GRB 250706B, GRB 250706C
GCN Circular 41087
Subject
GRB 250706B/C: 7DT Follow-up Observations
Date
2025-07-14T06:58:11Z (3 months ago)
From
Mankeun Jeong <jmk5040@gmail.com>
Via
Web form
Mankeun Jeong (SNU/SNUARC), Myungshin Im (SNU/SNUARC), Hyeonho Choi (SNU/SNUARC), and Seo-Won Chang (SNU/SNUARC) report on behalf of the 7-Dimensional Telescope collaboration
We observed the field of GRB 250706B/C (Palmerio et al., GCN 40989; Zhu et al., GCN 40991; Frederiks et al., GCN 41013; Longo et al., GCN 41019) using the 7-Dimensional Telescope (7DT). Observations were conducted on 2025 July 8 in multiple broadband and medium-band filters.
We performed forced photometry at the position of the UVOT optical counterpart (GCN 41015; RA = 02:44:53.43, Dec = −50:03:40.2, J2000), using a coadded image from all filters for source detection. Photometric calibration was performed using Gaia XP synthetic photometry (Gaia Collaboration et al. 2022).
We report a marginal detection only in the i-band, with a magnitude of i = 19.68±0.38 AB mag at T-T0 ≈ 1.84 days. The following table lists our photometric measurements (AB magnitudes). For non-detections, we provide 5σ upper limits:
Filter Mag (AB) Exp(s) UT Date-Obs T−T₀ (days)
-----------------------------------------------------------------------
g >19.90 300 2025-07-08 07:21:36 1.842
r >19.88 300 2025-07-08 07:26:58 1.846
i 19.68 ± 0.38 1200 2025-07-08 07:38:17 1.841
m400 >18.19 300 2025-07-08 07:27:01 1.846
m450 >18.85 300 2025-07-08 07:21:33 1.842
m475 >18.94 300 2025-07-08 07:26:58 1.846
m550 >18.65 300 2025-07-08 07:21:40 1.842
m575 >18.69 300 2025-07-08 07:27:08 1.846
m600 >18.71 300 2025-07-08 07:21:41 1.842
m625 >18.56 300 2025-07-08 07:27:15 1.846
m650 >18.81 300 2025-07-08 07:21:34 1.842
m675 >18.78 300 2025-07-08 07:26:57 1.846
m700 >18.62 300 2025-07-08 07:21:29 1.841
m725 >18.46 300 2025-07-08 07:26:50 1.846
m750 >18.31 300 2025-07-08 07:21:36 1.842
m775 >17.94 300 2025-07-08 07:26:59 1.846
m800 >17.70 300 2025-07-08 07:21:43 1.842
m825 >17.63 300 2025-07-08 07:27:13 1.846
m850 >17.06 300 2025-07-08 07:21:31 1.842
m875 >16.84 300 2025-07-08 07:21:37 1.842
GCN Circular 41049
Subject
GRB 250706B/C: Insight-HXMT detection
Date
2025-07-09T09:05:53Z (4 months ago)
From
Chenwei Wang at IHEP <cwwang@ihep.ac.cn>
Via
Web form
Chen-Wei Wang, Shao-Lin Xiong, Cheng-Kui Li, Chao Zheng, Wen-Jun Tan, Jia-Cong Liu, Hao-Xuan Guo, Xiao-Bo Li and Wang-Chen Xue report on behalf of the Insight-HXMT team:
At 2025-07-06T16:45:26.900 (T0), Insight-HXMT/HE was triggered on-ground by the long bright burst GRB 250706B/C, which is also detected by Konus-Wind (D. Frederiks, GCN #41013, GCN #41027) , SVOM (Jesse Palmerio et al., GCN #40989, GCN #41026) and Fermi/LAT (F. Longo, GCN #41019).
The Insight-HXMT/HE light curve mainly consists of numerous pulses with a T90 of 36.6 +/- 0.1s.
The 1s peak rate, measured from T0+36.150 s, is 18784 cnts/sec.
The total counts from this burst is 293497 counts.
The HXMT/HE light curve can be found here:
https://www.bursthub.cn//admin/static/hxmtgrb250706C.png
All measurements above are made with the CsI detectors operating in the regular mode with the energy range of about 60-900 keV (deposited energy). Only gamma-rays with energy greater than about 200 keV can penetrate the spacecraft and leave signals in the CsI detectors installed inside of the telescope.
