EP261007b
GCN Circular 45876
Subject
EP261007b: SVOM/ECLAIRs Upper Limit
Date
2026-10-09T14:43:57Z (2 days ago)
From
SVOM_group <svomgroup@bao.ac.cn>
Via
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H. Yang, O. Godet (IRAP)
report on behalf of the SVOM team:
We performed a targeted search to the X-ray transient EP261007b, detected by EP-WXT at 2026-10-07T15:05:16 UTC (T0) (EP team, GCN 45847), with the SVOM/ECLAIRs event-by-event data downloaded through the X-band ground station.
At T0, the transient position was within the ECLAIRs field of view. No statistically significant signal was identified in the [T0 - 3000 s, T0 + 1000 s] interval by the ECLAIRs offline trigger.
For the interval from T0 to T0 + 16 s, we derive a 3-sigma upper limit of 1.2e-8 erg/cm^2/s in the 4-120 keV energy band, assuming a power-law spectrum with a photon index of -1.9, as measured by EP-FXT for the uncatalogued source (GCN 45872). Alternatively, assuming a “soft” spectral template (a Band function with Epeak = 25 keV, alpha = -1.5, beta = -3.0), we derive a 3-sigma upper limit of 1.1e-8 erg/cm^2/s in the 4-120 keV energy band. We note that the EP/WXT flux of 7.9e-11 erg/cm^2/s in the 0.5-4 keV (GCN 45872) is more than two orders of magnitude lower than our 3-sigma upper limit.
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. ECLAIRs was developed jointly by APC, CEA, CNES, and IRAP.
The SVOM/ECLAIRs point of contact for this event is : Hui Yang (hui.yang@cnrs.fr)
GCN Circular 45873
Subject
EP261007b: J-band upper limit by SYSU 80cm infrared telescope
Date
2026-10-09T12:29:35Z (2 days ago)
From
sunjk7@mail2.sysu.edu.cn
Via
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Jing-Kai Sun, Jin-Ji Li, Zhong-Nan Dong, Jia-Qi Lin, Pu Lin, Yun Shi, Hao-Nan Yang, Rui-Chen Gao, Hao-Ran Zhang, P H Thomas Tam, Rong-Feng Shen, Bin Ma (Sun Yat-sen University) report on behalf of the SYSU 80cm infrared telescope team:
We observed the field of the X-ray transient EP261007b (Wen et al., GCN 45847) using the Sun Yat-sen University (SYSU) 80cm infrared telescope. Our observations were carried out on 2026 Oct 08 from 17:09 to 18:10 UT, 26.1 hours after the EP/WXT trigger, using stacked images in the J band (180 x 20 s, 1hr total exposure).
We do not detect any new source within the FXT localization (Wen et al., GCN 45847), nor at the position of the variable source reported by Quirola-Vasquez et al. (GCN 45864) at RA, Dec = 23:45:22.41, +11:20:25.8 (J2000), in our stacked J-band image. We obtain a 5-sigma upper limit of J ~ 20.0 mag (Vega).
Our non-detection is consistent with the non-detections reported by Ma et al. (GCN 45849), Zhang et al. (GCN 45860), Mohan et al. (GCN 45861), Shi et al. (GCN 45862), and Quirola-Vasquez et al. (GCN 45864).
The SYSU 80 cm infrared telescope is operated and managed by the Department of Astronomy, Sun Yat-sen University.
GCN Circular 45872
Subject
EP261007b: Refined analysis of the EP-WXT and autonomous EP-FXT observations
Date
2026-10-09T11:18:00Z (2 days ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
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W.F. Wen (SZTU), Q.H. Lao (NAO, CAS), L.N. Xu (USTC), H.Y. Liu (NAO, CAS), X.H. Tang (THU), and C.C. Jin (NAO, CAS) report on behalf of the Einstein Probe (EP) team:
The X-ray transient EP261007b triggered the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission at 2026-10-07T15:05:16
(Wen et al., GCN 45847). Subsequent follow-up observations were reported
by Ma et al. (GCN 45849), Zhang et al. (GCN 45860), Mohan et al. (GCN 45861), Shi et al. (GCN 45862), Quirola-Vasquez et al. (GCN 45864), and Li et al. (GCN 45870). Here we present the analysis of the EP-WXT and autonomous EP-FXT observations of EP261007b.
