GRB 260519C

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

SVOM/ECLAIRs team: Feliu Lacreu (IAP)
Report on behalf of the SVOM team:

SVOM/GRM was triggered in-flight by GRB 260519C (SVOM trigger reference: sb26051904) at 2026-05-19T13:45:38.000 UTC (T0), which is also detected by Konus-Wind (Svinkin, GCN #44677
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 ), and NuSTAR (Waratkar et al., GCN #44672
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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 multiple pulses with a T90 of 26.5 +1.0/-1.0 s in the 15-5000 keV band.

The SVOM/GRM light curve can be found here:
https://www.bursthub.cn//admin/static/svgrb260519C.png

With this localization, the time-averaged spectrum from T0-5 to T0+30 s is best fitted by a power law function with an exponential high-energy cutoff. The power law index is -0.78 +0.08/-0.09 and the cutoff energy, parameterized as Epeak, is 135 +7/-6 keV. The event fluence (10-1000 keV) in this time interval is (2.07 +0.07/-0.06)E-05 erg/cm^2.

The 1s peak spectrum, measured from T0+12.5 to T0+13.5 s, if fitted by a power law function with an exponential high-energy cutoff, the power law index is -0.60 +0.19/-0.20 and the cutoff energy, parameterized as Epeak, is 93.47 +6.95/-5.93 keV. The flux (10-1000 keV) in this time interval is (1.10 +0.21/-0.06)E-06 erg/cm^2/s.

The localization of GRB 260519C in the 'Amati' relation diagram is shown at: 
https://www.bursthub.cn//admin/static/svgrb260519C_amati.png
The localization of GRB 260519C in the 'Yonetoku' relation diagram is shown at: 
https://www.bursthub.cn//admin/static/svgrb260519C_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)

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

The long-duration GRB 260519C
(NuSTAR-ACS detection: Waratkar et al., GCN 44672
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 )
triggered Konus-Wind at T0=49536.443 s UT (13:45:36.443).

The burst light curve shows a multipeaked structure
which starts at ~T0-3.5 s and has a total duration of ~27 s.
The emission is seen up to ~2 MeV.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB260519_T49536/

As observed by Konus-Wind, the burst
had the total fluence of 2.47(-0.15,+0.13)x10^-5 erg/cm2,
and the 64-ms peak flux, measured from T0+2.192 s,
of 2.45(-0.46,+0.46)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+24.832 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 = -0.78(-0.16,+0.18),
the high energy photon index beta = -3.17(-0.94,+0.28),
the peak energy Ep = 119(-7,+8) keV
(chi2 = 96/96 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 = -0.70(-0.11,+0.11)
and Ep = 209(-10,+11) keV (chi2 = 91/78 dof).
Fitting by a GRB (Band) model yields the same alpha and Ep,
and an upper limit on the high energy photon index: beta < -3.6
(chi2 = 91/77 dof).

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

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 260519C 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 2026-05-19T13:45:37.0 UTC, shows a detection of GRB 260519C, also detected by Konus-Wind (GCN Notices) & SVOM/GRM (GCN Notices).

The NuSTAR CsI shield data are recorded at 1 Hz. We detect a 25-s long burst consistent with the Konus-Wind lightcurve. The peak count rate is ~2000-cps with a baseline rate of ~1000-cps during this time period. We do not see evidence in the signal above 100 keV in the CZT detectors.

Lightcurves and analysis for this GRB can be found here:
https://nustarsoc.caltech.edu/NuSTAR_Public/grbs/reports/2026/260519C 

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.