Insight-HXMT is the first Chinese space X-ray telescope, which was funded jointly by the China National Space Administration (CNSA) and the Chinese Academy of Sciences (CAS). More information about it could be found at: http://www.hxmt.org.
GCN Circular 41041
Subject
IPN triangulation of GRB 250706C (consistent with the GRB 250706B)
Date
2025-07-08T19:30:06Z (4 months ago)
From
Anna Ridnaia at Ioffe Institute <ridnaia@mail.ioffe.ru>
Via
Web form
A.S. Kozyrev, D.V. Golovin, M.L. Litvak, I.G. Mitrofanov, and A.B. Sanin
on behalf of the HEND/Mars Odyssey team,
A. Ridnaia, D. Frederiks, A. Lysenko, D. Svinkin,
and T. Cline on behalf of the Konus-Wind team,
E. Burns on behalf of the IPN,
and
W. Boynton, C. Fellows, K. Harshman, H. Enos, R. Starr,
and A.S. Gardner on on behalf of the GRS-Odyssey GRB team,
report:
The very bright, long-duration GRB 250706C
(Konus-Wind detection: Frederiks et al., GCNs 41013, 41027;
Fermi-LAT detection: Longo et al., GCN 41019)
was detected by Konus-Wind and Mars-Odyssey (HEND)
at about 60322 s UT (16:45:22), ~1200 s before GRB 250706B
(SVOM detection: Palmerio et al, GCNs 40989, 41026)
We have triangulated it to a Konus-HEND annulus centered at
RA(2000)=342.841 deg (22h 51m 22s) Dec(2000)=-8.288 deg (-8d 17' 19"),
whose radius is 63.671 +/- 0.024 deg (3 sigma).
This localization may be improved.
The SVOM/ECLAIRs source localization reported for GRB 250706B (GCN 40989)
is consistent with the Konus-HEND annulus, the Konus-Wind ecliptic latitude
response and the Fermi-LAT position (GCN 41019), indicating that GRB 250706B and
GRB 250706C are distinct emission episodes of the same GRB.
A triangulation map and HEALPix FITS file are posted at
http://www.ioffe.ru/LEA/GRBs/GRB250706_T60322/IPN/
The HEALPix triangulation map is the multi-order HEALPix in units of
probability density.
GCN Circular 41027
Subject
GRB 250706B/C: Konus-Wind analysis of the prompt emission
Date
2025-07-08T13:55:05Z (4 months ago)
From
Dmitry Frederiks at Ioffe Institute <fred@mail.ioffe.ru>
Via
email
D. Frederiks, A.Lysenko, A. Ridnaia, D. Svinkin,
A. Tsvetkova, M. Ulanov, and T. Cline,
on behalf of the Konus-Wind team, report:
We report on a further analysis of the KW detection
of the very bright, long-duration GRB 250706C
(Frederiks et al., GCN 41013), which common origin with
the SVOM/ECLAIRs GRB 250706B (Palmerio et al., GCN 40989)
was confirmed from the FERMI/LAT observation (Longo et al., GCN 41019).
GRB 250706C triggered Konus-Wind (KW) at T0=60322.626 s UT (16:45:22.626),
~1200 s before the SVOM/ECLAIRs trigger on GRB 250706B.
The burst light curve shows a very bright, multi-peaked
emission pulse with the duration of ~50 s (20-1500 keV).
This pulse is followed by a weaker, smoothly decaying emission tail,
visible up to the end of the KW triggered data record (~T0+250 s).
The emission in the main pulse is seen up to ~15 MeV.
The KW light curve is available at
http://www.ioffe.ru/LEA/GRBs/GRB250706_T60322/
As observed by Konus-Wind, the burst had
the total fluence of (5.12 ± 0.06)x10^-4 erg/cm^2 and
a 16-ms peak energy flux, measured from T0 + 36.800 s,
of (4.16 ± 0.22)x10^-5 erg/cm^2/s (both in the 20 keV - 10 MeV energy range).
The time-integrated spectrum (measured from T0 to T0+51.968 s)
is best fit in the 20 keV - 15 MeV range by a GRB (Band) function
with the following model parameters:
the low-energy photon index alpha = -1.00 (-0.01,+0.01),
the high energy photon index beta = -3.65 (-0.44,+0.23),
the peak energy Ep = 850(-15,+15) keV,
chi2 = 125/96 dof.
