GRB 260618B
GCN Circular 45268
Subject
GRB 260618B: SVOM/VT continued optical observations indicated a steeper and redder decay
Date
2026-08-02T05:29:56Z (a month ago)
From
Huali Li at at NAOC, SVOM <lhl@nao.cas.cn>
Via
Web form
H. L. Li, Y. N. Ma, L. P. Xin, Y. L. Qiu, C. Wu, Z. H. Yao, J. R. Xu, X. H. Han, J. Wang, Y. Xu, P. P. Zhang, W. J. Xie, Y. J. Xiao, H. B. Cai, J. S. Deng, J. Y. Wei (NAOC), and J. T. Palmerio (CEA/Irfu) report on behalf of the SVOM/VT team.
SVOM/VT performed further observations of GRB 260618B detected by Fermi/GBM (Fermi GBM team et al., GCN 44979; Veres et al., GCN 45010), Fermi/LAT (Gupta et al., GCN 44983), and Konus-Wind (Ridnaia et al., GCN 45038). The observation started at 2026-07-29T02:00:37 UTC, about 40.38 d post trigger in the VT_B (400-650 nm) and VT_R (650-1000 nm) channels simultaneously.
The optical candidate (Li et al., GCN 45001, GCN 45151; Shi et al., GCN 45014; Ma et al., GCN 45173) showed a steeper decay with a slope of -0.8 in both channels starting from about 21.6 d post trigger, and its color became redder. The following measurements are in AB magnitudes and are not corrected for Galactic extinction:
Mid time (d) | Exposure Time (s) | Band | Brightness (mag)
40.43 | 60*70 | VT_B | 22.9 +/- 0.2
40.43 | 58*70 | VT_R | 22.2 +/- 0.2
The nature of the source remains unclear. More follow-up observations are encouraged.
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 Circular 45173
Subject
GRB 260618B: EP-FXT follow-up observation upper limit
Date
2026-07-16T11:58:22Z (2 months ago)
From
Yinuo Ma <mayn@bao.ac.cn>
Via
Web form
Y. N. Ma (NAOC), A. Li (BNU), Y. L. Qiu, H. L. Li, L. P. Xin (NAOC), Y. L. Hua (PMO), M. J. Liu (NAOC), X. H. Tang (Tsinghua Univ.), W. D. Zhang (NAOC) on behalf of SVOM and EP missions:
EP-FXT performed a follow-up observation of GRB 260618B detected by Fermi (Fermi GBM team, GCN 44979; Gupta et al., GCN 44983; Veres et al., GCN 45010) and Konus-Wind (Ridnaia et al., GCN 45038). The follow-up observation started at 2026-07-15T10:43:17 (UTC), about 26.7 days after the Fermi trigger, with a total exposure time of around 2.9 ks.
No significant X-ray source was detected at the position of the reported optical candidate (Li et al., GCN 45001; Shi et al., GCN 45014; Li et al., GCN 45151). The upper limit for the 0.5-10 keV band is about 5.0e-14 ergs/cm^2/s (90% C.L.).
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 Circular 45151
Subject
GRB 260618B: SVOM/VT optical observations indicate shallow decay and reddening
Date
2026-07-14T02:57:05Z (2 months ago)
From
Huali Li at at NAOC, SVOM <lhl@nao.cas.cn>
Via
Web form
H. L. Li, L. P. Xin, Y. L. Qiu, C. Wu, Y. N. Ma, Z. H. Yao, J. R. Xu, X. H. Han, J. Wang, Y. Xu, P. P. Zhang, W. J. Xie, Y. J. Xiao, H. B. Cai, J. S. Deng, J. Y. Wei (NAOC) and J. T. Palmerio (CEA/Irfu) report on behalf of the SVOM/VT team.
SVOM/VT performed ToO observation for GRB 260618B detected by Fermi-LAT (Gupta et al., GCN 44983), Fermi-GBM (Fermi GBM team, GCN 44979; Veres et al., GCN 45010) and Konus-Wind (Ridnaia et al., GCN 45038). The latest observation started at 2026-07-10T09:59:57 UTC, about 21.71 d post trigger in the VT_B (400-650 nm) and VT_R (650-1000 nm) channels simultaneously.
