GRB 250704B, EP250704a
GCN Circular 41060
Subject
GRB 250704B / EP250704a: 1.3 GHz MeerKAT Detection
Date
2025-07-10T20:58:22Z (a year ago)
From
Genevieve Schroeder at Cornell University <genevieveschroeder@u.northwestern.edu>
Via
Web form
G. Schroeder (Cornell), L. Rhodes (TSI/McGill), W. Fong (Northwestern), T. Laskar (Utah), E. Berger (Harvard) report:
We observed the location of the short-duration GRB 250704B/EP 250704A (Wang et al., GCN 40940; Li et al., GCNs 40941, 40956, Frederiks et al., GCN 40972; Wang et al., GCN 40978; Shimizu et al., GCN 41025) with the MeerKAT radio telescope under program MKT-24032 (PI Schroeder) at a central frequency of 1.3 GHz for a total of 2 hours with a mid time of 2025 July 10 at 00:07 UT (5.7 days post burst).
In preliminary analysis, we detect the radio afterglow (Schroeder et al., GCN 41038; Ricci et al., GCN 41046), and measure a flux density of ~70 microJy. Further observations are planned.
We thank the staff at the South African Radio Astronomy Observatory for scheduling these observations.
GCN Circular 41050
Subject
IPN triangulation of GRB 250704B (short/hard, consistent with EP 250704a)
Date
2025-07-09T15:26:13Z (a year 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,
Y. Zhang, C. Wang, S. Xiong, J. Wei, and B. Cordier
on behalf of the SVOM-GRM team,
C. Wang, S. Xiong, S. Zheng and Y. Zhang,
on behalf of the Insight-HXMT team,
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 bright, short-duration GRB 250704B
(SVOM/GRM observation: Wang et al., GCN 40940;
Konus-Wind detection: Frederiks et al., GCN 40972;
Insight-HXMT detection: Wang et al., GCN 40978;
CALET/CGBM detection: Shimizu et al., GCN 41025)
was detected by SVOM(GRB), Konus-Wind, Insight (HXMT),
CALET (CGBM), and Mars-Odyssey (HEND) at about 29791 s UT (08:16:31).
We have triangulated it to a preliminary, 3 sigma error box
whose coordinates are:
---------------------------------------------
RA(2000), deg Dec(2000), deg
---------------------------------------------
Center:
300.880 (20h 03m 31s) +12.053 (+12d 03' 11")
Corners:
300.926 (20h 03m 42s) +12.075 (+12d 04' 30")
300.872 (20h 03m 29s) +12.109 (+12d 06' 33")
300.834 (20h 03m 20s) +12.031 (+12d 01' 52")
300.888 (20h 03m 33s) +11.997 (+11d 59' 49")
---------------------------------------------
The error box area is 19 sq. arcmin, and its maximum
dimension is 7 arcmin (the minimum one is 4 arcmin).
The Sun distance was 141 deg.
This localization may be improved.
The position of EP250704a (Li et al., GCNs 40941, 40956)
is consistent with the IPN localization, supporting
the association of the GRB and EP 250704a.
A triangulation map and HEALPix FITS file are posted at
http://www.ioffe.ru/LEA/GRBs/GRB250704_T29791/IPN/
The HEALPix triangulation map is the multi-order HEALPix in units of
probability density.
GCN Circular 41046
Subject
EP250704a/GRB 250704B: 10 GHz VLA detection
Date
2025-07-09T04:57:07Z (a year ago)
From
Roberto Ricci at INAF-IRA <ricci@ira.inaf.it>
Via
Web form
Roberto Ricci, Rosa L. Becerra, Eleonora Troja (Rome U.) report on behalf of the ERC BHianca team:
We observed the field of GRB 250704B/EP250704a (Wang et al. GCN 40940; Li et al. GCN 40491) with the Jansky Very Large Array (VLA) at the center frequency of 10 GHz (with a bandwidth of 4 GHz) in C-array configuration on 2025 July 8th at mid observing time 07:53 UT (3.98 days after burst).
A radio source was detected within the error circle of optical transient position (Schneider et al. GCN 40942) with a flux density of 92 +/- 7 microJy.
This is consistent with the radio detection reported by Schroeder et al. GCN 41038.
Further observations are planned.
We thank the VLA staff for executing the observations.
GCN Circular 41038
Subject
GRB 250704B / EP250704a: VLA radio detection
Date
2025-07-08T17:46:51Z (a year ago)
From
Genevieve Schroeder at Cornell University <genevieveschroeder@u.northwestern.edu>
Via
Web form
G. Schroeder (Cornell), J. Rastinejad, W. Fong (Northwestern), T. Laskar (Utah) report:
We observed the location of the short-duration GRB 250704B/EP 250704A (Wang et al., GCN 40940; Li et al., GCNs 40941, 40956, Frederiks et al., GCN 40972; Wang et al., GCN 40978; Shimizu et al., GCN 41025) with the Karl G. Jansky Very Large Array (VLA) in C configuration under program 25A-063 (PI Schroeder) at a mid time of 2025 July 8 at 09:57 UT (4.1 days post-burst) for 1.25 hours at a mean frequency of 6 GHz.
In preliminary analysis, we detect a 20-sigma radio source with a flux density of ~150 microJy at the position:
RA(J2000) = 20:03:29.511
Dec(J2000) = +12:01:23.31
with an uncertainty of ~0.2" in each coordinate. This position is consistent with the X-ray position (Li et al., GCNs 40941, 40956; Evans et al. GCN 40951, Salvaggio et al. GCN 40987) and optical position (Schneider et al., GCN 40942). At z=0.661 (An et al. GCN 40966), this corresponds to a rest-frame luminosity of ~ 2e30erg/s/Hz, on the bright end of typical short-duration Gamma-ray burst radio afterglow luminosities at a similar rest-frame time (e.g., Laskar et al. 2022). Further observations are planned to assess the variability of the radio source and its connection to GRB 250704B and EP250704A.
We thank the VLA staff for quickly approving and executing these observations.
