GCN 44210: GRB 260330B: SVOM/ECLAIRs refined analysis
2026-04-03T19:30:32.225Z | rev 0
Authors: S. Guillot, M. Brunet, F. Triot, H. Yang, O. Godet, G. Lan (IRAP) report on behalf of the SVOM/ECLAIRs team
Using the event-by-event data downloaded through the X-band ground station, we report further analysis of SVOM/ECLAIRs observations of GRB 260330B (SVOM burst-id sb26033003 – GCN 44157, trigger time T0 = 2026-03-30T14:19:57 UTC), which was also detected by Fermi GBM as a sub-threshold event (GCN 44159).
The burst that triggered ECLAIRs onboard shows a multiple-peak lightcurve. The burst duration is T90 = 58.7 (+9.4)(-4.9) s s in the 4-120 keV energy band.
The time-averaged spectrum from T0-23.0 s to T0+43.04 s in the energy range 4-120 keV is best fitted by a cutoff powerlaw model with alpha = -1.21 (+0.18)(-0.170), with Epeak = 24.9 (+11.5)(-8.8) keV. With this model, the 4-120 keV fluence is 1.32 (0.02)(-0.32) e-6 erg/cm^2 and the 4-120 keV photon flux is 0.85 +0.01/-0.14 ph/cm^2/s.
All the quoted errors are at the 68% confidence level.
The Space-based multi-band astronomical Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA), French Space Agency (CNES), and the Chinese Academy of Sciences (CAS), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. ECLAIRs was developed jointly by CNES, CEA-IRFU, CNRS-IRAP, CNRS-APC.
The SVOM/ECLAIRs point of contact for this burst is: Sebastien Guillot (IRAP) (sebastien.guillot1 at utoulouse.fr)
GCN 44205: GRB 260330B: SVOM/VT optical candidate
2026-04-03T12:11:54.444Z | rev 0
H. L. Li, Y. N. Ma, C. Wu, Z. H. Yao, Y. L. Qiu, J. R. Xu, L. P. Xin, X. H. Han, J. Wang, Y. Xu, P. P. Zhang, W. J. Xie, Y. J. Xiao, H. B. Cai, L. Lan, J. S. Deng, J. Y. Wei (NAOC), J. Palmerio (CEA/Irfu) report on behalf of the SVOM/VT team.
After the initial automatic follow-up (Li et al., GCN 44168), SVOM/VT performed ToO observations of the field of GRB 260330B detected by SVOM/ECLAIRs (sb26033003, Lan et al., GCN 44157), SVOM/GRM (Lan et al., GCN 44157) and Fermi/GBM with sub-threshold detection (Trigg et al., GCN 44159). The observations started at 2026-03-31T07:44:21 UTC, approximately 17.44 hours post trigger in the VT_B (400-650 nm) and VT_R (650-1000 nm) channels simultaneously.
A catalogued optical candidate within the error box of the EP-FXT source #1 (Guillot et al., GCN 44203) was observed brightening in VT_R within 19 hours post trigger. It overlaps a faint source in the Legacy Survey, with magnitudes g=25.00, r=23.96, z=22.16. The position is at R.A., Dec. = 252.637034, 36.841008 degrees, equivalent to:
R.A. (J2000) = +16:50:32.89
Dec. (J2000) = +36:50:27.63
with an uncertainty of 0.5 arcsec.
The measurements in AB magnitude are as follows:
Mid time | Band | Exposure Time | Brightness
1.00 h VT_R 34*50 sec >22.5 mag (5 sigma upper limit)
4.94 h VT_R 57*100 sec 22.67+/-0.20 mag
19.04 h VT_R 56*70 sec 22.12+/-0.15 mag
Our photometry was not corrected for Galactic extinction.
Further follow-up observation is scheduled.
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. VT was jointly developed by Xi'an Institute of Optics and Precision Mechanics (XIOPM), CAS and National astronomical observatories (NAOC), CAS.