The spectrum near the peak count rate (measured from T0+36.608 s to T0+37.120 s)
is best fit in the 20 keV - 15 MeV range by a GRB (Band) function
with the following model parameters:
the low-energy photon index alpha = -1.10 (-0.04,+0.04),
the high energy photon index beta = -2.91 (-0.60,+0.30),
the peak energy Ep = 1005(-91,+104) keV,
chi2 = 69/58 dof.
The spectrum of the decaying tail (measured from T0+51.968 s to T0+232.192)
is best fit by a simple power law (PL) function with the PL photon index
of (-2.0 ± 0.06), chi2 = 92/99 dof.
Assuming the redshift z=0.942 (Palmerio et al., GCN 41022)
and a standard cosmology with H_0 = 67.3 km/s/Mpc, Omega_M = 0.315,
and Omega_Lambda = 0.685 (Planck Collaboration, 2014),
we estimate the burst isotropic energy release E_iso to (1.26 ± 0.02)x10^54 erg,
the isotropic peak luminosity L_iso to (2.00 ± 0.11)x10^53 erg/s, and
the rest-frame peak spectral energy Epi,z to 1650(-30,+30) keV.
With the obtained estimates, GRB 250706B/C is consistent with 68% prediction bands
of both 'Amati' and 'Yonetoku' relations for the sample of >300 long KW GRBs
with known redshifts (Tsvetkova et al., 2017; Tsvetkova et al., 2021),
see http://www.ioffe.ru/LEA/GRBs/GRB250706_T60322/GRB250706BC_rest_frame.pdf
All the quoted errors are estimated at the 68% confidence level.
All the presented results are preliminary.
GCN Circular 41026
Subject
GRB 250706B: SVOM observations interpreted as the afterglow of GRB 250706C
Date
2025-07-08T13:38:21Z (4 months ago)
From
SVOM_group <svomgroup@bao.ac.cn>
Via
Web form
J. T. Palmerio, D. Adrien (CEA), M. G. Bernardini (INAF-OAB, LUPM), M. Brunet, J.-L. Atteia, O. Godet (IRAP), P. Maggi (ObAS), L. P. Xin (NAOC), D. B. Malesani (DAWN/NBI and Radboud), C. W. Wang (IHEP), A. Saccardi, D. Götz (CEA), F. Cangemi (APC) report on behalf of the SVOM mission team:
Following the detection of GRB 250706C reported by Konus-Wind (Frederiks et al., GCN 41013) and Fermi/LAT (Longo et al., GCN 41019), we discuss below the interpretation of GRB 250706B detected by SVOM (Palmerio et al., GCN 4989) as the hard X-ray afterglow of GRB 250706C.
At the Konus-Wind trigger time (Frederiks et al., GCN 41013), the position of GRB 250706B was occulted by the Earth for SVOM/ECLAIRs. The SVOM/ECLAIRs trigger for GRB 250706B occurred 21 min after the Konus-Wind trigger, right after the source was no longer occulted by the Earth. SVOM/ECLAIRs shows a featureless and long-lasting (~1000 s) emission in the 4-50 keV energy band. The time-averaged spectrum after the slew (from 134 s to 1000 s after the ECLAIRs trigger time, GCN 4989) in the energy range 5-120 keV is best fitted by a power law model with index 2.01 +/- 0.11 (68% c.l. errors). With this model, the total 4-120 keV fluence is 2.6e-6 erg/cm^2. There is no significant signal in the SVOM/GRM VHF data.
Taking the Konus-Wind detection time as T0 (16:45:22 UT), the SVOM/VT lightcurve appears as a single, unbroken power-law with index ~1.2. Assuming the same power-law decay held continuously before the VT observations, extrapolating the optical light curve to T0 + 1 min yields a magnitude of roughly 10 in VT_R. We encourage all-sky monitors to check for any emission at this location around the Konus-Wind trigger time.
Extrapolating the SVOM/MXT lightcurve to T0 + 1 min yields a 0.3-10 keV flux of 6.5x10^-8 erg/cm^2/s, assuming the same power-law decay held continuously before the MXT observations reported by Maggi et al. (GCN 41009).
We encourage further follow-up of this burst.