The time-averaged WXT spectrum in the 0.5–4 keV band was fitted with an absorbed power-law model. The hydrogen column density was fixed at the Galactic value of NH = 3.59 × 10^20 cm^-2 at the source position. The best-fit photon index is 1.9 (+0.6/-0.4). The average unabsorbed 0.5–4 keV flux is 7.9 (+2.8/-2.3) × 10^-11 erg s^-1 cm^-2.
The Follow-up X-ray Telescope (FXT) on board EP began its autonomous observation at 2026-10-07T15:10:24 UTC, 308 s after the WXT trigger. FXT-A detected two X-ray sources within the WXT error region reported in GCN 45847:
EPF_J234524.0+112110: R.A. = 356.3500 deg, Dec. = 11.3526 deg (J2000).
EPF_J234521.3+112424: R.A. = 356.3388 deg, Dec. = 11.4067 deg (J2000).
Each position has a 90% confidence error radius of approximately 10 arcsec, including statistical and systematic uncertainties.
For EPF_J234524.0+112110, we jointly fitted the FXT-A and FXT-B spectra in the 0.5–10 keV band with an absorbed power-law model. The hydrogen column density was fixed at the Galactic value. The best-fit photon index is 2.0 ± 0.4. The average unabsorbed 0.5–10 keV flux is 3.5 (+1.3/-0.8) × 10^-13 erg s^-1 cm^-2.
For EPF_J234521.3+112424, we fitted the FXT-B spectrum in the 0.5–10 keV band with an absorbed power-law model, fixing NH at the Galactic value. The best-fit photon index is 1.8 (+1.3/-1.0). The average unabsorbed 0.5–10 keV flux is 6.3 (+12.4/-3.8) × 10^-14 erg s^-1 cm^-2.
We note that a variable source was detected outside the FXT error circle by NOT (Quirola Vasquez et al., GCN 45864) and SVOM/VT (Li et al., GCN 45870). We have examined the FXT images and found no X ray source present at this position. More follow-up observations are encouraged.
All quoted uncertainties on the spectral parameters and fluxes are at the 90% confidence level.
Launched on January 9, 2024, EP is a space-based X-ray observatory designed to monitor the soft X-ray sky and perform X-ray follow-up observations (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics).
GCN Circular 45870
Subject
EP261007b: SVOM/VT optical detection of the NOT transient
Date
2026-10-09T02:31:07Z (2 days ago)
Edited On
2026-10-09T19:38:21Z (2 days ago)
From
Huali Li at at NAOC, SVOM <lhl@nao.cas.cn>
Edited By
Vidushi Sharma at NASA GSFC/UMBC <vidushi.sharma@nasa.gov> on behalf of Huali Li at at NAOC, SVOM <lhl@nao.cas.cn>
Via
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H. L. Li, Y. N. Ma, Y. L. Qiu, C. Wu, L. P. Xin, Z. H. Yao, J. R. Xu, L. J. Chen, X. H. Han, Y. Xu, J. Wang, 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/VT team.
Following the report from NOT observations (Quirola-Vasquez et al., GCN 45864), we reexamined the SVOM/VT images (Ma et al., GCN 45849) of EP261007b (Wen et al., GCN 45847). The optical transient reported therein was also detected in our stacked images.
The measurements below are in AB magnitudes and are not corrected for Galactic extinction:
Mid time | Band | Exposure Time | Brightness
1.24 hr | VT_B | 26*50 s | 21.9+/-0.2 mag
1.24 hr | VT_R | 26*50 s | 21.6+/-0.2 mag
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 45864
Subject
EP261007b: NOT optical observations, variable source outside the FXT region
Date
2026-10-08T15:56:21Z (3 days ago)
From
Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani@nbi.ku.dk>
Via
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J. A. Quirola-Vasquez (Radboud), A. Martin-Carrillo (UCD), D. B. Malesani (DAWN/NBI and Radboud), G. Corcoran (UCD), R. A. J. Eyles-Ferris (Leicester), P. G. Jonker (Radboud), A. J. Levan (Radboud and Warwick), J. Chacon (PUC), and F. E. Bauer (SSI and UTA) report on behalf of a larger collaboration:
We observed the field of the X-ray transient EP261007b (Wen et al., GCN 45847) using the ALFOSC camera on the Nordic Optical Telescope (NOT). Observations consisted of a sequence of 5 x 300 s exposures in the SDSS r-band, and started on 2026-10-08 at 01:57:25 UT (~10.9 hr after the EP/WXT trigger time; Wen et al., GCN 45847).
