GRB 250806A
GCN Circular 41550
Subject
GRB 250806A: HiPERCAM/GTC limits on a possible supernova
Date
2025-08-26T16:15:04Z (a month ago)
From
Antonio de Ugarte Postigo at LAM, CNRS <adeugartepostigo@gmail.com>
Via
email
A. de Ugarte Postigo (LAM), D. B. Malesani (DAWN/NBI and Radboud), A. Martin-Carrillo (UCD), C. C. Thoene (AbAO), J. F. Agui Fernandez (CAHA), M. A. Aloy (UV), J. P. U. Fynbo (DAWN/NBI), L. Galbany (IEEC-CSIC), S. Geier (GTC), L. Izzo (INAF/OACN and DARK/NBI), G. Lombardi (GTC), N. A. Rakotondrainibe (LAM), B. Schneider (LAM), N. R. Tanvir (Univ. Leicester), and D. Gonzalez Gonzalez (GTC) report:
We observed the field of GRB 250806A (Xie et al., GCN 41243) with the 5-band HiPERCAM imager mounted on the 10.4 m GTC telescope, at the Roque de los Muchachos Observatory (La Palma, Spain). The observation consisted of 20 x 60 s exposures in the u, g, r, i, and z bands, with mean epoch 2025-08-23 01:51:07 UT (16.75 days after the burst). Our images have 3-sigma detection limits of u > 26.1, g > 26.4, r > 25.9, i > 25.6, and z > 24.7 mag as compared to SDSS field stars (all the magnitudes in this GCN are in the AB system).
Within the refined XRT error box (Campana et al., GCN 41249) we clearly detect the two galaxies mentioned by Malesani et al. (GCN 41279), the brightest of which has been proposed as the host of GRB 250806A and has a measured redshift of 0.367 (Sevilla et al., GCN 41499). Under the assumption that the GRB is indeed associated with this galaxy, its low redshift prompted us to perform a search for a possible emerging supernova (SN). The time of our observation would correspond to 14.7 days after the burst in the rest frame, close to the time of expected maximum light.
No other significant new source is detected within the XRT error box beyond the mentioned galaxies. Image subtraction with respect to the Legacy Survey (LS) does not reveal any significant residuals, although we note that the reference images are significantly shallower than our data. A deeper search for the SN component will require obtaining deeper templates once the possible SN has faded away.
Assuming as limits for the subtraction images the depth of the Legacy Survey, we would not be detecting a SN component down to r > 25.0 mag and i > 24.5 mag. At a redshift of 0.367, this is equivalent to absolute magnitudes of M_4500AA (g-band) > -16.1 and M_5500AA (V-band) > -16.6, or approximately 2.7 magnitudes fainter than SN 1998bw in each of these bands. This could indicate a SN intrinsically fainter than SN 1998bw by a comparable amount, or might indicate line-of-sight extinction of A_V > 2.7 mag. The latter option would be consistent with the lack of optical/NIR afterglow detection (Wu et al., GCN 41244; Fortin et al., GCN 41245; Freeberg et al., GCN 41247; Xin et al., GCN 41250; Zheng et al., GCN 41251; Schneider et al., GCN 41253; Malesani et al., GCN 41279; Pankov et al., GCN 41285).
As an alternative, the galaxy at z = 0.367 might be of course a chance association, and the GRB might be simply further away.
GCN Circular 41499
Subject
GRB 250806A: Gemini observations of host galaxy candidate
Date
2025-08-22T16:12:48Z (a month ago)
From
gsriniv2@umd.edu
Via
Web form
C. Sevilla (Cornell), G. Srinivasaragavan (UMD), D. A. Perley (LJMU), A. Y. Q. Ho (Cornell), A. Bochenek (LJMU) report:
We observed the candidate host galaxy of GRB 250806A (Xie et al., GCN 41243, Malesani et al. GCN 41279) beginning UTC 2025-08-13 13:14:40 with the Gemini Multi-Object Spectrograph on Gemini North under program GN-2025B-Q-125 (PI: Srinivasaragavan). We use a 1 arcsecond slit, the R400 grating, and the GG455 filter. We obtained four exposures of 450 seconds each, two at a central wavelength of 520 nm and two at 525 nm.
