GRB 250706B, GRB 250706C

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.

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
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 ) 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
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 ) 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.

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
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 ). 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
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 ).

  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.

On 2025-07-06 I observed the field of GRB 250706B (Palmerio et al.,
GCN Circ. 40989
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 ) 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).

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
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 ), 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
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 ) and from SVOM/VT
(GCN 40992
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 ).

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.

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
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 ) 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
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 ; Romanov, GCN 40995
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 ) 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.

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
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 ) 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
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 ).

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.

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
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 ),
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.

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
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 ). A source consistent with the XRT position (Ferro et al., GCN Circ. 40996
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 ) and the optical transient (Zhu et al., GCN Circ. 40991
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 ; Palmerio et al., GCN Circ. 40992
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 ; Romanov, GCN Circ. 40995
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 ) 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).

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
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 ), SVOM/Eclairs (named GRB 250706B in GCN 40989
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 ), Swift/XRT (named GRB 250706B in GCN 40996
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 ) and Swift/UVOT (named GRB 250706B in GCN 41015
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 ).

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.