GCN 44203: GRB 260330B: EP-FXT follow-up and identification of candidate afterglow
2026-04-03T10:26:00.527Z | rev 0
S. Guillot, G. X. Lan (IRAP), D. Turpin, A. Saccardi (CEA/Irfu), report on behalf of the SVOM and EP collaborations:
EP-FXT performed two follow-up observations of the SVOM/ECLAIRs-detected burst GRB 260330B (SVOM/sb26033003; Lan et al., GCN 44157), also detected as a sub-threshold event by Fermi/GBM (Trigg et al., GCN 44159). The first epoch started on 2026-03-31 at 16:39:42 UT, approximately 26.3 hours after the SVOM trigger, with a total exposure time of 3174 s. The second exposure started on 2026-04-02 at 19:48:54 UT, i.e., 77.5 hours after the SVOM trigger, with a total exposure time of 3200 s.
Three uncataloged sources are detected within the ECLAIRs error circle in the first epoch:
Source 1: EPF_J165032.7+365028 at RA, Dec (J2000) = 252.6363, 36.8413 with Fx=(1.20+/-0.32)e-13 erg/sec/cm2 from the FXT-B telescope.
Source 2: EPF_J165010.3+364729 at RA, Dec (J2000) = 252.5430, 36.7914 with Fx=(2.40+/-0.43)e-13 erg/sec/cm2 from the FXT-B telescope.
Source 3: EPF_J164951.2+365709 at RA, Dec (J2000) = 252.4635 36.9525 with Fx=(1.96+/-0.41)e-13 erg/sec/cm2 from the FXT-B telescope.
All three sources detected by EP/FXT during the first visit are detected in the second epoch data set. However, in the second epoch EPF_J165032.7+365028 has faded by a factor ~4, down to a flux of (4.0 +/- 2.3)e-14 erg/sec/cm2, while the flux of the other sources remained constant within their uncertainties.
The fading behavior of this X-ray source confirms it as the afterglow of GRB 260330B, localized at RA, Dec (J2000) = 252.6363, 36.8413 with an error radius of 10 arcsec.
All fluxes are calculated in the 0.5-10 keV range with 1-sigma uncertainties.
Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics).
GCN 44168: GRB 260330B: SVOM/VT optical upper limits
2026-03-31T15:25:54.249Z | rev 0
H. L. Li, Y. N. Ma, C. Wu, Z. H. Yao, Y. L. Qiu, J. R. Xu, L. P. Xin, X. H. Han, J. Wang, Y. Xu, P. P. Zhang, W. J. Xie, Y. J. Xiao, H. B. Cai, L. Lan, J. S. Deng, J. Y. Wei (NAOC), J. Palmerio (CEA/Irfu), G. X. Lan, S. Guillot (IRAP) report on behalf of the SVOM/VT team.
SVOM/VT performed observations with automatic slew to the field of GRB 260330B detected by SVOM/ECLAIRs and SVOM/GRM (sb26033003, Lan et al., GCN 44157). It was also a sub-threshold detection by Fermi/GBM (Trigg et al., GCN 44159). The observation started at 2026-03-30T15:06:18 UTC, i.e., about 46 minutes post trigger in the VT_B (400-650 nm) and VT_R (650-1000 nm) channels simultaneously.
No credible sources were detected within the error box of ECLAIRs (Lan et al., GCN 44157), compared to the Legacy Survey. The 5 sigma upper limits in AB magnitude are derived as follows:
Mid time | Band | Exposure Time | 5 sigma upper limit
1.03 hour VT_B 38*50 sec > 22.7 mag
1.00 hour VT_R 34*50 sec > 22.5 mag
Our photometry was in AB magnitude and was not corrected for Galactic extinction.
This result is consistent with the nondetection report by LCO (Turpin et al., GCN 44158). More detailed analysis is ongoing.
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. VT was jointly developed by Xi'an Institute of Optics and Precision Mechanics (XIOPM), CAS and National astronomical observatories (NAOC), CAS.
GCN 44159: Fermi GBM Sub-Threshold Detection of GRB 260330B
2026-03-30T19:48:13.025Z | rev 0
A.C. Trigg (NPP ORAU, NASA MSFC) reports on behalf of the Fermi-GBM Team:
SVOM/ECLAIRs detected GRB 260330B on 2026-03-30 at 14:19:57 UTC (GCN 44157). There was no Fermi-GBM onboard trigger around this event time. An automated, blind search for gamma-ray bursts below the onboard triggering threshold in Fermi-GBM identified no candidates.