The optical candidate (Li et al., GCN 45001; Shi et al., GCN 45014) became fainter in both channels and redder relative to earlier observations (Li et al., GCN 45001). The following measurements are in the AB magnitude and are not corrected for Galactic extinction:
Mid time | Band | Exposure Time | Brightness
21.73 d VT_B 37*70 s 22.3 +/- 0.2 mag
21.73 d VT_R 34*70 s 21.6 +/- 0.1 mag
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 Circular 45038
Subject
Konus-Wind detection of GRB 260618B
Date
2026-06-24T17:14:43Z (3 months ago)
From
Anna Ridnaia at Ioffe Institute <ridnaia@mail.ioffe.ru>
Via
Web form
A. Ridnaia, D. Frederiks, A. Lysenko, D. Svinkin,
A. Tsvetkova, M. Ulanov, and T. Cline,
on behalf of the Konus-Wind team, report:
The long-duration GRB 260618B
(Fermi-GBM detection: Fermi GBM team, GCN 44979;
Veres & Meegan, GCN 45010;
Fermi-LAT detection: Gupta et al., GCN 44983)
triggered Konus-Wind at T0=60907.637 s UT (16:55:07.637).
The burst light curve shows a multipeaked structure
which starts at ~T0-1.6 s and has a total duration of ~61.2 s.
The emission is seen up to ~4 MeV.
The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB260618_T60907/
As observed by Konus-Wind, the burst
had the total fluence of 2.26(-0.34,+0.43)x10^-5 erg/cm2,
and the 64-ms peak flux, measured from T0+0.378 s,
of 4.90(-0.98,+1.01)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+57.600 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 = -0.83(-0.16,+0.19)
and Ep = 1066(-241,+365) keV (chi2 = 94/98 dof).
Fitting by a GRB (Band) model yields the same alpha and Ep,
and an upper limit on the high energy photon index: beta < -2.7
(chi2 = 94/97 dof).
The spectrum near the maximum count rate
(measured from T0 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 = -0.39(-0.14,+0.15)
and Ep = 817(-97,+119) keV (chi2 = 96/88 dof).
Fitting by a GRB (Band) model yields the same alpha and Ep,
and an upper limit on the high energy photon index: beta < -3.2
(chi2 = 96/87 dof).
All the quoted errors are at the 68% confidence level.
All the quoted values are preliminary.
GCN Circular 45014
Subject
GRB 260618B: LCO optical observations
Date
2026-06-20T09:11:54Z (3 months ago)
From
SVOM_group <svomgroup@bao.ac.cn>
Via
Web form
Y.Y. Shi (GXU), A. Saccardi (CEA/Irfu), S.J.Yin (GXU), C. Wu (NAOC), L.P. Xin (NAOC), D. Turpin(CEA/Irfu) report on behalf of the SVOM mission team:
We observed the field of the GRB 260618B detected by Fermi-GBM (Fermi GBM team, GCN 44979; Veres et al. GCN 45010) and Fermi-LAT (Gupta et al., GCN 44983) with the LCO 1m telescope at South African Astronomical Observatory equipped with the Sinistro instrument.
Our observation started on 2026-06-20 at 00:01:12 (about 31.1 hr after the trigger) and we obtained 5x200 s exposures in the SDSS r and 5x200 s exposures in the Pan-STARRS z filters.
The optical counterpart (Li et al., GCN 45001) is detected in our images. We measure the following magnitudes calibrated against the DESI DR10 catalog, and not corrected for Galactic extinction:
r = 20.49 +/- 0.06 AB (mid-time 31.25 hr after the trigger);
z > 20.9 AB (3-sigma, mid-time 32.35 hr after the trigger).
This project is funded by the SVOM collaboration.