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 SVOM point of contact for this burst is Jesse Palmerio: palmerio@cea.fr
Please contact the BA by email if you require additional information regarding the SVOM follow-up of this burst.
GCN Circular 41022
Subject
GRB 250706B/C: VLT/X-shooter redshift z = 0.942
Date
2025-07-08T12:04:56Z (4 months ago)
From
Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani@nbi.ku.dk>
Via
Web form
J. T. Palmerio (CEA/Irfu), B. Schneider (LAM), A. Saccardi (CEA/Irfu), M. Garnichey (LUX-Paris Obs.), G. Corcoran (UCD), V. D'Elia (ASI/SSDC), M. De Pasquale (Univ. Messina), L. Izzo (INAF/OACn and DARK/NBI), D. B. Malesani (DAWN/NBI and Radboud), P. O'Brien (Leicester), N. R. Tanvir (Univ. Leicester), S. D. Vergani (LUX-Paris Obs.) report on behalf of the Stargate collaboration:
We observed the optical afterglow (Zhu et al., GCN 40991) of the SVOM GRB 250706B (Palmerio et al., GCN 40989), an extension of GRB 250706C detected by Konus-Wind and Fermi/LAT (Frederiks et al., GCN 41013; Longo et al., GCN 41019), using the ESO VLT UT3 (Melipal) equipped with the X-shooter spectrograph. Our spectra cover the wavelength range 3000-25000 AA, and consist of 2 exposures of 600 s each. The observation mid time was 2025 Jul 8.40 UT (1.70 days after the Konus-Wind trigger).
In a 30 s image taken with the acquisition camera on Jul 8.39 UT (1.69 days after the Konus-Wind trigger), the optical afterglow is clearly detected, and we measure a magnitude r = 20.37 +- 0.02 AB (calibrated against a single star from the SkyMapper catalog).
In a preliminary reduction of the spectra, continuum is visible over the entire covered wavelength range. From the detection a multitude of absorption lines, which we interpret as due to Al II, Ni II, Si II, Si II*, Al III, Cr II, Zn II, Ni II*, Fe II, Fe I, Mn II, Mg II, Mg I, Ca II, Ca I, we infer a redshift of z = 0.942. We also detect two emission lines which we identify as the [O III] 4959,5007 doublet at the same redshift. The detection of fine-structure lines, as well as of nebular emission, allows us to securely identify z = 0.942 as the redshift of GRB 250706B/C.
We acknowledge excellent support from the ESO observing staff in Paranal, in particular Diego Parraguez. The analysis of this spectrum was carried out with the help of the zHunter tool (https://doi.org/10.5281/zenodo.15189495).
GCN Circular 41019
Subject
GRB 250706B/C: Fermi-LAT detection
Date
2025-07-07T20:48:10Z (4 months ago)
From
N. Di Lalla at Stanford University <niccolo.dilalla@stanford.edu>
Via
email
F. Longo (University and INFN, Trieste), N. Di Lalla (Stanford Univ.), A. Holzmann Airasca (UniTrento and INFN Bari), E. Bissaldi (Politecnico and INFN Bari), R. Gupta (NASA/GSFC), S. Lopez (CNRS / IN2P3) and J. Racusin (NASA/GSFC) report on behalf of the Fermi-LAT Collaboration:
On July 06, 2025, Fermi-LAT detected high-energy emission from GRB 250706B/C, which was also detected by Konus-Wind (named GRB 250706C in GCN 41013), SVOM/Eclairs (named GRB 250706B in GCN 40989), Swift/XRT (named GRB 250706B in GCN 40996) and Swift/UVOT (named GRB 250706B in GCN 41015).
The best LAT on-ground location is found to be:
RA, Dec = 41.17, -50.04 (J2000)
with an error radius of 0.07 deg (90 % containment, statistical error only). This was 87 deg from the LAT boresight at the time of the Konus-Wind trigger (T0 = 16:45:22.626 UT).
The data from the Fermi-LAT shows a significant increase in the event rate starting about 600 s after the Konus-Wind trigger, as soon as the GRB position entered the LAT field of view. The LAT emission is spatially and temporally correlated with both the Konus-Wind (GRB 250706C) and SVOM (GRB 250706B) emission with high significance, which are therefore to be considered part of the same burst. The photon flux above 100 MeV in the time interval 600 - 10000 s after the Konus-Wind trigger is (3.52 ± 0.59) E-6 ph/cm2/s. The estimated photon index above 100 MeV is -1.87 ± 0.12. The highest-energy photon is a 21 GeV event which is observed ~ 8 ks seconds after the Konus-Wind trigger.