The GBM Targeted Search [1], the most sensitive coherent search for GRB-like signals in GBM, identified a transient approximately 16 s after the ECLAIRs best image SNR time of 2026-03-30T14:19:31. The Targeted Search candidate was found most significantly on the 32 s timescale using the "soft" spectral template (i.e., Band function with Epeak = 70 keV, alpha = -1.9, beta = -3.7) and has a false alarm rate of 1.0-04 Hz. The Fermi-MET of this transient is 796573176.806 s. The Targeted Search localization is spatially consistent with the ECLAIRs location.
[1] Goldstein et al. 2019 arXiv:1903.12597
GCN 44158: GRB 260330B: LCO optical upper limits
2026-03-30T18:58:32.131Z | rev 0
D. Turpin (CEA/Irfu), A. Saccardi (CEA/Irfu), G. X. Lan (IRAP), S. Guillot (IRAP), report on behalf of the SVOM mission team:
We observed the field of GRB 260330B detected by SVOM (Lan et al., GCN 44157) with the LCO 1m telescope at Siding Spring Observatory equipped with the Sinistro instrument.
Our observation started on 2026-03-30 at 17:05:41 UT (about 2.76 hr after the trigger) and we obtained 3x200 s exposures in the SDSS r and 3x200 s exposures in the Pan-STARRS z filters.
In the stacked image, we do not detect any new source within the SVOM/ECLAIRs error box. We measure the following upper limit calibrated against the Pan-STARRS DR1 catalog, and not corrected for Galactic extinction:
r > 20.4 AB (5-sigma, mid-time 2.85 hr after the trigger);
z > 19.2 AB (5-sigma, mid-time 2.86 hr after the trigger).
This project is funded by the SVOM collaboration.
GCN 44157: GRB 260330B: SVOM detection of a burst
2026-03-30T14:50:52.860Z | rev 0
G. X. Lan, S. Guillot, H. Yang, O. Godet (IRAP), report on behalf of the SVOM mission team:
At 2026-03-30T14:19:57 UTC (T0), SVOM/ECLAIRs triggered and located the gamma-ray burst GRB 260330B (SVOM burst-id sb26033003).
The following trigger information was received on the ground with low latency by the SVOM VHF Alert Network.
The burst was detected both by the Count-Rate Trigger (CRT) and the Image Trigger (IMT), which produced a sequence of 9 alerts. IMT provided the alert with the best signal-to-noise-ratio in the image (SNR) of 12.13 in the [5-20] keV energy band over a time window of 40.96 seconds starting at 2026-03-30T14:19:31.
The localization of the best alert is R.A., Dec. 252.555, 36.862 degrees:
R.A. (J2000) = 16h50m13s
Dec. (J2000) = 36d51m44s
with a 90% confidence level (C.L.) radius of 6.66 arcmin (including systematic error of 2 arcmin added in quadrature).
The SVOM/ECLAIRs light curve showed multiple broad peak structures with a T90 duration of about 20.844 (-3.025 / +7.55) s.
This burst was also detected by SVOM/GRM with a significance of 9.10.
SVOM slewed to the burst.
No X-ray observation could be performed by SVOM/MXT for the time being.
No optical observation could be performed by SVOM/VT for the time being.
The Space-based multi-band astronomical Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA), French Space Agency (CNES), and the Chinese Academy of Sciences (CAS), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. SVOM/ECLAIRs was developed jointly by CNES, CEA-IRFU, CNRS-IRAP, CNRS-APC. SVOM/GRM was developed by the Institute of High Energy Physics (IHEP) of CAS. SVOM/MXT was developed jointly by CNES, CEA-IRFU, CNRS-IJCLab, University of Leicester, MPE.
The Burst Advocate (BA) on shift for this alert is Guangxuan Lan: glan@irap.omp.eu.
Please contact the BA by email if you require additional information.