We obtain a redshift of 0.367 from galactic Hβ, [O II], and [O III] emission lines. At this redshift, using the 4 - 120 keV flux from Bouchet et al. (GCN 41287) and observed burst time of 20 seconds, we estimate the burst has an isotropic equivalent energy of ~ 8e49 erg.
We thank the Gemini Contact Scientists and the Gemini Helpdesk for their assistance in observing and data reduction.
GCN Circular 41288
Subject
GRB 250806A: SVOM/MXT refined analysis
Date
2025-08-08T14:08:03Z (2 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 250806A (Xie et al. GCN 41243) was observed by SVOM/MXT after an automatic SVOM slew, starting at T0 = 2025-08-06T08:00:11, 117 s after trigger time Tb. MXT observed for the remainder of the orbit for 297s.
Using the full X-band dataset, the position of the MXT candidate afterglow is refined to:
R.A. (J2000) = 23h13m38.10s
Dec (J2000) = +01d21m59.0s
with a 90% C.L. radius of 66” (including 25 arcseconds systematic error added in quadrature). This is just outside the Swift/XRT afterglow identified by Evans et al. (GCN 41249) and observed with Einstein Probe/FXT (Liang et al., GCN 41260), but the light curve analysis below confirms that this is the same source.
We analysed the time-averaged spectrum, subtracting an appropriately-scaled blank sky background spectrum from the Lockmann Hole. Modelled with an absorbed power-law, a soft spectrum is found, with a photon index Gamma >2.2 at 90% C.L and an absorbing column NH = 4.1 (+4.5/-3.2) x1e21 cm2 on top of Galactic NH = 4.6 x 1e20 /cm2.
We derive a count rate conversion factor of 1 cps = 9.9e-11 erg/s/cm2
The light curve exhibits an initial fast decay with temporal index alpha~-3.5+/-0.6 (with count rate proportional to t^alpha). At t >Tb + 250s the source is no longer detected by MXT. Extending the light curve using Swift/XRT data in Photon Counting mode from the automatic follow-up suggests a transition to a shallower decay occurred at about 5 minutes post-trigger.
The source is below the MXT detection limit in subsequent orbits.
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 Wenjin Xie (xiewj@bao.ac.cn).
Please contact the BA by email if you require additional information regarding the SVOM follow-up of this burst.
GCN Circular 41287
Subject
GRB 250806A: SVOM/ECLAIRs refined analysis
Date
2025-08-08T13:18:05Z (2 months ago)
From
SVOM_group <svomgroup@bao.ac.cn>
Via
Web form
L. Bouchet (IRAP), Y. Nourlil (CEA), S. Schanne (CEA), N. Dagoneau (CEA), J.-L. Atteia (IRAP) report on behalf of the SVOM/ECLAIRs team:
We performed further analysis of GRB 250806A (SVOM burst-id sb25080601), triggered by ECLAIRs onboard SVOM (Xie et al. GCN 41243).
Using the ECLAIRs event-by-event data downloaded through the X-band ground station, we confirm that the burst consists of a single peak, with a duration of about 20 s. The time-averaged spectrum from T0-10 s to T0+10 s (T0 = 2025-08-06T07:58:14 UTC) in the energy range 4-120 keV is best fitted by a powerlaw model with index -2.07 (-0.12, +0.12). With this model, the total flux in 4-120 keV is 1.2e-8 erg/cm^2/s. All quoted errors are at the 68% confidence level.
This prompt emission spectrum suggests that GRB 250806A is an X-Ray Flash (a low energy GRB with a soft spectrum) located at low redshift. This interpretation is consistent with the Swift/UVOT observation of the afterglow (GCN 41264) and the identification of a host galaxy candidate by NOT (GCN 41279).
We note that the calibration of SVOM/ECLAIRs is ongoing thus these results are preliminary.
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 GCN circular is: Laurent Bouchet (laurent.bouchet@irap.omp.eu).
