401A, consistent with Fermi/GBM (Fermi GBM team, GCN Circ. 44174, MAXI/GSC (Usuki et al., GCN Circ. 44176), CALET/GBM (Cannady et al., GCN Circ. 44177) and SVOM/GRM (GCN Notices).

The NuSTAR CsI shield data are recorded at 1 Hz. We detect a single peak lasting for ~10-s which is consistent with the bright part of the burst as seen in the GBM lightcurve. The peak count rate is ~3000-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 CZT detectors. 

The GBM localization (GCN Circular 44174) at RA = 219.5, Dec = -17.4 implies an offset from the NuSTAR boresight of 42-deg (i.e. from the side of the instrument) and an offset from the geocenter of 73-deg.

Lightcurves and analysis for this GRB can be found here:
https://nustarsoc.caltech.edu/NuSTAR_Public/grbs/reports/2026/260401A/     

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.

N. Cannady (GSFC), 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, T. Tamura (Kanagawa U),
M. L. Cherry (LSU), S. Ricciarini (U of Florence), P. S. Marrocchesi (U of Siena),
and the CALET collaboration:

The long GRB 260401A (Fermi GBM Final Real-time Localization: Fermi GBM team,
GCN Circ. 44174; BALROG localization: Preis et al., GCN Circ 44175; 
MAXI/GSC detection: Usuki et al., GCN Circ 44176) triggered the CALET Gamma-ray
Burst Monitor (CGBM) at 10:53:31.52 UTC on 1 April 2026
(https://cgbm.calet.jp/cgbm_trigger/flight/1459075956/index.html).
The burst signal was seen by all CGBM detectors.  

The burst light curve shows a single pulse that starts
at T+1.4 sec, peaks at T+2.2 sec, and ends at T+6.3 sec.
The T90 and T50 durations measured by the SGM data are 4.4 +/- 0.6 sec
and 2.0 +/- 0.3 sec (40-1000 keV), respectively.

The ground-processed light curve is available at

https://cgbm.calet.jp/cgbm_trigger/ground/1459075956

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

T. Usuki (Ehime U.), K. Fujiwara (Kyoto U.), M. Serino, Y. Kawakubo (AGU), 
M. Nakajima, H. Negoro, K. Takagi, H. Nishio (Nihon U.),
T. Mihara, T. Tamagawa, N. Kawai, M. Matsuoka (RIKEN),
T. Sakamoto, S. Sugita, H. Hiramatsu, Y. Kondo, D. Iijima, A. Yoshida (AGU),
Y. Tsuboi, H. Sugai, N. Nagashima, Y. Ishihara (Chuo U.),
M. Shidatsu, C. Kang, T. Nakamoto, M. Uenishi, S. Yatsuzuka (Ehime U.),
I. Takahashi, Y. Yatsu (Science Tokyo),
S. Nakahira, S. Ueno, H. Tomida, S. Ogawa, M. Kurihara (JAXA),
Y. Ueda, S. Kobayashi (Kyoto U.),
M. Yamauchi, M. Nishio, C. Hiraizumi (Miyazaki U.),
K. Yamaoka (Nagoya U.),
M. Sugizaki (Kanazawa U.),
W. Iwakiri (Chiba U.),
T. Kawamuro (Osaka U.),
S. Yamada (Tohoku U)

report on behalf of the MAXI team:
The MAXI/GSC nova alert system triggered a bright uncatalogued X-ray transient source at 10:53:19 UT on 2026 April 1.
Assuming that the source flux was constant over the transit,
we obtain the source position at
(R.A., Dec) = (219.942 deg, -17.349 deg) = (14 39 46, -17 20 56) (J2000)
with a statistical 90% C.L. elliptical error region
with long and short radii of 0.06 deg and 0.02 deg, respectively.
The roll angle of long axis from the north direction is 15.0 deg counterclockwise.
There is an additional systematic uncertainty of 0.1 deg (90% containment radius).
The X-ray flux averaged over the scan was 5700 +- 200 mCrab
(4.0-10.0keV, 1 sigma error).
Without assumptions on the source constancy, we obtain a rectangular error
box for the transient source with the following corners:
(R.A., Dec) = (220.028, -17.391) deg = (14 40 06, -17 23 27) (J2000)
(R.A., Dec) = (220.046, -17.270) deg = (14 40 11, -17 16 11) (J2000)
(R.A., Dec) = (219.512, -17.198) deg = (14 38 02, -17 11 52) (J2000)
(R.A., Dec) = (219.494, -17.319) deg = (14 37 58, -17 19 08) (J2000)
There was no significant excess flux in the previous transit at 09:20 UT
and in the next transit at 12:26 UT with an upper limit of 20 mCrab for each.
This event is spatially and temporally consistent with GRB 260401A, which was also detected by Fermi/GBM (GCN #44174, #44175).

T. Preis (University of Innsbruck) & J. Greiner (MPE Garching) report:

The public trigdat data of the Fermi Gamma-ray Burst Monitor (GBM) trigger
796733611 at 10:53:26 on 01 April 2026 were automatically fitted for spectrum
and sky location with BALROG (Burgess et al. 2018, MNRAS 476, 1427;
Berlato et al. 2019, ApJ 873, 60).

The best-fit position is:
RA(2000.0) = 220.7 deg
Decl.(2000.0) = -16.9 deg
The 1 sigma statistical error radius is 1.3 deg.
We estimate an additional systematic error of 1 deg.

Further details are available at:
https://grb.mpe.mpg.de/grb/GRB260401454/

The Healpix map can be downloaded from:
https://grb.mpe.mpg.de/grb/GRB260401454/healpix

The location parameters are available as JSON at:
https://grb.mpe.mpg.de/grb/GRB260401454/json

                        

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

At 10:53:26 UT on 1 Apr 2026, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 260401A (trigger 796733611.002678 / 260401454).

The on-ground calculated location, using the Fermi GBM trigger data, is RA = 219.5, Dec = -17.4 (J2000 degrees, equivalent to J2000 14h 37m, -17d 23'), with a statistical uncertainty of 1.6 degrees.

The angle from the Fermi LAT boresight is 60.0 degrees.

The skymap can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260401454/quicklook/glg_skymap_all_bn260401454.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/bn260401454/quicklook/glg_healpix_all_bn260401454.fit

The GBM light curve can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260401454/quicklook/glg_lc_medres34_bn260401454.gif