GCN Circular 41030
Subject
EP250704a/GRB250704B : HCT optical and NIR upper limits
Date
2025-07-08T15:06:08Z (a year ago)
From
V. Swain at IIT Bombay <vishwajeet.s@iitb.ac.in>
Via
Web form
V. Swain (IITB), D. Eappachen (IIA), T. Mohan (IITB), A.P. Saikia (IITB), D.K. Sahu (IIA),
A. Balasubramanian (IIA), G. C. Anupama (IIA), V. Bhalerao (IITB), S. Barway (IIA),
M. Nayana (IAO), S. Bandari (IAO) :
We observed the field of EP250704a (Li et al., GCN 40941) also detected by SVOM (GRB 250704B; Wang et al., GCN 40940), with the 2.0m Himalayan Chandra Telescope (HCT) of the Indian Astronomical Observatory (IAO). We obtained multiple exposures in J and r' filters. We obtained the following upper limits:
| JD (mid) | Date-Time (UTC)| Filter | Total Exposure (s) | tmid-t0 (days) | 5-sigma Upper Limit (AB) |
| ---------------- |---------------- | ------ | ------------------ | -------------- | ---------------- |
| 2460862.386666 | 2025-07-05T21:16:47.94 | J | 1170 | 1.54 | 18.9 |
| 2460864.216936 | 2025-07-07T17:12:23.27 | r' | 3600 | 3.37 | 21.8 |
The magnitudes are calibrated against PanSTARRS DR1 for SDDS r' filter (Chambers et al., 2016) and against 2MASS catalog (Skrutskie et al., 2006) for J-filter. The magnitudes are not corrected for Galactic extinction.
The result is consistent with other optical observations.
These observations were carried out under the ToO program HCT-2025-C2-P010. We thank the HCT staff for their support during the observations. The Indian Astronomical Observatory is operated by the Indian Institute of Astrophysics, Bengaluru, India.
GCN Circular 41025
Subject
GRB 250704B: CALET Gamma-Ray Burst Monitor detection
Date
2025-07-08T13:15:29Z (a year ago)
From
Yuta Kawakubo at Aoyama Gakuin University <kawakubo@phys.aoyama.ac.jp>
Via
Web form
Y. Shimizu (Kanagawa U), A. Yoshida, T. Sakamoto, S. Sugita, Y. Kawakubo (AGU),
K. Yamaoka (Nagoya U), S. Nakahira (JAXA), Y. Asaoka (ICRR), S. Torii, Y. Akaike,
K. Kobayashi (Waseda U), T. Tamura (Kanagawa U), N. Cannady (GSFC/UMBC),
M. L. Cherry (LSU), S. Ricciarini (U of Florence), P. S. Marrocchesi (U of Siena),
and the CALET collaboration:
The short GRB 250704B (SVOM/GRM observation: Wang et al., GCN 40940;
Konus-Wind detection: Frederiks et al., GCN 40972; Insight-HXMT detection:
Wang et al., GCN 40978) associated with the EP X-ray transient EP250704a
(Einstein Probe detection: Li et al., GCN 40941; refined analysis of the EP-WXT
and FXT follow-up observations: Li et al., GCN 40956) triggered the CALET Gamma-ray
Burst Monitor (CGBM) at 08:16:26.88 UTC on 4 July 2025 and the GCN/CALET Notice
was distributed near real-time
(https://cgbm.calet.jp/cgbm_trigger/flight/1435651911/index.html).
The burst signal was seen by only the SGM detector.
The burst light curve shows a double-peaked structure that starts
at T+0.21 sec, peaks at T+0.37 sec, and ends at T+0.51 sec.
The T90 and T50 durations measured by the SGM data are 0.28 +/- 0.03 sec
and 0.23 +/- 0.04 sec (40-1000 keV), respectively.
The ground-processed light curve is available at
https://cgbm.calet.jp/cgbm_trigger/ground/1435651911/
The CALET data used in this analysis are provided by
the Waseda CALET Operation Center located at Waseda University.
GCN Circular 41024
Subject
GRB 250704B / EP250704a: Mondy and AbAO optical observations
Date
2025-07-08T12:26:15Z (a year ago)
From
Alexei Pozanenko at IKI, Moscow <grb.alex@gmail.com>
Via
email
A. Volnova (IKI), A. Pozanenko (IKI), E. Klunko (ISTP), R. Ya. Inasaridze
(AbAO), N. Pankov (HSE) report on behalf of GRB IKI FuN:
We observed the field of the EP250704a (Li et al., GCN 40941) also detected
by SVOM (GRB 250704B; Wang et al., GCN 40940) with AZT-33IK telescope of
Mondy observatory in R-filter on 2025-07-05 and with AS-32 telescope of
Abastumani observatory on 2025-07-05. The optical afterglow (Li et al.,
GCNs 40941; Schneider et al., GCN 40942; Malesani et al., GCN 40945; Brivio
et al., GCN 40947; Evans et al., GCN 40951; Perez-Garcia et al., GCN 4957;
Gillanders et al., GCN 40958; Xin et al., GCN 40960; Mohan et al., GCN
40962; Moskvin et al., GCN 40963; Liu et al., GCN 40965; An et al., GCN
40966; Yang et al., GCN 4970; Martin-Carillo et al., GCN 40971; Frederiks
et al., GCN 40972,Malte Busmann et al., GCN 40974, Li et al., GCN40975, Li
et al., GCN 40986; An et al., GCN 40993; Wang et al., GCN 40999).
We detect the afterglow on 2025-07-05. Preliminary photometry of a stacked
image is the following
Date UT start t-T0 Exp. Filter OT Err. UL(3sigma) Tel.
(mid, days) (s)
2025-07-05|16:34:49| 1.36693| 30*120 |R|21.06 |0.05| 22.9 |AZT-33IK
2005-07-06|19:39:17| 2.51241| 52*60 |R|n/d |- | 21.2 |AS-32
The photometry is based on nearby reference stars from PS1 (Lupton
transformations)
RA Dec R_Lupton
20:03:37.87 +12:01:59.6 17.385
20:03:26.50 +12:02:18.5 17.436
20:03:24.72 +11:59:09.1 16.41
GCN Circular 40999
Subject
EP250704A: NOWT Detection
Date
2025-07-07T03:57:17Z (a year ago)
From
Abdusamatjan Iskandar at XAO,CAS <abudu@xao.ac.cn>
Via
Web form
Letian Wang, Abdusamtjan Iskandar, Ali Esamdin, Yu Zhang, Jinzhong Liu, report on behalf of optical group of the XAO:
We observed the filed of EP250704a (Li et al., GCNs 40941, 40956; Schneider et al., GCN 40942; Malesani et al., GCN 40945; Brivio et al., GCN 40947; Evans et al., GCN 40951; Perez-Garcia et al., GCN 4957; Gillanders et al., GCN 40958; Xin et al., GCN 40960; Mohan et al., GCN 40962; Moskvin et al., GCN 40963; Liu et al., GCN 40965; An et al., GCN 40966; Yang et al., GCN 4970; Martin-Carillo et al., GCN 40971; Frederiks et al., GCN 40972,Malte Busmann et al., GCN 40974, Li et al., GCN40975, Li et al., GCN 40986) with the Nanshan One-meter Wide-field Telescope (NOWT) at Nanshan Station of Xinjiang Astronomical Observatory (XAO). The observations were started at 2025-07-04T18:33:06.0 (UTC, 0.43 days after the trigger), and obtained a series of V band images.