After performing image subtraction on our stacked image against the Legacy Survey, no residuals are found within the EP-FXT error circle, although we note that the bright nearby star Gaia DR3 2763566336899590528 leaves some subtraction imperfections, affecting a small part of the EP-FXT area. We obtain an upper limit in our difference image within the EP-FXT region of r > 23.8 (AB), calibrated using nearby stars from the Pan-STARRS catalogue, and not corrected for Galactic extinction.
We do however note the presence of a transient source outside (51" away) of the FXT region at coordinates (J2000) RA = 23:45:22.41, Dec= +11:20:25.8. For this source we measure a difference magnitude of r = 22.09 +/- 0.10. A faint object can be seen in the Legacy Survey at this position with r = 23.62 +/- 0.20 (AB). Given the low S/N of this archival detection, no morphological properties can be reliably inferred.
It is not expected for the FXT localization to be so far off the actual transient position, so the association between this object and EP261007b cannot be confirmed at the present time, but we encourage further monitoring as this source remains consistent with the EP/WXT localization.
GCN Circular 45862
Subject
EP261007b: SVOM/C-GFT optical upper limits
Date
2026-10-08T15:08:18Z (3 days ago)
From
SVOM_group <svomgroup@bao.ac.cn>
Via
Web form
Y.Y. Shi (GXU), Z. Kang (CHO), C. Wu , L.P. Xin, X.H. Han, P.P. Zhang, X.M. Lu (NAOC), Z.W. Li, Y. Lv (CHO), R.S. Zhang, Y.J. Xiao, Y.L. Qiu, J. Wang, J.S. Deng, L. Huang, J.Y. Wei (NAOC), report on behalf of the SVOM/C-GFT team:
We observed the field of EP261007b detected by EP/WXT (Wen et al., GCN 45847) with LATIOS on SVOM/C-GFT. Our observations started at 2026-10-07T15:23:10 UTC, ~ 17.9 min after the trigger.
We obtained follow-up imaging in g, r, and i bands. After preliminary analysis, no credible candidate was detected within the error box provided by EP/FXT (Wen et al., GCN 45847) in our images. We measure the following upper limits calibrated against the Pan-STARRS DR1 catalog, and not corrected for Galactic extinction:
g > 21.4 AB (3-sigma, mid-time 1.96 hr after the trigger).
r > 20.9 AB (3-sigma, mid-time 2.05 hr after the trigger).
i > 20.4 AB (3-sigma, mid-time 0.59 hr after the trigger).
Our non-detection is consistent with the non-detections by Ma et al. (GCN 45849), Zhang et al. (GCN 45860), and Mohan et al. (GCN 45861).
We thank the observation assistants B.-W. Li and H.-X. Xue at Jilin observatory for their excellent support.
The Chinese Ground Follow-up Telescope (C-GFT) for the SVOM mission is located at Jilin Station, Changchun Observatory, National Astronomical Observatories, CAS. It features two instruments: (1) CATCH at the Cassegrain focus with a 21 arcmin x 21 arcmin FOV for simultaneous g/r/i-band imaging, and (2) LATIOS, a 4k x 4k CMOS camera at the prime focus with a 1.28 deg x 1.28 deg FOV that images in g, r, and i bands via filter switching.
GCN Circular 45861
Subject
EP261007b: J-band upper limit with WINTER
Date
2026-10-08T13:48:40Z (3 days ago)
From
tanishk.mohan@iitb.ac.in
Via
Web form
Tanishk Mohan (IITB), Geoffrey Mo (Caltech/Carnegie), Viraj Karambelkar (Columbia), Benjamin Schneider (LAM), Tomas Ahumada (NOIRLab), Robert Stein (UMD), Danielle Frostig (CfA), Nathan Lourie (MIT), Robert Simcoe (MIT), and Mansi Kasliwal (Caltech) report:
We observed the field of EP261007b (Wen et al., GCN 45847) in the near-infrared J band with the Palomar 1-m telescope, equipped with the 1.2-square degree WINTER camera (Lourie et al. 2020, Frostig et al. 2024).