GCN Circular 41285
Subject
GRB 250806A: Mondy optical observations, and nearby bright galaxy photometry
Date
2025-08-08T12:08:44Z (2 months ago)
From
Nicolai Pankov at HSE, IKI RAS <colinsergesen@gmail.com>
Via
Web form
N. Pankov (HSE, IKI), A. Pozanenko (IKI), E. Klunko (ISTP) report on behalf of IKI-GRB-FuN:
We observed the field of GRB 250806A detected by SVOM/ECLAIRs (Xie et. al, GCN 41243) with the 1.5m AZT-33IK telescope of the Solar Sayan Observatory (Mondy). The series of 28x120 s images were ingested in the R-filter starting on 2025-08-06 at 16:59 UT, i.e. ~0.39 days after the SVOM trigger. In the co-add image of 26x120 we do not detect the optical source reported in (Breeveld, GCN 41264). We note the presence of a nearby SDSS-DR12 galaxy at the coordinates (J2000) 23:13:43.45 01:21:21.9 located at the photometric redshift ~ 0.3 and has the SDSS photometry r = 21.383 +/- 0.141, and i = 21.113 +/- 0.163. This SDSS galaxy is the brighter one of the Legacy Survey galaxies reported in Malesani et al., GCN 41279. The preliminary photometry of the galaxy in the co-add image is presented below:
Date UT start t-T0 Exp. Filter Mag Err UL
(mid, days) (n x s) (3sigma)
2025-08-06 16:59:03 0.39362 26x120 R 21.27 0.22 22.4
The photometry was calibrated using nearby stars from the SDSS-DR12 catalog (R mags were obtained via Lupton 2005 transformations) and not corrected for the Galactic extinction. At present we cannot say anything about the change in brightness of the galaxy. The second Legacy Survey faint galaxy (RA = 23:13:43.42, Dec = +01:21:19.8) is not detected in our image.
Our upper limits do not contradict the non-detections by other observations (Wu et. al, GCN 41244; Fortin et. al, GCN 41245; Freeberg et. al, GCN 41247; Xin et. al, GCN 41250; Zheng et. al, GCN 41251; Schneider et. al, GCN 41253).
Indeed the bright galaxy might be the host of GRB 250806A (Malesani et al., GCN 41279). Further comparison of the early observation is necessary and the SN monitoring.
GCN Circular 41279
Subject
GRB 250806A: NOT z-band upper limit and host galaxy candidate
Date
2025-08-08T08:32:02Z (2 months ago)
From
Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani@nbi.ku.dk>
Via
Web form
D. B. Malesani (DAWN/NBI and Radboud), L. Izzo (INAF/OAC and DARK/NBI), A. Martin-Carrillo (UCD), G. Corcoran (UCD), Dimple (Birmingham), A. Saccardi (CEA/Irfu), B. Schneider (LAM), D. Xu (NAOC) report on behalf of a larger collaboration:
We observed the location of the SVOM/ECLAIRs GRB 250806A (Xie et al., GCN 41243) using the Nordic Optical Telescope (NOT) equipped with the ALFOSC camera. A total of 25 exposures by 120 s each were secured in the SDSS z band, with mean epoch 2025 Aug 7.00 UT (16.05 hr after the trigger).
There are two galaxies from the Legacy Survey that are consistent with the X-ray position currently listed at https://www.swift.ac.uk/xrt_positions/ (but slightly outside the preliminary error circle reported by Campana et al., GCN 41249).
The first one (RA = 23:13:43.51, Dec = +01:21:22.0) is relatively bright, with (AB) magnitudes from the Legacy survey g = 22.64, r = 21.75, i = 21.45, z = 21.15, and a photometric redshift z = 0.51 +/- 0.09. This object is marginally detected in our NOT stacked z-band image.
The second one (RA = 23:13:43.42, Dec = +01:21:19.8) is fainter (r = 24.10), and is not detected in the NOT image.
No other objects are seen consistent with the XRT position in the NOT z-band images, down to a limiting magnitude z > 22.4 AB, calibrated against nearby Pan-STARRS objects.
Given its low (<2%) chance-association probability with the X-ray source, the brighter galaxy is a promising host galaxy candidate for GRB 250806A, and we invite other observers (e.g., Wu et al., GCN 41244; Fortin et al., GCN 41245; Freeberg et al., GCN 41247; Zheng et al., GCN 41251; Schneider et al., GCN 41253) to examine their data and to monitor this galaxy in order to check for variability. A SN should be also detectable in the next couple of weeks, if the photometric redshift is approximately correct and there is no significant extinction.
We acknowledge expert support from the NOT observing staff, in particular Tapio Pursimo and Laura Fuglsang.