The optical afterglow was clearly detected at the position reported by Schneider et al.(GCN 40942) in the our stacked image, the preliminary photometric magnitudes is V=19.98 +- 0.16 mag (MJD 60860.7729, 6X300s).
The photometric calibration was performed using nearby stars from the GAIA synthesis catalog and the magnitude is not corrected for Galactic extinction.
GCN Circular 40993
Subject
EP250704a / GRB 250704B: end of optical plateau phase and rapid fading of the counterpart
Date
2025-07-06T20:50:24Z (a year ago)
From
Antonio Martin-Carrillo at UCD,Space Science Group <antonio.martin-carrillo@ucd.ie>
Via
Web form
J. An (NAOC), A. Martin-Carrillo (UCD), D. B. Malesani (DAWN/NBI and Radboud), X. Liu (NAOC), A. K. Haris-Kiss (Helsinki Univ.), M. Korpi-Lagg (Aalto Univ.), S. Wedemeyer (Univ. Oslo), A. Casasbuenas Corral (IAC), report on behalf of a larger collaboration:
We performed further observations of the field of the EP250704a (Li et al., GCN 40941) also detected by SVOM (GRB 250704B; Wang et al., GCN 40940), using the StanCam camera mounted on the Nordic Optical Telescope (NOT). We obtained 3x300 s exposures in the Bessel-R and I bands, starting at 2025-07-05 at 23:49:00 UT (~39.54 hr after the EP trigger).
In our stacked image, the optical afterglow detected by COLIBRÍ (Schneider et al., GCN 40942), BOOTES-5 (Perez-Garcia et al., GCN 40957), Pan-STARRS (Gillanders et al., GCN 40958), SVOM/VT (Xin et al., GCN 40960), GROWTH-India (Mohan et al., GCN 40962), SAO RAS (Moskvitin et al., GCN 40963), JinShan (Liu et al., GCN 40965), VLT/X-shooter (An et al., GCN 40966), VLT/HAWK-I (Yang et al., GCN 40970), NOT (Martin-Carrillo et al., GCN 40971), FTW (Busmann et al., GCN 40974) and GSP (Li et al., GCN 40975), the optical counterpart is detected at a preliminary magnitude of:
i = 21.56 +/- 0.14 (AB),
calibrated against nearby Pan-STARRS sources and not corrected for Galactic extinction.
Compared to our previous measurement (Martin-Carrillo et al., GCN 40971), the new magnitude indicates a rapid decay (effective power-law index ~ 1.8), in stark contrast with the flat / rising behavior observed over the first ~16 hr after the trigger (e.g., Gillanders et al., GCN 40958; Yang et al., GCN 40970; Martin-Carrillo et al., GCN 40971; Busmann et al., GCN 40974).
GCN Circular 40987
Subject
GRB 250704B: Swift-XRT refined Analysis
Date
2025-07-06T11:00:24Z (a year ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
Via
email
C. Salvaggio (INAF-OAB), M. Ferro (INAF-OAB), R. Brivio (INAF-OAB), S.
Dichiara (PSU), J.A. Kennea (PSU), D.N. Burrows (PSU), K.L. Page (U.
Leicester), A.P. Beardmore (U. Leicester),
E. Ambrosi (INAF-IASF PA) and P.A. Evans (U. Leicester) reports on
behalf of the Swift-XRT team:
We have analysed 3.3 ks of XRT data for GRB 250704B, from 2.0 ks to
97.7 ks after the Einstein Probe/WXT trigger. The data are entirely in
Photon Counting (PC) mode. Using 1717 s of PC mode data and 1 UVOT
image, we find an enhanced XRT position (using the XRT-UVOT alignment
and matching UVOT field sources to the USNO-B1 catalogue): RA, Dec =
300.87238, +12.02301 which is equivalent to:
RA (J2000): 20h 03m 29.37s
Dec(J2000): +12d 01' 22.9"
with an uncertainty of 2.7 arcsec (radius, 90% confidence).
The light curve can be modelled with a power-law decay with a decay
index of alpha=0.9 (+0.4, -0.3).
A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.96 (+0.16, -0.13). The
best-fitting absorption column is consistent with the Galactic value
of 1.5 x 10^21 cm^-2 (Willingale et al. 2013). The counts to observed
(unabsorbed) 0.3-10 keV flux conversion factor deduced from this
spectrum is 3.6 x 10^-11 (4.3 x 10^-11) erg cm^-2 count^-1.
A summary of the PC-mode spectrum is thus:
Total column: 1.47 (+/-0.25) x 10^21 cm^-2
Galactic foreground: 1.5 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index: 1.96 (+0.16, -0.13)
The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00019908.
This circular is an official product of the Swift-XRT team.
GCN Circular 40982
Subject
GRB 250704B: Swift/UVOT Upper Limits
Date
2025-07-05T18:41:26Z (a year ago)
Edited On
2025-07-07T13:30:09Z (a year ago)
From
Mike Siegel at PSU/Swift MOC <mhs18@psu.edu>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of 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 250704B 2.0 ks after the SVOM/GRM trigger (Wang et al., GCN Circ. 40940). No optical afterglow consistent with the XRT position (Evans et al., GCN Circ. 40951) or the reported optical counterparts (Malesani et al., GCN Circ. 40945; Gillanders et al., GCN Circ. 40958; Xin et al., GCN Circ. 40960) is detected in the initial UVOT exposures.
Preliminary 3-sigma upper limits using the UVOT photometric system
(Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) are:
Filter T_start(s) T_stop(s) Exp(s) Mag
u 2024 7891 1944 >19.59
The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.115 in the direction of the burst
(Schlegel et al. 1998).
GCN Circular 40978
Subject
GRB 250704B: Insight-HXMT detection
Date
2025-07-05T15:09:12Z (a year ago)
From
Chenwei Wang at IHEP <cwwang@ihep.ac.cn>
Via
Web form
Chen-Wei Wang, Shao-Lin Xiong, Cheng-Kui Li, and Chao Zheng report on behalf of the Insight-HXMT team:
At 2025-07-04T08:16:27.100 (T0), Insight-HXMT/HE detected the short burst GRB 250704B/ EP250704a, which is also detected by SVOM/GRM (Wang et al., GCN #40940), Konus-Wind (Dmitry Frederiks et al., GCN # 40972) and EP (Li et al., GCN # 40941).