The Fermi-LAT point of contact for this burst is Dimakatso Maheso (d.j.maheso@gmail.com).
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 41015
Subject
GRB 250706B: Swift/UVOT Detection
Date
2025-07-07T18:35:19Z (4 months ago)
From
Mike Siegel at PSU/Swift MOC <mhs18@psu.edu>
Via
Web form
M. H. Siegel (PSU) reports on behalf of the Swift/UVOT team:
The Swift/UVOT began settled observations of the field of GRB 250706B 12.5 ks after the SVOM/ECLAIRs trigger (Palmerio et al., GCN Circ. 40989). A source consistent with the XRT position (Ferro et al., GCN Circ. 40996) and the optical transient (Zhu et al., GCN Circ. 40991; Palmerio et al., GCN Circ. 40992; Romanov, GCN Circ. 40995) is detected in the UVOT exposures.
The preliminary UVOT position is:
RA (J2000) = 02:44:53.43 = 41.23097 (deg.)
Dec (J2000) = -50:03:40.2 = -50.06116 (deg.)
with an estimated uncertainty of 0.42 arc sec. (radius, 90% confidence).
Preliminary detections and 3-sigma upper limits using the UVOT photometric system
(Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the early exposures are:
Filter T_start(s) T_stop(s) Exp(s) Mag
u 12471 18805 2023 16.49+/-0.03
The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.026 in the direction of the burst
(Schlegel et al. 1998).
GCN Circular 41013
Subject
GRB 250706C: Konus-Wind detection of a very bright GRB ~1200 s before GRB 250706B
Date
2025-07-07T17:17:34Z (4 months ago)
From
Dmitry Frederiks at Ioffe Institute <fred@mail.ioffe.ru>
Via
email
D. Frederiks, A.Lysenko, A. Ridnaia, D. Svinkin,
A. Tsvetkova, M. Ulanov, and T. Cline,
on behalf of the Konus-Wind team, report:
The long-duration GRB 250706C triggered Konus-Wind (KW)
at T0=60322.626 s UT (16:45:22.626).
The burst light curve shows a very bright, multi-peaked
emission pulse with the duration of ~50 s (20-1500 keV).
This pulse is followed by a weaker, smoothly decaying emission tail,
visible at lower energies (20-100 keV) up to the end
of the KW triggered data record (~T0+250 s).
A preliminary plot of the KW light curve is available at
http://www.ioffe.ru/LEA/GRBs/GRB250706_T60322/KW20250706_60322_BG.png
The KW ecliptic latitude response suggest the source location
in the southern ecliptic hemisphere, at moderate-to-high ecliptic latitude.
We note that the KW trigger time is ~1200 s before SVOM/ECLAIRs trigger
on GRB 250706B at 2025-07-06T17:05:23 (Palmerio et al., GCN 40989),
in the [5-20] keV energy band, and localized to R.A., Dec. 41.1337, -50.1172 deg,
which is consistent with the KW ecliptic latitude response.
This may suggest that the KW GRB 250706C is an initial, prompt emission
episode of GRB 250706B.
The KW analysis of GRB 250706C is ongoing and its detailed results will
be reported in a separate GCN.
GCN Circular 41009
Subject
GRB 250706B: refined SVOM/MXT data analysis
Date
2025-07-07T15:24:57Z (4 months ago)
From
SVOM_group <svomgroup@bao.ac.cn>
Via
Web form
P. Maggi (ObAS), D. Götz (CEA), H. Goto (Kanazawa University/CEA), M. Moita (CEA), C. Plasse (CEA), F. Robinet (IJCLab), C. Van Hove (IJCLab) report of behalf of the SVOM/MXT Team:
GRB 250706B (Palmerio et al. GCN 40989) was observed by SVOM/MXT after an automatic SVOM slew, starting at T0 = 2025-07-06T17:08:42.6 (158 s after trigger time Tb). MXT observed during 6 orbits for 13.6 ks effective exposure time.
Using the full X-band dataset, the X-ray source afterglow is located at R.A., Dec = 41.239, -50.065 degrees:
R.A. (J2000) = 02h44m57.45s
Dec. (J2000) = -50d03m55.6s
with a 90% C.L. radius of 31 arcseconds (including 30 arcseconds systematic error added in quadrature) and consistent with the reported optical VT position (Palmerio et al. GCN 40992).
