EP261007b
GCN Circular 45870
Subject
EP261007b: SVOM/VT optical detection of the NOT transient
Date
2026-10-09T02:31:07Z (2 hours ago)
From
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 of EP261007b (Ma et al., GCN 45849). 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 (12 hours 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 (13 hours 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 (14 hours ago)
From
tanishk.mohan@iitb.ac.in
Via
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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 (14 hours 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 (a day 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 (a day 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).