The Insight-HXMT/HE light curve mainly consists of double pulses with a T90 of 0.68 +0.34/-0.48 s.
The total counts from this burst is 2046 counts.
The HXMT/HE light curve can be found here:
https://www.bursthub.cn//admin/static/hxmtgrb250704B.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 40975
Subject
EP250704a/GRB 250704B: GSP observations of the optical counterpart
Date
2025-07-05T12:56:03Z (a year ago)
From
Wenxiong Li at NAOC <liwenxiong1992@gmail.com>
Via
Web form
W. X. Li, S. J. Xue (NAOC), M. Andrews, J. Farah, D. A. Howell, M. Newsome, E. Padilla Gonzalez, C. McCully, and G. Terreran (Las Cumbres Observatory), on behalf of a larger collaboration, report:
Following the detection of the fast X-ray transient EP250704a/GRB 250704B (Li et al., GCN 40491, 40956; Wang et al., GCN 40940; Frederiks et al., GCN 40972), we initiated observations of the fast X-ray transient location starting on July 05 at 4:16 UT (~20 hours after the EP/WXT trigger) in the g, r and i bands. These observations were conducted using the 1-meter telescope at the Las Cumbres Observatory node located at the Cerro Tololo Inter-American Observatory in Chile.
The optical counterpart, detected by COLIBRÍ (Schneider et al., GCN 40942), VLT/FORS2 (Malesani
et al., GCN 40945), BOOTES-5 (Pérez-García et al., GCN 40957), Pan-STARRS (Gillanders et al., GCN 40958), SVOM/VT (Xin et al., GCN 40960), GROWTH-India (Mohan et al., GCN 40962), SAO RAS (Moskvitin et al., GCN 40963), JinShan (Liu et al., GCN 40965), VLT/X-shooter (An et al., GCN 40966), VLT/HAWK-I (Yang et al., GCN 40970), and NOT (Ricci et al., GCN 40970), is clearly detected in all LCO exposures, with preliminary magnitudes of g ≈ 20.2, r ≈ 20.1, and i ≈ 20.0 (AB mag).
These observations were taken as part of the Global Supernova Project.
GCN Circular 40974
Subject
EP250704a/GRB 250704B: FTW optical and NIR observations of the counterpart
Date
2025-07-05T12:53:11Z (a year ago)
From
Malte Busmann at Ludwig-Maximilians-Universität München <m.busmann@physik.lmu.de>
Via
Web form
Malte Busmann (LMU), Xander Hall (Carnegie Mellon U.), Brendan O’Connor (Carnegie Mellon U.), Daniel Gruen (LMU), and Antonella Palmese (Carnegie Mellon U.) report:
We observed the counterpart of EP250704a/GRB 250704B (Wang et al., GCN 40940; Li et al., GCNs 40941, 40956; Schneider et al., GCN 40942; Malesani et al., GCN 40945; Brivio et al., GCN 40947; Evans et al., GCN 40951; Perez-Garcia et al., GCN 4957; Gillanders et al., GCN 40958; Xin et al., GCN 40960; Mohan et al., GCN 40962; Moskvin et al., GCN 40963; Liu et al., GCN 40965; An et al., GCN 40966; Yang et al., GCN 4970; Martin-Carillo et al., GCN 40971; Frederiks et al., GCN 40972) with the Three Channel Imager (3KK) at the Fraunhofer Telescope at Wendelstein Observatory (FTW) in the r, i and J band simultaneously for 10 x 180 s starting at 2025-07-04T20:50:45 UT and again at 2025-07-05T01:28:10 UT (0.52 and 0.72 days after the trigger). The counterpart is detected in the individual images. In our second observation we detect it at:
r = (20.2 +/- 0.1) mag,
i = (19.9 +/- 0.1) mag,
J = (19.4 +/- 0.2) mag.
Consistent with previous observations, we do not see a significant evolution in time or color between our two epochs.
The r and i band magnitudes are calibrated against the PS1 catalog and the J band is calibrated with the 2MASS Catalog. All magnitudes are provided in the AB system and are not corrected for Galactic extinction.
We thank Silona Wilke from the Wendelstein Observatory staff for obtaining these observations.
GCN Circular 40972
Subject
Konus-Wind detection of short/hard GRB 250704B/ EP250704a
Date
2025-07-05T11:37:01Z (a year 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 short-duration GRB 250704B
(SVOM/GRM observation: Wang et al. GCN 40940;)
associated with the EP X-ray transient EP250704a
(Li et al. GCN 40491, 40956) triggered Konus-Wind (KW)
at T0=29791.045 s UT (08:16:31.045).
The burst shows multiple emission pulses and has a total duration of ~0.4 s.
The emission is seen up to ~8 MeV.
The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB250704_T29791/
As observed by Konus-Wind, the burst had
a fluence of (4.24 ± 0.65)x10^-6 erg/cm^2 and
a 16-ms peak energy flux, measured from T0 + 0.240 s,
of (5.82 ± 0.89)x10^-5 erg/cm^2/s (both in the 20 keV - 10 MeV energy range).
The time-integrated spectrum (measured from T0 to T0+0.256 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 = -1.17 (-0.08,+0.09),
the high energy photon index beta = -2.48 (-1.91,+0.39),
the peak energy Ep = 935(-197,+305) keV,
chi2 = 53/44 dof.
Assuming the redshift z=0.661 (An et al., GCN 40966)
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 (5.15 ± 0.79)x10^51 erg,
the isotropic peak luminosity L_iso to (1.17 ± 0.18)x10^53 erg/s, and
the rest-frame peak spectral energy Ep,z to 1550(-330,+510) keV.
With the obtained estimates, the position of GRB 250704B
is consistent with Type I (short/hard) GRB population in both 'Amati' and 'Yonetoku'
diagrams for the sample of KW GRBs with known redshifts
(Tsvetkova et al., 2017; Tsvetkova et al., 2021),
see http://www.ioffe.ru/LEA/GRBs/GRB250704_T29791/GRB250704B_rest_frame.pdf
All the quoted errors are estimated at the 68% confidence level.
All the presented results are preliminary.
GCN Circular 40971
Subject
EP250704a / GRB 250704B: NOT observations of the optical afterglow
Date
2025-07-05T11:11:01Z (a year ago)
From
Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani@nbi.ku.dk>
Via
Web form
A. Martin-Carrillo (UCD), D. B. Malesani (DAWN/NBI and Radboud), X. Liu (NAOC), A. K. Haris-Kiss (Helsinki Univ.), M. Korpi-Lagg (Aalto Univ.), S. Wedemeyer (Univ. Oslo), A. Casasbuenas Corral (IAC), report on behalf of a larger collaboration:
We observed the field of the EP250704a (Li et al., GCN 40941) also detected by SVOM (GRB 250704B; Wang et al., GCN 40940), using the StanCam camera mounted on the Nordic Optical Telescope (NOT). We obtained 6x300 s exposures in the i band, starting at 2025-07-05 at 00:03:42.7 UT (15.78 hr after the EP trigger).