Observations began on 2026-10-08 at 02:38:26.332 UTC in the J band (11.55 hr after the WXT trigger), consisting of 30*120 sec exposures. 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).
We do not detect any new source in the FXT localization and obtain the following 5-sigma upper limit: J = 19.6 mag (AB).
We also note early optical upper limits by Ma et. al., GCN 45849.
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 45860
Subject
The EP-WXT trigger 01709321008/EP261007b：Xinglong optical upper limit
Date
2026-10-08T13:46:42Z (3 days ago)
From
Xinglong Observatory at National Astronomical Observatories (NAOC), Chinese Academy of Sciences (CAS) <xinglong@nao.cas.cn>
Via
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Junbo-Zhang (NAOC), Junjie-Jin (NAOC), Rui-Zhu (NAOC), Haiyang-Mu (NAOC), Feng-Xiao (NAOC), Yuguang-Sun (NAOC), Jie-Zheng (NAOC), Dong-Xu (NAOC), Zhou-Fan (NAOC), Hong-Wu (NAOC) report on behalf of a large collaboration
Following the detection of EP261007b by EP-WXT (Wen et al., GCN 45847), we observed the field of EP261007b using the 0.8‑m and 2.16‑m telescopes at Xinglong Observatory, NAOC.
We obtained 3 x 300 s g‑band frames with the 0.8‑m telescope at a median time of 2026‑10‑07T15:32:59 UTC, i.e., 0.46 hr after the EP trigger. The stacked g-band image has a 5-sigma limiting magnitude of ~20.66 mag, calibrated with the Pan-STARRS g-band field. For the 2.16‑m telescope, we obtained 3 x 300 s clear‑band frames with a median time of 2026‑10‑07T15:33:09 UTC, i.e., 0.46 hr after the EP trigger. The stacked clear-band image using the 2.16-m telescope has a 5-sigma limiting magnitude of ~22.21 mag, calibrated with the Pan-STARRS g-band field. No optical counterpart candidate was detected in our 0.8‑m and 2.16‑m observations.
We summarize our observation results as follows:
Obs. No. | Start Time (UTC) | End Time (UTC) |Exposure Time (s)|Upper limit mag| Telescope
1 | 2026-10-07 15:27:53 | 2026-10-07 15:43:05 | 3x300 s | 20.66 | Xinglong 0.8-m Telescope
2 | 2026-10-07 15:28:02 | 2026-10-07 15:43:41 | 3x300 s | 22.21 | Xinglong 2.16-m Telescope
GCN Circular 45849
Subject
EP261007b: SVOM/VT optical upper limits
Date
2026-10-08T06:45:49Z (3 days ago)
From
Yinuo Ma <mayn@bao.ac.cn>
Via
Web form
Y. N. Ma, H. L. Li, L. P. Xin, Y. L. Qiu, C. Wu, Z. H. Yao, J. R. Xu, L. J. Chen, X. H. Han, Y. Xu, J. Wang, 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/VT team.
SVOM/VT performed a Target of Opportunity observation of EP261007b detected by EP/WXT (Wen et al., GCN 45847). SVOM/VT began observing the field at 2026-10-07T16:05:33 UTC, 1.005 hours after the trigger, in the VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously.
No uncatalogued candidate was identified in our stacked images within the error box provided by EP/FXT position (Wen et al., GCN 45847), compared to the Legacy Survey.
The following measurements are in AB magnitudes and are not corrected for Galactic extinction:
Mid time | Band | Exposure Time | 3-sigma upper limit
1.23 hr | VT_B | 26*50 s | > 23.3 mag
1.23 hr | VT_R | 26*50 s | > 23.0 mag
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 45847
Subject
EP261007b: Einstein Probe detection of an X-ray transient
Date
2026-10-08T03:02:37Z (3 days ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
W.F. Wen (SZTU), L.N. Xu (USTC), H.Y. Liu (NAO, CAS), Q.H. Lao (NAO, CAS), X.H. Tang (THU) and C.C. Jin (NAO, CAS) 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 EP261007b. The transient triggered EP-WXT (ID: 01709321008) at 2026-10-07T15:05:16(UTC). The WXT position of the source is R.A. = 356.349 deg, DEC = 11.369 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. = 356.3481 deg, DEC = 11.3539 deg (J2000) with an uncertainty of 20 arcsec in radius (90% C.L. statistical and systematic).
Further information will be updated when the telemetry data are 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).