GCN Circular 41264
Subject
GRB 250806A: Swift/UVOT U-band detection
Date
2025-08-07T11:15:10Z (2 months ago)
From
Alice Breeveld at MSSL-UCL <a.breeveld@ucl.ac.uk>
Via
email
A.A. Breeveld (UCL-MSSL) and Paul Kuin (UCL-MSSL) report on behalf of the Swift/UVOT team:
The Swift/UVOT began settled observations of the field of the SVOM/ECLAIRS detected burst GRB 250806A (Xie et al., GCN Circ. 41243) 354 s after the SVOM trigger. The X-ray afterglow reported by Campana et al. (GCN Circ. 41249) and Liang et al. (GCN Circ. 41260) is detected in the initial U-band exposure, but has faded in the later white band exposures.
Preliminary magnitude and 3-sigma upper limit using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the initial exposures is:
Filter T_start(s) T_stop(s) Exp(s) Mag
u 354 1370 1000 20.5 ± 0.3
white 63123 64122 975 > 21.7
The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V)=0.044 in the direction of the burst (Schlegel et al. 1998).
GCN Circular 41260
Subject
GRB 250806A: EP-FXT observation
Date
2025-08-07T09:46:25Z (2 months ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
R. D. Liang, H. N. Yang (NAO, CAS), Y. J. Zhang (THU), W. D. Zhang (NAO, CAS) report on behalf of the Einstein Probe team:
GCN #41243 reported the detection of GRB 250806A with SVOM on 2025-08-06T08:00:11 UTC (T0) and the X-ray conterpart was detected with swift-XRT (GCN #41249).
The Follow-up X-ray Telescope (FXT) on board Einstein Probe observed GRB 250806A about 4 ks (2025-08-06T09:04:02, UTC) after T0, with an exposure time of 790 seconds. On-ground analysis of the FXT data found an uncatalogued source at R.A. = 348.4308, DEC =1.3556 (J2000) with an uncertainty of about 10 arcsec (radius, 90% C.L. statistical and systematic), which is consistent positionally with the swift-XRT transient (GCN #41249). The angular distance between the FXT position and SVOM-MXT detection (GCN #41243) is 106 arcseconds. The average 0.5-10 keV spectrum can be fitted with an absorbed power law with a Galactic hydrogen column density of 4.5 x 10^20 cm^-2 and a photon index of 1.25 (-1.14/+1.11). The derived average unabsorbed 0.5-10 keV flux is 1.49 (-0.98/+6.08) x 10^(-12) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters.
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 41253
Subject
GRB 250806A: J-band upper limit from WINTER
Date
2025-08-06T20:53:38Z (2 months ago)
Edited On
2025-08-07T13:18:19Z (2 months ago)
From
Benjamin Schneider at MIT <bschn@mit.edu>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Benjamin Schneider at MIT <bschn@mit.edu>
Via
Web form
Benjamin Schneider (LAM), Robert Stein (UMD), Geoffrey Mo (MIT), Tomas Ahumada (Caltech), Viraj Karambelkar (Caltech), Danielle Frostig (CfA), Nathan Lourie (MIT), Robert Simcoe (MIT), and Mansi Kasliwal (Caltech) report:
We observed the field of the SVOM GRB 250806A (Xie et al., GCN 41243; Wu et al., GCN 41244; Fortin et al., GCN 41245; Freeberg et al., GCN 41247; Campana et al., GCN 41249; Xin et al., GCN 41250; Zheng et al., GCN 41251) in the near-infrared with the Palomar 1-m telescope, equipped with the 1-square degree WINTER camera (Lourie et al. 2020, Frostig et al. 2024). Observations started on 2025-08-06 at 08:16:51 UT (18.62 min after the trigger) and consisted of 15 exposures of 120 s in the J-band.
In the stacked image, we do not detect any new source at the X-ray afterglow position reported by Xie et al., GCN 41243 and Campana et al., GCN 41249 down to the following 3-sigma AB magnitude:
J > 19.7
The images were processed using the WINTER data reduction pipeline implemented with mirar (https://github.com/winter-telescope/mirar, https://doi.org/10.5281/zenodo.13352565). The photometric calibration was performed using nearby stars from the 2MASS catalog and the magnitude is not corrected for Galactic extinction.