The optical afterglow detected by COLIBRÍ (Schneider et al., GCN 40942), BOOTES-5 (Perez-Garcia et al., GCN 40957), Pan-STARRS (Gillanders et al., GCN 40958), SVOM/VT (Xin et al., GCN 40960), GROWTH-India (Mohan et al., GCN 40962), SAO RAS (Moskvitin et al., GCN 40963), JinShan (Liu et al., GCN 40965), VLT/X-shooter (An et al., GCN 40966), and VLT/HAWK-I (Yang et al., GCN 40970) is well visible in all individual images with a magnitude
i = 19.78 +/- 0.05 (AB),
calibrated against nearby Pan-STARRS sources and not corrected for Galactic extinction.
Our i-band magnitude suggests a possible re-brightening when compared to the value reported by Liu et al. (GCN 40958), recorded about 10.29 hr after the trigger, and in general confirms the flat, and unusual, behavior of the early optical light curve.
GCN Circular 40970
Subject
EP250704a/GRB 250704B VLT/HAWK-I NIR afterglow detection
Date
2025-07-05T10:56:20Z (a year ago)
From
Roberto Ricci at INAF-IRA <ricci@ira.inaf.it>
Via
Web form
Yu-Han Yang, Niccolo Passaleva, Rosa L. Becerra, Roberto Ricci, Eleonora Troja (U. Rome) report on behalf of the ERC BHianca team:
We observed the field of GRB 250704B/EP250704a (Wang et al. GCN 40940; Li et al. GCN 40491) with the HAWKI imager on the ESO VLT UT4 (Yepun) camera. Our observations were carried out approximately at T_0+20 hr in the filters J and Ks.
At the position of the optical afterglow (Schneider et al. GCN 40942; Malesani et al. 40945), the NIR counterpart is clearly detected in all filters at a preliminary magnitude of J~19.5 AB,
calibrated to nearby stars from 2MASS catalog (Skrutskie et al. 2006) and not corrected by the Galactic extinction.
This result is consistent with the relatively flat light curve inferred from other observations (Perez-Garcia et al., GCN 40957; Gillanders et al., GCN 40958; Xin et al., GCN 40960;
Mohan et al., GCN 40962).
We thank the staff at the VLT, especially Thomas Szeifert, for the rapid execution of these observations.
GCN Circular 40966
Subject
EP250704a / GRB 250704B: VLT/X-shooter spectroscopic redshift z = 0.661
Date
2025-07-05T06:15:41Z (a year ago)
From
J. An <jiean0813@foxmail.com>
Via
Web form
J. An (NAOC), D. B. Malesani (DAWN/NBI and Radboud), D. Xu, X. Liu, Z.P. Zhu (NAOC), B. Schneider (LAM), M. De Pasquale (Univ. of Messina), V. D’Elia (SSDC & INAF-OAR), A. L. Thakur (INAF/IAPS), D. Pieterse (Radboud), A. J. Levan (Radboud and Warwick), N. R. Tanvir (Univ. Leicester) report on behalf of the Stargate collaboration:
We observed the optical counterpart of EP250704a detected by EP/WXT (Li et al., GCN 40941), also likely by SVOM/GRM as GRB 250704B (Wang et al., GCN 40940), using the 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 1200 s each. The observation mid time was 2025-07-05 03:51:50.164 UT (19.58 hr after the GRB).
In the acquisition camera, the optical counterpart (first discovered by Schneider et al., GCN 40942) is well detected in the Sloan r- band, and we measure a magnitude r ~ 20.0 AB, at a mid time Jul 5.13 UT (18.8 hr after trigger), calibrated against nearby Pan-STARRS DR2 stars.
In a preliminary reduction of the spectra, we detect a continuum over the entire covered wavelength range (down to ~3100 AA at the blue end). From the detection of multiple absorption lines, which we interpret as due to the Mg II doublet, several Fe II lines and Mg I, we infer a redshift of z = 0.661. No emission lines are detected at this redshift.
We acknowledge excellent support from the ESO observing staff in Paranal, in particular Cedric Ledoux, Diego Parraguez, and Robert Klement.
GCN Circular 40965
Subject
EP250704a / GRB 250704B: JinShan optical observations
Date
2025-07-05T05:07:19Z (a year ago)
From
liuxing@nao.cas.cn
Via
Web form
X. Liu, J. An, Z.P. Zhu, S.Q. Jiang, L.B. He, D. Xu (NAOC), S.Y. Fu (HUST), J.Z. Liu (XAO) report on behalf of a large collaboration:
We observed the field of EP250704a detected by EP/WXT (Li et al., GCN 40941), also likely by SVOM/GRM as GRB 250704B (Wang et al., GCN 40940), using the 100cm-C telescope (100C) of the JinShan project, located at Altay, Xinjiang, China. Observations started at 18:01:00 UT on 2025-07-04, i.e., 9.74 hr after the trigger, and 12 x 300 s frames in Sloan i-band were obtained at a mid-time of 10.29 hr post-trigger.
The optical afterglow (e.g., Schneider at el., GCN 40942; Malesani at el., GCN 40945; Brivio at el., GCN 40947, etc.) was clearly detected in our stacked image, with a preliminary magnitude of i ~ 19.9, calibrated with nearby Pan-STARRS stars and without the Galactic extinction correction.
We acknowledge the excellent support from T.Q. Chen and J.L. He for enabling these observations.
GCN Circular 40963
Subject
EP250704a / GRB 250704B: SAO RAS optical observations
Date
2025-07-05T01:03:17Z (a year ago)
From
Alexander Moskvitin at SAO RAS <mosk@sao.ru>
Via
Web form
A. Moskvitin, O. Spiridonova (SAO RAS),
report on behalf of a GRB follow-up collaboration.
We observed the field of the GRB 250704B (Wang et al., GCN 40940),
EP250704a (Li et al., GCN 40941) with SAO RAS 1-m telescope
Zeiss-1000 equipped with CCD-photometer. We obtained 6 x 300 sec.
exposures in Rc band on July 4, 22:55:08--23:38:07 UT
(t_mid - T0 = 15.00 hours).