The spectrum is modelled with an absorbed power law. Across the full data set the absorption column NH is consistent within uncertainties, between 2 and 3 x 1e22 /cm2 (all uncertainties are at the 90% C.L.). The photon index evolves from 2.25 (-0.16/+0.18) within 1200s of trigger time, to 2.53 (-0.25/+0.28) at Tb + 5 ks < t < Tb+ 20 ks, and softens further to 3.43 (-1.04/+1.45) at t > Tb + 25 ks. The 0.3-10 keV flux is 8.24e-10 (-0.91/+0.82) x 1e-10 erg/cm2/s, 8.9 (-1.0/+1.2) x 1e-11 erg/cm2/s, and down to <1.0 x 1e-11 erg/cm2/s during the same intervals.
We observe a complex light curve with at least 3 power-law segments with count rate proportional to t^alpha, with:
alpha = -0.42 +/-0.02 until Tb+5031s +/-277s
alpha = -1.57 +/-0.07 until Tb+24326s +/-3479s
alpha = -3.05 +/-1.62 afterwards
The SVOM/MXT light curve can be found here: https://seafile.unistra.fr/f/909c01d644774417897e/. With the spectrum reported in the first interval, a count rate of 1 count/s corresponds to a flux of 1.89x1e-10 erg/cm2/s.
Follow-up observations at other wavelengths are encouraged.
The Space 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. MXT was developed jointly by CEA, CNES, University of Leicester, IJCLab and MPE.
The SVOM point of contact for this burst is Jesse Palmerio: palmerio@cea.fr
Please contact the BA by email if you require additional information regarding the SVOM follow-up of this burst.
GCN Circular 41007
Subject
GRB 250706B: REM optical observations
Date
2025-07-07T10:23:35Z (4 months ago)
From
Riccardo Brivio at INAF-OAB <riccardo.brivio@inaf.it>
Via
Web form
R. Brivio, M. Ferro, P. D'Avanzo, S. Covino, D. Fugazza (INAF-OAB) report on behalf of the REM team:
We observed the field of GRB 250706B detected by SVOM/ECLAIRs (Palmerio et al., GCN 40989) with the REM 60 cm robotic telescope located at the ESO observatory of La Silla (Chile). The observations were carried in the g, r, i, and z bands, started on 2025 July 07 at 05:14:01 UT (i.e. 12.1 hr after the burst), and lasted for about 1 hour.
From preliminary inspection, we do not detect any counterpart at the position of the optical afterglow (Zhu et al., GCN 40991; Romanov, GCN 40995) down to the following 3sigma limit:
r > 19.2 (AB; calibrated against the SkyMapper catalogue),
at a mid-time of 13.0 hr after the trigger.
GCN Circular 40996
Subject
GRB250706B: Swift-XRT counterpart detection
Date
2025-07-06T21:33:59Z (4 months ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
Via
email
M. Ferro (INAF-OAB), R. Brivio (INAF-OAB), J.A. Kennea (PSU), D.N.
Burrows (PSU), M.A. Williams (PSU), A.P. Beardmore (U. Leicester), J.P.
Osborne (U. Leicester), A. D'Ai (INAF-IASFPA) and P.A. Evans (U.
Leicester) reports on behalf of the Swift-XRT team:
Swift has performed follow-up observations of the SVOM detected burst
GRB250706B (GCN 40989), collecting 1.6 ks of XRT data from 1.4 ks to
3.0 ks after the SVOM/ECLAIRs trigger. The data are entirely in Photon
Counting (PC) mode. A candidate X-ray counterpart has been found with a
position RA, Dec = 41.2296, -50.0604 which is equivalent to:
RA (J2000): 02 44 55.11
Dec(J2000): -50 03 37.5
with an uncertainty of 3.5 arcsec (radius, 90% confidence).
We consider the X-ray source the afterglow of GRB 250706B, even if this
position appears only marginally consistent with the optical
counterpart detected by TRT (Zhu et al., GCN 40991) and from SVOM/VT
(GCN 40992).
The light curve can be modelled with an initial power-law decay with an
index of alpha=0.93 (+0.17, -0.21), followed by a break at T+2679 s to
an alpha of 8.0 (+0.0, -1.4).