GCN Circular 45010
Subject
GRB 260618B: Fermi GBM Observation
Date
2026-06-19T17:39:30Z (3 months ago)
From
Peter Veres at University of Alabama in Huntsville <veresp@gmail.com>
Via
Web form
P. Veres (UAH) and C. Meegan (UAH) report on behalf of
the Fermi Gamma-ray Burst Monitor Team:
"At 16:55:04.71 UT on 18 June 2026, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 260618B (trigger 803494509/260618705),
which was also detected by Fermi/LAT (Gupta et al., GCN 44983) and SVOM/VT (Li et al., GCN 45001).
The Fermi GBM on-ground location is consistent with the LAT and the SVOM/VT position.
The angle from the Fermi LAT boresight is 16 degrees.
The GBM light curve consists of a single emission episode with a duration (T90)
of about 20 s (50-300 keV). The time-averaged spectrum
from T0 to T0+10 s is best fit by
a power law function with an exponential high-energy cutoff.
The power law index is -0.42 +/- 0.01 and the cutoff energy,
parameterized as Epeak, is 1030 +/- 20 keV.
The event fluence (10-1000 keV) in this time interval is
(9.4 +/- 0.2)E-06 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+0.96 s in the 10-1000 keV band is 4.3 +/- 0.2 ph/s/cm^2.
A Band function fits the spectrum equally well
with Epeak= 1000 +/- 20 keV, alpha = -0.41 +/- 0.02 and beta = -3.2 +/- 0.2.
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/"
GCN Circular 45001
Subject
GRB 260618B: SVOM/VT optical candidate
Date
2026-06-19T13:38:11Z (3 months ago)
From
Huali Li at at NAOC, SVOM <lhl@nao.cas.cn>
Via
Web form
H. L. Li, L. P. Xin, Y. L. Qiu, C. Wu, J. R. Xu, Y. N. Ma, Z. H. Yao, X. H. Han, J. Wang, Y. Xu, P. P. Zhang, W. J. Xie, Y. J. Xiao, H. B. Cai, J. S. Deng, J. Y. Wei (NAOC), J. T. Palmerio (CEA/Irfu) report on behalf of the SVOM/VT team.
SVOM/VT performed ToO observations for GRB 260618B detected by Fermi-LAT (Gupta et al., GCN 44983) and Fermi-GBM (the Fermi GBM team, GCN 44979). The observation started at 2026-06-19T01:44:32 UTC, i.e., 8.83 hours post trigger in the VT_B (400-650 nm) and VT_R (650-1000 nm) channels simultaneously.
An uncatalogued optical candidate was clearly detected in both channels compared with the Legacy Survey, with an angular separation of 8.7 arcminutes from the Fermi-LAT position (Gupta et al., GCN 44983). The position is at R.A., Dec. = 354.333559, -44.735518 degrees, equivalent to:
R.A. (J2000) = 23:37:20.05
Dec. (J2000) = -44:44:07.87
with an uncertainty of 0.5 arcsec.
The following measurements are in the AB magnitude and are not corrected for Galactic extinction:
Mid time | Band | Exposure Time | Brightness
10.53 h VT_B 48*70 s 20.8 +/-0.1 mag
10.53 h VT_R 41*70 s 20.9 +/-0.1 mag
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 Circular 45000
Subject
GRB 260618B: SVOM/VT optical candidate (duplicate Submission of 45001)
Date
2026-06-19T13:04:05Z (3 months ago)
Edited On
2026-06-19T20:12:44Z (3 months ago)
From
Huali Li at at NAOC, SVOM <lhl@nao.cas.cn>
Edited By
Vidushi Sharma at NASA GSFC/UMBC <vidushi.sharma@nasa.gov> on behalf of Huali Li at at NAOC, SVOM <lhl@nao.cas.cn>
Via
Web form
H. L. Li, L. P. Xin, Y. L. Qiu, C. Wu, J. R. Xu, Y. N. Ma, Z. H. Yao, X. H. Han, J. Wang, Y. Xu, P. P. Zhang, W. J. Xie, Y. J. Xiao, H. B. Cai, J. S. Deng, J. Y. Wei (NAOC), J. T. Palmerio (CEA/Irfu) report on behalf of the SVOM/VT team.