WINTER (Wide-field INfrared Transient ExploreR) is a partnership between MIT and Caltech, housed at Palomar Observatory, and funded by NSF MRI, NSF AAG, the David and Lucile Packard Foundation, and the MIT Kavli Institute for Astrophysics and Space Research.
GCN Circular 41251
Subject
GRB 250806A: KAIT optical upper limit
Date
2025-08-06T17:32:12Z (2 months ago)
From
Weikang Zheng at UC Berkeley <weikang@berkeley.edu>
Via
email
WeiKang Zheng (UCB), Xuhui Han (NAOC), Pinpin Zhang (NAOC) and
Alexei V. Filippenko (UCB) report on behalf of the KAIT GRB team:
The 0.76-m Katzman Automatic Imaging Telescope (KAIT), located at
Lick Observatory, observed the field of SVOM GRB 250806A (Xie
et al., GCN 41243) starting at 08:00:40, Aug. 06 UT, 146 seconds
after the burst. Observations were performed in 3 x 3 tiling mode,
and a set of clear (roughly R) filter images were obtained.
Preliminary analysis do not reveal any new optical counterpart
candidate within the Swift/XRT error circles (Campana et al., GCN
41249), neither in single image, nor in the co-add images. Our
typical limiting magnitude of single image is about ~19.5 mag,
with the first image covered Swift/XRT position at 237s after the
burst. Our result is consistent with the upper limit reported by
other groups (Wu et al., GCN 41244; Fortin et al., GCN 41245;
Freeberg et al., GCN 41247; Xin et al., GCN 41250).
GCN Circular 41250
Subject
GRB 250806A: SVOM/VT refined analysis
Date
2025-08-06T15:26:59Z (2 months ago)
From
Liping Xin at NAOC, SVOM <xlp@nao.cas.cn>
Via
Web form
L. P. Xin, H. L. Li, Y. L. Qiu, C. Wu, 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. Palmerio (CEA) report on behalf of the SVOM mission team.
SVOM/VT performed an automatic slew on the burst triggered by SVOM/ECLAIRs (Xie et al., GCN 41243) in the VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously.
No uncatalogued candidates were detected in our single or stacked images within the errorbox of the Swift/XRT position (Campana et al., GCN 41249) in the X-band downlinked data, comparing to Legacy survey. The 3 sigma upper limits are:
[T-T0, mid-time] | exposure time (s) | band | upper limit (AB)
----------- -----|-------------------|------|-----------------
377 | 9*50 | VT_B | 23.3
3293 | 59*50 | VT_B | 23.6
377 | 9*50 | VT_R | 22.9
3293 | 59*50 | VT_R | 23.2
Our photometry are not corrected for the Galactic extinction.
Our results are consistent with non-detection of optical candidate (Wu et al., GCN 41244, Fortin et al., GCN 41245, Freeberg et al., GCN 41247)
NIR follow-ups are encouraged to investigate the nature of the burst.
The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA, China), National Centre 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.
The Burst Advocate (BA) on shift for this alert is Wenjin Xie (xiewj@bao.ac.cn)
GCN Circular 41249
Subject
GRB 250806A: Swift-XRT counterpart detection
Date
2025-08-06T15:03:16Z (2 months ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
Via
email
S. Campana (INAF-OAB), A. Melandri (INAF-OAR), T. Sbarrato (INAF-OAB),
D.N. Burrows (PSU), M.A. Williams (PSU), S. Dichiara (PSU), J.P.
Osborne (U. Leicester), K.L. Page (U. Leicester) and P.A. Evans (U.
Leicester) reports on behalf of the Swift-XRT team:
We have analysed 972 s of XRT data for GRB 250806A, from 343 s to 1.4
ks after the SVOM/ECLAIRs trigger. The data are entirely in Photon
Counting (PC) mode. We find an uncatalogued X-ray source at RA, Dec =
348.4298, +1.3550 which is equivalent to:
RA (J2000): 23 13 43.16
Dec(J2000): +01 21 18.1
with an uncertainty of 3.7 arcsec (radius, 90% confidence).
The light curve can be modelled with a power-law decay with a decay
index of alpha=1.9 (+/-0.4).
A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.5 (+0.4, -0.3). The
best-fitting absorption column is 1.7 (+1.6, -1.2) x 10^21 cm^-2,
consistent with the Galactic value of 5.4 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 4.6 x 10^-11 (5.3 x
10^-11) erg cm^-2 count^-1.