A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.50 (+/-0.07). The
best-fitting absorption column is 1.11 (+0.24, -0.23) x 10^21 cm^-2,
in excess of the Galactic value of 2.8 x 10^20 cm^-2 (Willingale et al.
2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum is 4.3 x 10^-11 (4.8 x 10^-11) erg
cm^-2 count^-1.
A summary of the PC-mode spectrum is thus:
Total column: 1.11 (+0.24, -0.23) x 10^21 cm^-2
Galactic foreground: 2.8 x 10^20 cm^-2
Excess significance: 5.9 sigma
Photon index: 1.50 (+/-0.07)
If the light curve continues to decay with a power-law decay index of
8.0, the count rate at T+24 hours will be 1.3 x 10^-11 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 5.6 x
10^-22 (6.2 x 10^-22) erg cm^-2 s^-1.
The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00019912.
This circular is an official product of the Swift-XRT team.
GCN Circular 40995
Subject
GRB 250706B: iTelescope optical observations
Date
2025-07-06T20:59:09Z (4 months ago)
From
Filipp Dmitrievich Romanov at Amateur astronomer <filipp.romanov.27.04.1997@gmail.com>
Via
email
On 2025-07-06 I observed the field of GRB 250706B (Palmerio et al.,
GCN Circ. 40989) remotely using the telescopes T33 (0.3-m f/9
reflector + CCD with Astrodon Series II Red filter) and T17 (0.43-m
f/6.8 reflector + CMOS with Astrodon Red-E filter) of iTelescope.Net
at Siding Spring Observatory, Australia.
The exposure times of the photographs were 300 seconds, observations
started at 19:56:23 UT (2h 50m after the trigger). I detected the
optical afterglow in all images with the position (J2000.0):
02:44:55.46 -50:03:40.1.
I measured the magnitudes (compared to r' magnitudes of nearby stars
from the APASS DR9 catalog: Henden et al., 2016):
Mid-time (UT) | Magnitude | Error | Telescope |
19:58:53.3 16.852 0.073 T33
20:04:55.9 16.790 0.046 T33
20:10:58.5 16.756 0.110 T33
20:17:00.3 16.752 0.071 T33
20:05:50.8 16.738 0.040 T17
20:11:47.9 16.875 0.054 T17
20:17:43.6 16.833 0.076 T17
Magnitudes were not corrected for Galactic extinction.
F. D. Romanov (AAVSO).
GCN Circular 40992
Subject
GRB 250706B: SVOM/VT optical observation
Date
2025-07-06T18:45:55Z (4 months ago)
Edited On
2025-07-07T13:29:29Z (4 months ago)
From
SVOM_group <svomgroup@bao.ac.cn>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of SVOM_group <svomgroup@bao.ac.cn>
Via
Web form
J. Palmerio (CEA), L. P. Xin, Y. L. Qiu, C. Wu, H. L. Li, Z. H. Yao, Y. N. Ma, X. H. Han, Y. Xu, J. Wang, P. P. Zhang, W. J. Xie, Y. J. Xiao, H. B. Cai, L. Lan, J. S. Deng, J. Y. Wei (NAOC) report on behalf of the SVOM/VT team
After the trigger by SVOM/ECLAIRs at 2025-07-06T17:06:04 UTC (T0), SVOM performed an automatic slew on the burst location (Palmerio et al., GCN 40989). SVOM/VT began observing the field at 2025-07-06T17:10:48, 283.83 seconds after T0, in the VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously.
From a preliminary analysis of the 1-bit subimage and source list downloaded via VHF network, at least one credible candidate is identified, the details of which are presented below.
VT_ID 50:
This candidate was flagged as un uncatalogued source whose magnitude varied with 3 sigma significance.
The position of this candidate is R.A., Dec. 41.2311, -50.0612 degrees, corresponding to:
R.A. (J2000) = 2h44m55.5s
Dec. (J2000) = -50d03m40.4s
with an uncertainty of 1 arcsec.
This location is within the R90 uncertainty region of the SVOM/MXT onboard localization.
This location is consistent with the optical afterglow reported by Zhu et al. (GCN 40991).