The OT (Schneider et al., GCN 40942; Malesani et al., GCN 40945;
Brivio et al., GCN 40947; Perez-Garcia et al., GCN 40957;
Gillanders et al., GCN 40958; Xin et al., GCN 40960;
Mohan et al., GCN 40962) is clearly detected in our stacked image
with the brightness of R = 20.80 +/- 0.18.
Preliminary photometry is based on the nearby USNO-B1.0 stars
and has not corrected for the Galactic extinction.
GCN Circular 40962
Subject
EP250704a / GRB 250704B : GROWTH-India Telescope optical observations
Date
2025-07-05T00:09:42Z (a year ago)
From
V. Swain at IIT Bombay <vishwajeet.s@iitb.ac.in>
Via
Web form
T. Mohan, V. Swain, A.P. Saikia, V. Bhalerao (IITB), G.C. Anupama, S. Barway (IIA) and K. Angail (IAO) report on behalf of the GIT team:
We observed the field of EP250704a (Li et al., GCN 40941) also detected by SVOM (GRB 250704B; Wang et al., GCN 40940), with 0.7m GROWTH-India Telescope (GIT). We obtained two epochs of multiple exposures in r' and i' filters. The first and second epochs started at 8.05 hours and 14.2 hours after the SVOM trigger. We detected a source at position reported by COLIBRÍ (Schneider et al., GCN 40942) and is within the uncertainty region of EP FXT localization (Li et al., GCN 40956). The photometry results are as follows:
| MJD (mid) | Filter | t-t0 (in hours) | Total Exposure Time (sec) | Magnitude (AB) |
| ------------ | ------ | --------------- | ------------------------- | -------------- |
| 60860.739443 | r' | 9.47 | 11 x 300 | 20.12 +/- 0.08 |
| 60860.936365 | r' | 14.2 | 5 x 300 | 19.80 +/- 0.09 |
| 60860.919386 | i' | 13.79 | 4 x 300 | 19.81 +/- 0.09 |
The measurement is calibrated against PanSTARRS DR1 (Chambers et al., 2016) and not corrected for Galactic extinction.
We see that the source has brightned in the r' band between our observations at 9.5 to 14.2 hours. The r'-i' colour is consistent with zero at 14.2 hours. The same trend is also seen when considering other reported values (Schneider et al., GCN 40942, Malesani et al., GCN 40945, Brivio et al., GCN 40947, Perez-Garcia et al., GCN 40957, Gillanders et al., GCN 40958, Xin et al., GCN 40960).
Given the proximity of the source to the galactic plane (latitude -10.4 deg), we cannot rule out the possibility that this may be a galactic source rather than a distant cosmological afterglow. Spectroscopic observations are strongly encouraged to ascertain the nature of this source.
The GROWTH India Telescope (GIT; Kumar et al. 2022) is a 70-cm telescope with a 0.7-degree field of view, set up by the Indian Institute of Astrophysics (IIA) and the Indian Institute of Technology Bombay (IITB) with funding from DST-SERB and IUSSTF. It is located at the Indian Astronomical Observatory (Hanle), operated by IIA. We acknowledge funding by the IITB alumni batch of 1994, which partially supports the operations of the telescope. Telescope technical details are available at https://sites.google.com/view/growthindia/.
GCN Circular 40960
Subject
EP250704a / GRB 250704B: SVOM/VT optical blue counterpart
Date
2025-07-04T21:08:26Z (a year 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, 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), J.X Cao (GXU), Y.H Cheng (SWIFAR, YNU) report on behalf of the SVOM/VT team.
SVOM/VT performed a Target of Opportunity observation of GRB 250704B（Wang et al., GCN 40940) and EP250704a (Li et al., GCN 40941) . The observation began at 2025-07-04T12:58:50 UTC , 4.68 hours after the trigger, in the VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously.
The afterglow (Schneider et al. GCN 40942, Malesani et al., GCN 40945, Gillanders et al., GCN 40958) within the errorbox of Swift/XRT (Evens et al., GCN 40951) and EP/FXT (Li et al., GCN 40956) was detected in both VT_B and VT_R images.
The brightness was about 20.4 mag in AB magnitude in both bands with a VT color of roughly zero, indicating a blue optical source.
Our photometry was not corrected for Galactic extinction.
More follow-ups are encouraged.
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.
GCN Circular 40958
Subject
EP250704a / GRB 250704B: Pan-STARRS iz-band imaging and photometry
Date
2025-07-04T19:49:50Z (a year ago)
From
James Gillanders at University of Oxford <jhgillanders.astro@gmail.com>
Via
Web form
J. H. Gillanders (Oxford), M. Huber, K. C. Chambers (IfA, Univ. Hawaii), S. J. Smartt, K. W. Smith (Oxford, QUB), S. Srivastav, F. Stoppa (Oxford), M. Nicholl, D. Young, M. Fulton (QUB), T.-W. Chen (NCU, Taiwan) A. S. B. Schultz, T. de Boer, J. Fairlamb, G. Paek, C. C. Lin, T. Lowe, E. Magnier, P. Minguez, I. A. Smith, R. J. Wainscoat (IfA, Univ. Hawaii).
We observed the optical counterpart of EP250704a (Li et al., GCN 40941) and GRB 250407B (Wang et al., GCN 40940), first reported by Schneider et al. (GCN 40942), using the Pan-STARRS telescope system (Chambers et al., 2016, arXiv e-prints, 1612.05560) on MJD 60860.60 (2025-07-04 14:24 UTC), approximately 6.1 hours after the SVOM/GRM detection (Wang et al., GCN 40940). The Pan-STARRS system consists of two 1.8-m telescope units located at the summit of Haleakala on the Hawaiian island of Maui, employing an SDSS-like filter system denoted as grizy, and a broad w-filter, which is a composite of the gri-filters.
Our observation consisted of 10x90s exposures in both the i and z filters. The images were processed with the Pan-STARRS pipeline. After astrometric and photometric calibration, reference images were subtracted from the target stacked images (Magnier et al., 2020a, ApJS, 251, 3; Magnier et al., 2020b, ApJS, 251, 6; Waters et al., 2020, ApJS, 251, 4).
From these difference images, we measure the following AB magnitudes:
m_i = 20.20 +/- 0.03,
m_z = 20.01 +/- 0.07.
Our i-band detection is ~0.26 mags brighter than that recorded ~5.8 hours earlier by Schneider et al. (GCN 40942), while our z-band detection closely matches that measured ~5.4 hours earlier by Malesani et al. (GCN 40945). Further multi-band observations with Pan-STARRS are planned.