A summary of the PC-mode spectrum is thus:
Total column: 1.7 (+1.6, -1.2) x 10^21 cm^-2
Galactic foreground: 5.4 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index: 1.5 (+0.4, -0.3)
If the light curve continues to decay with a power-law decay index of
1.9, the count rate at T+24 hours will be 2.4 x 10^-5 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.1 x
10^-15 (1.3 x 10^-15) erg cm^-2 s^-1.
The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00019983.
This circular is an official product of the Swift-XRT team.
GCN Circular 41247
Subject
GRB 250806A: Kilonova-Catcher optical upper limits
Date
2025-08-06T13:12:51Z (2 months ago)
From
Damien Turpin at CEA-Saclay <dturpin-astro@hotmail.com>
Via
Web form
M. Freeberg, R. Hellot (KNC), D. Turpin (CEA-Saclay/Irfu), C. Andrade(UMN), M. Pillas (ULiege), A. Simon, D. Zazubyk (ASPD TShNU of Kyiv), S. Antier (OCA/IJCLAB) on behalf of the GRANDMA/Kilonova-Catcher collaboration:
We observed the field of GRB 250806A (Xie et al., GCN 41243) detected by SVOM/ECLAIRs with the GRANDMA citizen science project Kilonova-catcher (KNC). Our observations were performed with the iTelescope T72 and TEC160FL telescopes operated by M. Freeberg and the CDK17 telescope located at AITP San Pedro Chile Observatory operated by R. Hellot. Our observations started at T0+43min.
In our stacked frames, subtracted from the PanSTARRS DR2 template image, we do not detect any optical counterpart inside the SVOM/MXT position (Xie et al., GCN 41243).
We report our follow-up results in the table below:
+---------------+-----------+------------+---------------------+--------------+
| Tmid-TGRB (hr)| Exp (s) | Filter | Magnitude | Instrument |
+===============+===========+============+=====================+==============+
| 1.39 | 5 x 600s | sdssr (AB) | 20.4 (U.L., 5 sigma) | CDK17-AITP |
| 1.46 | 16 x 180s | Rc (Vega) | 21.0 (U.L., 5 sigma) | iT72 |
| 2.27 | 18 x 300s | sdssr (AB) | 21.3 (U.L., 5 sigma) | TEC160FL |
+---------------+-----------+------------+----------------------+-------------+
All the data have been reduced by a single data processing pipeline, STDPipe (Karpov et al., 2022). Images obtained with the sloan filters were calibrated using the PanSTARRS DR1 catalog. Images obtained with the Johnson-cousins filters were calibrated using the GAIA DR3 synphot catalog.
We use the SkyPortal application (skyportal.io) to monitor our observational campaign (Coughlin et al. 2023).
GRANDMA is a worldwide telescope network (grandma.ijclab.in2p3.fr) devoted to the observation of transients in the context of multi-messenger astrophysics (Antier et al. 2020 MNRAS 497, 5518). Kilonova-Catcher (KNC) is the citizen science program of GRANDMA (http://kilonovacatcher.in2p3.fr/).
GCN Circular 41245
Subject
GRB 250806A: SVOM/COLIBRÍ (FM-GFT) optical upper limit
Date
2025-08-06T09:54:10Z (2 months ago)
From
F. Fortin at IRAP <ffortin.sci.edu@gmail.com>
Via
Web form
Francis Fortin (IRAP), Camila Angulo (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Alan M. Watson (UNAM), Fredd Alvarez (UNAM), Jean-Luc Atteia (IRAP), Dalya Akl (AUS), Sarah Antier (OCA), Rosa L. Becerra (U Roma), Nathaniel R. Butler (ASU) , Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Leonardo García García (UNAM), Noémie Globus (UNAM), Kin Ocelotl López (UNAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Benjamin Schneider (LAM), Antonio de Ugarte Postigo (LAM), W. J. Xie (NAOC), L. P. Xin (NAOC)
We imaged the field of the SVOM GRB 250806A (Xie et al., GCN Circ. 21243) using the DDRAGO wide-field imager on the SVOM/COLIBRÍ (FM-GFT) telescope. We observed from 2025-08-06 07:59:05 to 08:58:07 UTC (from 51 seconds to 59 minutes after the onboard trigger) and obtained 2560 seconds of exposure in the i-band filter.