The source was detected in both VT_R and VT_B, and was fading between the first 2 VT observing sequences. The candidate's magnitudes are:
| date-obs (UTC) | mid-time | exposure | band | mag(AB) |
| -------------------- | ----------- | --------- | ----- | ------------- |
| 2025-07-06T17:10:48 | 7.23 min | 6*50 sec | VT_B | 14.90 ± 0.01 |
| 2025-07-06T17:10:48 | 7.23 min | 6*50 sec | VT_R | 14.48 ± 0.01 |
| 2025-07-06T17:15:48 | 12.23 min | 6*50 sec | VT_B | 15.17 ± 0.01 |
| 2025-07-06T17:15:48 | 12.23 min | 6*50 sec | VT_R | 14.65 ± 0.01 |
Magnitudes were not corrected for dust extinction.
Given the VT colour of the counterpart, it might be a low redshift gamma-ray burst.
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/VT was jointly developed by Xi'an Institute of Optics and Precision Mechanics (XIOPM), CAS and National astronomical observatories (NAOC), CAS.
The SVOM/VT point of contact for this burst is Jesse Palmerio: palmerio@cea.fr.
GCN Circular 40991
Subject
GRB 250706B: TRT bright optical counterpart detection
Date
2025-07-06T18:32:19Z (4 months ago)
From
Dong Xu at NAOC/CAS <dxu@nao.cas.cn>
Via
Web form
Z.P. Zhu (NAOC), K. Noysena, S. Tinyanont, K. Chanchaiworawit (NARIT), S.Q. Jiang, X. Liu, J. An, L.B. He, D. Xu (NAOC), S.Y. Fu (HUST) report on behalf of a large collaboration:
We observed the field of GRB 250706B detected by SVOM/ECLAIRs (Palmerio et al., GCN 40989) using the 0.7-m telescope of the Thai Robotic Telescope network (TRT), located at New South Wales, Australia (SBO). Observations started at 17:41:58 UTC on 2025-07-06, i.e., 35.9 min after the SVOM trigger and a series of frames in the R band were obtained.
An uncatalogued and varying optical transient is detected within the SVOM/MXT error circle (Palmerio et al., GCN 40989) at coordinates
R.A. (J2000) = 02:44:55.45
Dec. (J2000) = -50:03:40.32
with an uncertainty of ~0.5 arcsec. It has R ~ 15.5 mag in the first R-band frame, calibrated with Legacy Survey DR-10 and not corrected for Galactic extinction.
We thus think this is the optical afterglow of the GRB.
GCN Circular 40989
Subject
GRB 250706B: SVOM detection of a long burst
Date
2025-07-06T17:33:22Z (4 months ago)
From
SVOM_group <svomgroup@bao.ac.cn>
Via
Web form
Jesse Palmerio, Dylan Adrien, Nicolas Dagoneau (CEA), Marius Brunet (IRAP), Maria Grazia Bernardini (INAF-OAB, LUPM), Charlotte Van Hove (IJCLab), Pierre Maggi (ObAS) report on behalf of the SVOM mission team:
At 2025-07-06T17:06:04 UTC (T0), SVOM/ECLAIRs triggered and located the gamma-ray burst GRB 250706B (SVOM burst-id sb25070609).
The following trigger information was received on the ground with low latency by the SVOM VHF Alert Network.
The burst was only detected by the Image Trigger (IMT), which produced a sequence of 7 alerts. IMT provided the alert with the best signal-to-noise-ratio in the image (SNR) of 10.09 in the [5-20] keV energy band over a time window of 81.92 seconds starting at 2025-07-06T17:05:23.
The localization of the best alert is R.A., Dec. 41.1337, -50.1172 degrees (J2000) with a 90% confidence level (C.L.) radius of 7.89 arcmin (including systematic error of 2 arcmin added in quadrature).
SVOM slewed to the burst.
SVOM/MXT began observing the field at 2025-07-06T17:08:42 UTC, 158 seconds after T0. Using onboard processed data we found an uncatalogued X-ray source located at R.A., Dec. 41.2300, -50.0621 degrees:
R.A. (J2000) = 2h44m55.20s
Dec. (J2000) = -50d03m43.51s
with a 90% C.L. radius of 38 arcseconds.
This location is 4.97 arcminutes from the ECLAIRs onboard position. This position may be improved as more data is received.
VT began observing the field after the slew. The analysis of the data will be published in a future circular.
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 Jesse Palmerio: palmerio@cea.fr.
Please contact the BA by email if you require additional information.