GRB 250704A
GCN Circular 40934
Subject
GRB 250704A: SVOM detection of a burst
Date
2025-07-04T05:09:08Z (a day ago)
From
SVOM_group <svomgroup@bao.ac.cn>
Via
Web form
J.X Cao (GXU), Y.H Cheng (SWIFAR, YNU), W.J Xie (NAOC), D.H Zhao (NAOC), N. Dagoneau (CEA), L. Zhang (IHEP), C. Van Hove (IJCLab), P. Maggi (ObAS) report on behalf of the SVOM mission team:
At 2025-07-04T03:42:22 UTC (T0), SVOM/ECLAIRs triggered and located the gamma-ray burst GRB 250704A (SVOM burst-id sb25070404).
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 11 alerts. CRT provided the alert with the best signal-to-noise-ratio (SNR) in the image of 11.88 in the [8-120] keV energy band over a time window of 5.10 seconds starting at 2025-07-04T03:42:20.
The localization of the best alert is R.A., Dec. 17.1325, -17.2798 degrees (J2000) with a 90% confidence level (C.L.) radius of 6.78 arcmin (including systematic error of 2 arcmin added in quadrature).
This burst also triggered SVOM/GRM at 2025-07-04T03:42:20 with a SNR of 9.20. The SVOM/GRM light curve showed a single peak structure with a duration of about 8 seconds.
The SVOM/GRM light curve can be found here: https://www.bursthub.cn//admin/static/svgrb250704A.png.
SVOM/MXT began observing the field at 2025-07-04T03:45:35 UTC, 192 seconds after T0. Using data rapidly transmitted to the ground via the VHF network, we found an uncatalogued X-ray source located at R.A., Dec. 17.1760, -17.3394 degrees:
R.A. (J2000) = 1h08m42.24s
Dec. (J2000) = -17d20m21.8s
with a 90% C.L. radius of 102 arcseconds (including a systematic uncertainty of 35 arcseconds added in quadrature).
This location is 4.36 arcminutes from the ECLAIRs onboard position. This position may be improved as more data is received in the full X-band dataset.
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 Jiaxin Cao: cjx@st.gxu.edu.cn.
Please contact the BA by email if you require additional information.
GCN Circular 40944
Subject
GRB 250704A: REM retraction of optical afterglow detection and updated limit
Date
2025-07-04T10:52:18Z (a day ago)
From
Riccardo Brivio at INAF-OAB <riccardo.brivio@inaf.it>
Via
Web form
R. Brivio, M. Ferro (INAF-OAB), A. Rossi (INAF-OAS), P. D'Avanzo, S. Covino, D. Fugazza (INAF-OAB) report on behalf of the REM team:
Further analysis of the optical data obtained with REM (Brivio et al. GCN Circ. 40938) reveals that the source identified as the optical afterglow of GRB 250704A (Cao et al., GCN Circ. 40934) is spurious, likely resulting from a cosmic ray. From further analysis of the field, we obtain a 3sigma upper limit of:
r > 18.1 (AB; calibrated against the Pan-STARRS catalogue),
at a mid-time of 1.7 hr after the trigger.
We apologize for any confusion this may have caused.
GCN Circular 40950
Subject
GRB 250704A: Swift-XRT afterglow detection
Date
2025-07-04T14:00:05Z (a day ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
Via
email
J.P. Osborne (U. Leicester), B. Sbarufatti (INAF-OAB), C. Salvaggio
(INAF-OAB), M. Ferro (INAF-OAB), M.A. Williams (PSU), S. Dichiara
(PSU), J.A. Kennea (PSU), K.L. Page (U. Leicester), A.P. Beardmore (U.
Leicester) and P.A. Evans (U. Leicester) reports on behalf of the
Swift-XRT team:
We have analysed 2.0 ks of XRT data for GRB 250704A, from 2.9 ks to 8.7
ks after the SVOM/ECLAIRs trigger. The data are entirely in Photon
Counting (PC) mode. A bright, fading source is detected inside the
SVOM/ECLAIRs error region (Cao et al., GCN 40934), which we propose as
the GRB afterglow. Using 2029 s of PC mode data and 2 UVOT images, we
find an enhanced XRT position (using the XRT-UVOT alignment and
matching UVOT field sources to the USNO-B1 catalogue): RA, Dec =
17.19414, -17.32397 which is equivalent to:
RA (J2000): 01h 08m 46.59s
Dec(J2000): -17d 19' 26.3"
with an uncertainty of 2.8 arcsec (radius, 90% confidence).