The data were reduced and coadded with the COLIBRÍ pipeline and analyzed with STDWeb/STDPipe (Karpov 2025). The photometry was calibrated using nearby stars from the PanSTARRS DR1 catalog, is in the AB system, and is not corrected for Galactic extinction.
In the stacked image, we do not detect any new source at the MXT source position (Xie et al., GCN Circ. 21243) down to the following 3-sigma limit:
i > 23.44
This result is consistent with the non detection reported by SVOM/VT (Wu et al. GCN Circ.41244).
Further observations and analysis are ongoing.
We thank the staff of the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir and the COLIBRÍ and DDRAGO engineering teams.
COLIBRÍ is an astronomical observatory developed and operated jointly by France (AMU, CNES and CNRS) and Mexico (UNAM and SECIHTI). It is located at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir, Baja California, Mexico.
GCN Circular 41244
Subject
GRB 250806A: SVOM/VT optical upper limit
Date
2025-08-06T09:24:01Z (2 months ago)
Edited On
2025-08-06T14:11:13Z (2 months ago)
From
Huali Li at at NAOC, SVOM <lhl@nao.cas.cn>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Huali Li at at NAOC, SVOM <lhl@nao.cas.cn>
Via
Web form
C. Wu, L.P. Xin, Y. L. Qiu, H.L. Li, Z.H. Yao, Y.N. Ma, X.H. Han, J. Wang, W.J. Xie, Y. Xu, H.B. Cai, J. Y. Wei (NAOC), J. Palmerio (CEA) report on behalf of the SVOM team.
SVOM/VT performed an automatic slew on the burst triggered by SVOM/ECLAIRs (Xie et al., GCN 41243). SVOM/VT began observing the field automatically at 2025-08-06T08:02:01.000 UTC, 227 seconds after the T0, with the slew of the platform triggered on-board, in VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously.
With downlinked VHF data, no any candidates were found in stacked images within the errorbox of SVOM/MXT (Xie et al., GCN 41243) compared to Legacy survey.
The 3 sigma limit in AB magnitude was derived as follows:
---------------------------------------------------------
(T-T0)_mid(s) exptime(s) Band upperlim (3sigma)
377 300 VT_R 21.5
More deeper photometry in near-inferred is encouraged to investigate the nature of the burst.
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 41243
Subject
GRB 250806A: SVOM detection of a soft burst
Date
2025-08-06T08:39:15Z (2 months ago)
From
SVOM_group <svomgroup@bao.ac.cn>
Via
Web form
W. J. Xie (NAOC), L. P. Xin (NAOC), S. Schanne (CEA), L. Bouchet (IRAP), L. Zhang (IHEP), H. Goto (Kanazawa Univ.), P. Maggi (ObAS), report on behalf of the SVOM mission team:
At 2025-08-06T07:58:14 UTC (T0), SVOM/ECLAIRs triggered and located the gamma-ray
burst GRB 250806A (SVOM burst-id sb25080601). 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 15 alerts. IMT provided the alert with the best signal-to-noise-ratio in the image (SNR) of 10.48 in the 5-20 keV energy band over a time window of 20.48 seconds starting at 2025-08-06T07:58:04 (Tb).
The localization of the best alert is R.A., Dec. 348.4237, 1.3791 degrees (J2000) with a 90% confidence level (C.L.) radius of 7.62 arcmin (including systematic error of 2 arcmin added in quadrature). The transmitted ECLAIRs VHF light curve shows a single peak of a duration of about 20 seconds mainly in 5-20 keV and the GRM VHF light curve shows no clear signal. Future X-band data will provide more information on the burst duration and spectrum.
SVOM slewed to the burst.
SVOM/MXT began observing the field at 2025-08-06T08:00:11 UTC, 117 seconds after T0. Using onboard processed data we found an uncatalogued X-ray source located at R.A., Dec. 348.4214, 1.3820 degrees:
R.A. (J2000) = 23h13m41.1s
Dec. (J2000) = 01d22m55s
with a 90% C.L. radius of 85 arcseconds (including a systematic uncertainty of 25” added in quadrature).
This location is 0.22 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 Wenjin Xie (xiewj@bao.ac.cn).
Please contact the BA by email if you require additional information.