GCN Circular 40957
Subject
EP 250704a: BOOTES-5 optical detection
Date
2025-07-04T18:00:16Z (a year ago)
From
I. Perez-Garcia at Institute of Astrophysics of Andalusia <ipg@iaa.es>
Via
Web form
I. Perez-Garcia (IAA-CSIC, Granada), Y.-D. Hu (GXU), E. J. Fernandez-Garcia, M. D. Caballero-Garcia, S.-Y. Wu, R. Sanchez-Ramirez, S. Guziy and A. J. Castro-Tirado (IAA-CSIC, Granada), I. M. Carrasco (SMA), M. Gritsevich (Univ. of Helsinki), C. Perez del Pulgar (Univ. of Malaga), S. Jeong (ADD, Daejeon), G. Garcia-Segura and D. Hiriart (IA-UNAM, Ensenada), D. R. Xiong (Yunnan Observatories of CAS, Kunming), and W. H. Lee (UNAM, Mexico DF), on behalf of a larger collaboration, report:
Following the detection of EP 250704a by the Einstein Probe (Li et al., GCN 40941), the 0.6m BOOTES-5/JGU robotic telescope at San Pedro Martir Observatory (Mexico) automatically responded to this high-energy event starting on July 4, 08:33:01 UT (i.e., 16 min after detection). Series of images in clear filter were gathered and we detect an optical source consistent with the one reported by Becerra et al. and Bjørn et al. (GCN 40942, 40945). Using GaiaDR3 Gmag as a reference, we measure an initial magnitude of 20.1 +/- 0.2. Further analysis of the additional images is ongoing.
We would like to thank the staff at San Pedro Martir Observatory for their excellent support.
GCN Circular 40956
Subject
EP250704a: refined analysis of the EP-WXT and FXT follow-up observations
Date
2025-07-04T16:47:42Z (a year ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
A. Li (BNU), Y.H Cheng (SWIFAR,YNU), C. Zhou (HUST), G.Y. Zhao (SYSU), Y. J. Zhang (THU) and W. D. Zhang (NAOC) on behalf of the Einstein Probe (EP) team:
The X-ray transient EP250704a triggered the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (GCN 40941), and followed by Swift XRT (GCN 40951) and several optical telescopes (GCN 40942, GCN 40945, GCN 40947). The refined analysis of the WXT data shows that the event started at 2025-07-04T08:16:27 (UTC) and lasted for 10 s with the peak flux of 6 x 10^(-7) erg/s/cm^2, before the observation was interrupted by the autonomous follow-up observation. The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a fixed Galactic equivalent hydrogen column density of 8.0 x 10^20 cm^-2 and a photon index of 1.7 (-/+1.3). The derived average unabsorbed 0.5-4 keV flux is 1.3 (-1.1/+0.8) x 10^(-9) erg/s/cm^2 over the 10 seconds of the pulse.
The autonomous observation by the Follow-up X-ray Telescope (FXT) was performed at 2025-07-04 08:18:59 (UTC), about two minutes later. The exposure time of this observation is 495 s. The on-ground analysis shows that an uncatalogued source was detected at R.A., Dec. = 300.8721, 12.0230 deg (J2000) with an uncertainty of about 10 arcsec (radius, 90% C.L. statistical and systematic). The average FXT 0.5-10 keV spectrum can be fitted with an absorbed power law with a Galactic equivalent hydrogen column density of 8.0 x 10^20 cm^-2 and a photon index of 2.05 (-/+0.07). The derived average unabsorbed 0.5-10 keV flux is 9.0 (-/+0.5) x 10^(-11) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters.
Further follow-up observations with EP-FXT have been scheduled.
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 40951
Subject
EP250704a / GRB250704B: Swift-XRT counterpart detection
Date
2025-07-04T14:06:18Z (a year ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
Via
email
P.A. Evans (U. Leicester), J.A. Kennea (PSU), E. Ambrosi (INAF-IASFPA) , A.P.
Beardmore (U. Leicester), M.G. Bernardini (INAF-OAB), R. Brivio (INAF-OAB), D.N.
Burrows (PSU), S. Campana (INAF-OAB), M. Capalbi (INAF-OAR), A. D'Ai (INAF-IASFPA),
P. D'Avanzo (INAF-OAB), V. D'Elia (SSDC & INAF-OAR), S. Dichiara (PSU), M. Ferro
(INAF-OAB), A. Melandri (INAF-OAR), J.P. Osborne (U. Leicester), K.L. Page (U.
Leicester), M. Perri (SSDC & INAF-OAR), C. Salvaggio (INAF-OAB), T. Sbarrato
(INAF-OAB), B. Sbarufatti (INAF-OAB), M.A. Williams (PSU) report on behalf of the
Swift-XRT team:
Swift-XRT has performed follow-up observations of the Einstein Probe/WXT-detected
source EP250704a (temporally coincident with the SVOM GRB 250704B, Wang et al., GCN
40940), collecting 1.7 ks of Photon Counting (PC) mode data between T0+1.8 ks and
T0+3.7 ks after the trigger. A candidate counterpart has been found. The details of
this source are:
Source 2 (SWIFT J200329.1+120123):
==================================
RA (J2000.0): 300.8716 = 20h 03m 29.18s
Dec (J2000.0): +12.0233 = +12d 01' 23.9"
Error: 3.6 (arcsec, radius, 90% confidence).
Detect flag: REASONABLE
Distance: 24 arcsec from the Einstein Probe/WXT position.
Mean rate: 0.247 +/- 0.017 ct s^-1
Mean flux: (8.79 +/- 0.59)e-12 erg cm^-2 s^-1
Peak rate: 0.389 +/- 0.088 ct s^-1
Peak flux: (1.39 +/- 0.31)e-11 erg cm^-2 s^-1
ECF: 3.56e-11 erg cm^-2 ct^-1, assuming NH=1.47e+21 cm^-2,
gamma=1.97; determined from a spectral fit.
RASS UL: 2.9e-02 ct s^-1 (converted to XRT; 0.3-10 keV)
so the source is not above this 3-sigma upper limit.
The source may be fading, at the 1.7-sigma level.
There is 1 2MASS object within the source's 3-sigma error radius.
All fluxes are 0.3-10 keV, observed. For all flux conversions and comparisons with
catalogues and upper limits from other missions, we assumed a power-law spectrum
with NH=3x10^20 cm^-2 and photon index (Gamma)=1.7 unless otherwise stated.
The results of the XRT-team automatic analysis of the XRT observations, including a
position-specific upper limit calculator, are available at
https://www.swift.ac.uk/EP.
The detected afterglow position is consistent with the optical counterpart of the EP
trigger, reported by Schneider et al. (GCN 40942).