The light curve can be modelled with a power-law decay with a decay
index of alpha=1.3 (+0.6, -0.5).
A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.03 (+0.31, -0.28). The
best-fitting absorption column is 6.0 (+6.7, -4.5) x 10^20 cm^-2,
consistent with the Galactic value of 1.6 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 3.2 x 10^-11 (3.8 x
10^-11) erg cm^-2 count^-1.
A summary of the PC-mode spectrum is thus:
Total column: 6.0 (+6.7, -4.5) x 10^20 cm^-2
Galactic foreground: 1.6 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index: 2.03 (+0.31, -0.28)
If the light curve continues to decay with a power-law decay index of
1.3, the count rate at T+24 hours will be 2.4 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 7.9 x
10^-14 (9.1 x 10^-14) erg cm^-2 s^-1.
The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00019907.
This circular is an official product of the Swift-XRT team.
GCN Circular 40953
Subject
GRB 250704A: VLT/X-shooter afterglow discovery and spectroscopic redshift z = 1.091
Date
2025-07-04T14:56:37Z (a day ago)
From
Benjamin Schneider at MIT <bschn@mit.edu>
Via
Web form
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), D. B. Malesani (DAWN/NBI and Radboud), A. L. Thakur (INAF-IAPS), S. D. Vergani (LUX-Paris Obs.) report on behalf of the Stargate collaboration:
We observed the field of GRB 250704A detected by SVOM/ECLAIRs (Cao et al., GCN 40934) with ESO VLT UT3 (Melipal) equipped with the X-shooter spectrograph. Our spectra cover the wavelength range 3000-21000 AA, and consist of 4 exposures of 600 s each. The observation mid time was 2025 Jul 4.41 UT (6.10 hr after the GRB).
In images taken with the acquisition camera, we clearly detect a source inside the error circle of the Swift/XRT X-ray afterglow (Osborne et al., GCN 40950) at coordinates (J2000):
RA = 01:08:46.69
Dec = -17:19:25.87
The source is well detected in the g, r, and z bands, and we measure a magnitude r = 20.48 +- 0.05 AB, at a mid time Jul 4.39 UT (5.59 hr after trigger). The photometric calibration is preliminary as it was performed using a single nearby star from the Pan-STARRS catalog. This source is significantly brighter than existing archival imaging, and we thus conclude that this is the optical afterglow of GRB 250704A.
In a preliminary reduction of the spectra, we detect a continuum over the entire covered wavelength range. From detection in the blue down to 3080 AA, and the lack of hydrogen absorption, we set a redshift upper limit z < 1.53. Furthermore, we detect multiple absorption lines, which we interpret as Al II, Fe II and the Mg II doublet, at a common redshift of z = 1.091. Three intervening Mg II absorbers are also detected at z = 0.65, z = 0.71, and z = 0.91. No emission lines are observed across the spectrum.
Due to the lack of detection of fine-structure transitions, z = 1.091 is strictly a lower limit to the GRB redshift, but the absence of unidentified features and the reasonable S/N over the whole continuum suggest z = 1.091 as the actual value of the redshift.
We acknowledge excellent support from the ESO observing staff in Paranal, in particular Robert Klement, Diego Parraguez, and Leonel Rivas.
GCN Circular 40964
Subject
GRB 250704A: SVOM/VT optical observation
Date
2025-07-05T03:16:00Z (12 hours ago)
From
SVOM_group <svomgroup@bao.ac.cn>
Via
Web form
J. X. Cao(GXU), Y. H. Cheng(SWIFAR,YNU), 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), J. Palmerio (CEA) report on behalf of the SVOM/VT team
SVOM/VT performed an automatic slew on the burst GRB 250704A (sb25070404) triggered by SVOM/ECLAIRs (Cao et al., GCN 40934) in the VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously.
Part of X band data was received. The optical afterglow (Schneider et al., GCN 40953) within the errorbox of Swift/XRT (Osborne et al., GCN 40950) and SVOM/MXT (Cao et al., GCN 40934) was clearly detected in both VT_B and VT_R images.
The magnitudes are VT_B=20.80+/-0.05 mag and VT_R=20.52+/-0.05 mag in AB magnitude at the mid time of 8.3 hours post the burst.
Our photometry was not corrected for Galactic extinction.
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 SVOM/VT point of contact for this burst is: Jiaxin Cao: cjx@st.gxu.edu.cn.