SVOM/VT performed ToO observations for GRB 260618B detected by Fermi-LAT (Gupta et al., GCN 44983) and Fermi-GBM (the Fermi GBM team, GCN 44979). The observation started at 2026-06-19T01:44:32 UTC, i.e., 8.83 hours post trigger in the VT_B (400-650 nm) and VT_R (650-1000 nm) channels simultaneously.
An uncatalogued optical candidate was clearly detected in both channels compared with the Legacy Survey, with an angular separation of 8.7 arcminutes from the Fermi-LAT position (Gupta et al., GCN 44983). The position is at R.A., Dec. = 354.333559, -44.735518 degrees, equivalent to:
R.A. (J2000) = 23:37:20.05
Dec. (J2000) = -44:44:07.87
with an uncertainty of 0.5 arcsec.
The following measurements are in the AB magnitude and are not corrected for Galactic extinction:
Mid time | Band | Exposure Time | Brightness
10.53 h VT_B 48*70 s 20.8 +/-0.1 mag
10.53 h VT_R 41*70 s 20.9 +/-0.1 mag
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 Circular 44983
Subject
GRB 260618B: Fermi-LAT detection
Date
2026-06-18T22:53:02Z (3 months ago)
From
Rahul Gupta at NASA GSFC <rahul.gupta@nasa.gov>
Via
Web form
R. Gupta (NASA/GSFC), A. Holzmann Airasca (UniTrento and INFN Bari), and N. Di Lalla (Stanford University) report on behalf of the Fermi-LAT collaboration:
At 16:55:04.71 UT on June 18th, 2026, Fermi-LAT detected high-energy emission from GRB 260618B, which was also detected by Fermi-GBM (trigger 803494509/260618705, GCN #44979).
The best LAT on-ground location is found to be
RA, Dec = 354.35, -44.88 (J2000)
with an error radius of 0.10 deg (90 % containment, statistical error only).
This was 16 deg from the LAT boresight at the time of the trigger.
The data from the Fermi-LAT show a significant increase in the event rate that is spatially and temporally correlated with the trigger with high significance.
The photon flux above 100 MeV in the time interval 0 - 100 s after the GBM trigger is (1.99 ± 0.56)E-5 ph/cm2/s. The estimated photon index above 100 MeV is -1.82 ± 0.21. The highest energy photon has an energy of 6.7 GeV and occurs at 1.8 s after trigger time.
The Fermi-LAT point of contact for this burst is Samantha López (lopez@llr.in2p3.fr).
The Fermi-LAT is a pair conversion telescope designed to cover the energy band from 20 MeV to greater than 300 GeV. It is the product of an international collaboration between NASA and DOE in the U.S. and many scientific institutions across France, Italy, Japan and Sweden.
GCN Circular 44979
Subject
GRB 260618B: Fermi GBM Final Real-time Localization
Date
2026-06-18T17:07:30Z (3 months ago)
From
Fermi GBM Team at MSFC/Fermi-GBM <do_not_reply@GIOC.nsstc.nasa.gov>
Via
email
The Fermi GBM team reports the detection of a likely LONG GRB
At 16:55:04 UT on 18 Jun 2026, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 260618B (trigger 803494509.712104 / 260618705).
The on-ground calculated location, using the Fermi GBM trigger data, is RA = 351.1, Dec = -44.1 (J2000 degrees, equivalent to J2000 23h 24m, -44d 06'), with a statistical uncertainty of 1.8 degrees.
The angle from the Fermi LAT boresight is 18.0 degrees.
The skymap can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260618705/quicklook/glg_skymap_all_bn260618705.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/bn260618705/quicklook/glg_healpix_all_bn260618705.fit
The GBM light curve can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260618705/quicklook/glg_lc_medres34_bn260618705.gif