This circular is an official product of the Swift-XRT team.
GCN Circular 40947
Subject
EP250704a / GRB 250704B - REM observations
Date
2025-07-04T13:08:04Z (a year ago)
From
Matteo Ferro at INAF-OAB <matteo.ferro@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 EP250704a (Li et al., GCN 40941) / GRB 250704B (Wang et al., GCN 40940) 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, z, and H bands, started on 2025 July 04 at 08:28:07 UT (i.e. 11.3 min after the burst), and lasted for about 2 hours.
From preliminary inspection, we do not detect any counterpart at the position of the optical afterglow candidate (Schneider et al., GCN 40942, Malesani et al., GCN 40945) down to the following 3sigma limits:
r > 18.7 (AB; calibrated against the Pan-STARRS catalogue),
at a mid-time of 11.4 min after the trigger;
H > 14.8 (Vega; calibrated against the 2MASS catalogue),
at a mid-time of 11.3 min after the trigger.
GCN Circular 40945
Subject
EP250704a / GRB 250704B: VLT/FORS2 optical afterglow detection
Date
2025-07-04T11:22:17Z (a year ago)
From
Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani@nbi.ku.dk>
Via
Web form
Daniele B. Malesani (DAWN/NBI and Radboud), Rob Eyles-Ferris (U. of Leicester), Gregory Corcoran (UCD), Massimiliano De Pasquale (University of Messina), Peter G. Jonker (Radboud), Andrew J. Levan (Radboud), Antonio Martin-Carrillo (UCD), Aishwarya L. Thakur (INAF-IAPS), Dong Xu (NAOC) report on behalf of the Stargate collaboration:
We observed the field of EP250704a (Li et al., GCN 40941) / GRB 250704B (Wang et al., GCN 40940) using the ESO VLT UT1 (Antu) equipped with the FORS2 camera. Observations were carried out in the z band and were executed in rapid response mode (RRM).
The optical counterpart reported by Schneider et al. (GCN 40942) using COLIBRÍ/DDRAGO is clearly detected in our images. From a single 60-s exposure taken in the z band at a mean epoch of 2025 Jul 4.375 UT (42.8 min after the trigger), we measure
z = 20.12 +- 0.05 AB.
This is significantly brighter than the limit of the Pan-STARRS catalog, and well within the EP/FXT error circle. We thus concur with Schneider et al. (GCN 40942) that this is the likely afterglow of EP250704a.
We acknowledge expert support from the observing staff in Paranal, in particular Sam Kim, Robert Klement and Leonel Rivas.
GCN Circular 40942
Subject
EP250704a: COLIBRÍ optical counterpart candidate
Date
2025-07-04T09:58:19Z (a year ago)
From
Rosa L. Becerra at Tor Vergata, Roma <rosa.becerra@roma2.infn.it>
Via
Web form
Benjamin Schneider (LAM), Rosa L. Becerra (U Roma), Sarah Antier (OCA), William H. Lee (UNAM), Stéphane Basa (UAR Pytheas), Alan M. Watson (UNAM), Jean-Luc Atteia (IRAP), Fredd Alvarez (UNAM), Camila Angulo (UNAM), Dalya Akl (AUS), Nathaniel R. Butler (ASU) , Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (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), and Antonio de Ugarte Postigo (LAM):
We imaged the field the EP250704a (Li et al., GCN 40941) using the DDRAGO wide-field imager on the COLIBRÍ telescope. We observed from 2025-07-04 08:27 to 08:46 UTC (from 10.8 to 30.0 minutes after the trigger and starting 25 seconds after the arrival of the notice) and obtained 16 minutes of exposure in the i filter.
The data were reduced and coadded with the COLIBRÍ pipeline and analysed 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 our stacked image, we detect a new source not visible in the PanSTARRS catalogue at:
RA(J2000) = 20:03:29.51 = 300.8729 degrees
Dec(J2000) = +12:01:23.46 = 12.0232 degrees
with an uncertainty of 0.5 arcsec and within the EP/FXT region (Li et al., GCN 40941). We measure a preliminary magnitude of:
i = 20.46 +/- 0.06
We suggest this is the optical counterpart of EP250704a.
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 40941
Subject
EP250704a: Einstein Probe detection of an X-ray transient
Date
2025-07-04T09:50:47Z (a year ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
A. Li (BNU), Y.H Cheng (SWIFAR,YNU), C. Zhou (HUST), G.Y. Zhao (SYSU), Y. J. Zhang (THU) and W. D. Zhang (NAOC) on behalf of the Einstein Probe (EP) team:
We report on the detection of an X-ray transient by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission, designated EP250704a. The transient triggered EP-WXT (ID: 01709181144) at 2025-07-04T08:16:52 (UTC). The WXT position of the source is R.A. = 300.872 deg, DEC = 12.030 deg (J2000) with an uncertainty of 3 arcmin in radius (90% C.L. statistical and systematic).
A follow-up observation with the Follow-up X-ray Telescope (FXT) was performed automatically. Within the WXT error circle, an uncatalogued X-ray source was detected at R.A. = 300.8742, DEC = 12.0243 deg (J2000) with an uncertainty of 20 arcsec in radius (90% C.L. statistical and systematic).
This transient is temporally coincidence with GRB 250704B (GCN 40940). Further information will be updated when the telemetry data is received.
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 40940
Subject
GRB 250704B: SVOM/GRM observation of a short burst
Date
2025-07-04T09:10:04Z (a year ago)
From
Chenwei Wang at IHEP <cwwang@ihep.ac.cn>
Via
Web form
SVOM/GRM team: Chen-Wei Wang, Shi-Jie Zheng, Yue Huang, Shao-Lin Xiong, Shuang-Nan Zhang (IHEP)
SVOM/ECLAIRs team: Ulysse Jacob (LUPM)
Report on behalf of the SVOM team:
SVOM/GRM was triggered in-flight by a short burst GRB 250704B (SVOM trigger reference: sb25070406) at 2025-07-04T08:16:27.100 UTC (T0).
The real-time alert data and light curves of SVOM/GRM were downlinked to the ground through the VHF system with low latency. With the VHF data, the GRM light curve shows that this burst consists of two spikes with a T90 of 0.68 +0.16/-0.14 s in the 15-5000 keV band.
The SVOM/GRM light curve can be found here:
https://www.bursthub.cn//admin/static/svgrb250704B.png
At the time of the burst, ECLAIRs was not taking data.
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. GRM is developed by the Institute of High Energy Physics (IHEP) of CAS.
The SVOM point of contact for this burst is: Chen-Wei Wang (IHEP)(cwwang@ihep.ac.cn)