EP261006a
GCN Circular 45868
G. Corcoran (UCD), A. Saccardi (CEA/Irfu), J. An (NAOC), A. J. Levan (Radboud and Warwick), R. A. J. Eyles-Ferris (U. Leicester), D. Xu (NAOC), D. B. Malesani (DAWN/NBI and Radboud), A. Martin-Carrillo (UCD), report on behalf of the Stargate Collaboration:
We observed the optical counterpart AT 2026aetp (Eyles-Ferris et al., GCN 45831; Wu et al., GCN 45852; Bochenek et al., GCN 45853; Chen et al., GCN 45854) of the Einstein Probe fast X-ray transient EP261006a (Lao et al., GCN 45826) using the ESO VLT UT3 (Melipal) equipped with the X-shooter spectrograph.
A spectrum covering the wavelength range 3000-21000 AA was secured, with a total exposure time of 3x900 s in each of the UVB, VIS, and NIR arms. Our observations started on 2026 October 8 at 05:20:28 UT (1.48 days after the EP-WXT trigger).
In a preliminary reduction of the spectra, we detect a clear blue continuum over the entire wavelength range, consistent with the spectrum described by Jiang et al. (GCN 45833). From the detection of multiple narrow absorption features, consistent with Mg II, Mg I, and Ca II, we infer a redshift of z = 0.1853 (in excellent agreement with the Legacy Survey DR 10 photometric redshift of z = 0.1954 +/- 0.0109).
While the blue continuum is consistent with a tidal disruption event origin, we do not detect any significant emission features that would allow us to confirm this classification or propose an alternative one. Further photometric and spectroscopic observations are encouraged.
We acknowledge excellent support from the ESO staff in Paranal, in particular Celia Desgrange and Felipe Gaete.
GCN Circular 45857
Nicolas Monsalves, Muskan Yadav, Hira Waseem, and Eleonora Troja (U Rome) reports on behalf of the ERC BHianca team:
We observed the field of the fast X-ray transient EP261006a (Lao et al. GCN 45826) with the Neil Gehrels Swift Observatory. Observations (PI: Troja) started on 2026 Oct 7.74 UT (~24 hr after the trigger) for a total exposure of 1.8 ks in Photon Counting mode.
Within the EP/FXT localization (Lao et al. GCN 45826), a bright source is detected by the UVOT in all filters with uvw2~18.4 AB mag, not corrected for Galactic extinction. Its position is consistent with the nucleus of the galaxy Gaia DR3 2587044864659595392 and the candidate optical counterpart (Eyles-Ferris et al. GCN 45831; Wu et al. GCN 45852; Bochenek & Perley GCN 45853; Chen et al. GCN 45854). Archival GALEX images show only a weak source at this position with NUV ~ 23.5 AB mag, suggesting significant variability between the two epochs.
Swift/XRT observations detect an X-ray source at >99.99% significance at RA, Dec (J2000) = 01:27:09.74, +12:53:21.8 with an uncertainty of 6.7 arcsec (90% C.L.), 6 arcsec from the galaxy’s nucleus and the candidate optical counterpart. The limited number of counts does not allow for an enhanced XRT position.
The XRT count rate of 0.010 (+0.004, -0.003) ct/s corresponds to an unabsorbed 0.3-10 keV flux of ~4e-13 erg cm^-2 s^-1 (photon index 1.64 and Galactic N_H as in Lao et al. GCN 45826), suggesting substantial fading with respect to the earlier EP/FXT observations. Using a simple power-law to describe the decay rate as t^alpha, we derive a temporal index alpha ~- 1. The X-ray fading does not match the behavior of the candidate optical counterpart, still blue and brightening at a similar epoch (r = 18.7 at ~28 hr, Bochenek & Perley GCN 45853). Based on the marginal positional coincidence and different temporal behavior, the association between the X-ray afterglow and candidate optical and UV counterpart cannot be confirmed at present.
We thank the Swift team for rapidly scheduling these observations.
GCN Circular 45854
Z.-Y. Chen, A. Aryan, T.-W. Chen, C.-S. Lin (all NCU), A. K. H. Kong (NTHU), S. J. Smartt, J. H. Gillanders (both Oxford), S. Yang (HNAS), A. Sankar.K, K. N.-T. Ho, A. Dutta, M.-H. Lee, Y.-H. Lee, Y.-C. Pan, C.-C. Ngeow, C.-H. Lai, H.-C. Lin, H.-Y. Hsiao, W.-J. Hou, J.-K. Guo (all NCU), Z. N. Wang, D. C. Qiang, L. L. Fan (all HNAS), Y. J. Yang (NYUAD), H.-W. Lin (UMich), K. W. Smith, H. F. Stevance, S. Srivastav, L. Rhodes (all Oxford), M. Nicholl, M. Fulton, C. Angus, A. Aamer (all QUB), T. Moore (STScI), A. Schultz and M. Huber (both IfA, Hawaii) report:
We observed the field of the fast X-ray transient EP261006a (Lao et al., GCN 45826) using the 1m LOT at the Lulin observatory, as part of the Kinder collaboration (Chen et al. 2025, ApJ, 983, 86, doi:10.3847/1538-4357/adb428). The first LOT epoch of observations in r-band started at 15:11 UTC on the 07th of October 2026 (MJD 61320.6327), 21.35 hr after the EP-WXT detection.
We used the Python-based AutoPhOT package (Brennan & Fraser 2022, A&A, 667, A62) to perform template subtraction with the Pan-STARRS1 3Pi archive image (Chambers et al. 2016 arXiv:1612.05560) using the 'SFFT' (Hu et al. 2022, ApJ, 936, 157) algorithm on individual frames. The optical counterpart (Eyles-Ferris et al., GCN 45831; Wu et al., GCN 45852; Bochenek et al., GCN 45853) was detected in each individual frame.
Moreover, we further used AutoPhOT to perform PSF photometry. The details of the observations and the measured photometry from the first frame are as follows:
Telescope | Filter | MJD (start) | t-t0 (hr) | Exposure (s) | Magnitude | Seeing | Airmass
LOT | r | 61320.6327 | 21.35 | 300 * 1 | 19.096 +/- 0.039 | 1".3 | 1.06
Our measurements confirm the reported rise by Eyles-Ferris et al. (GCN 45831); however, the source does not vary rapidly over 25 minutes of observations, remaining at ~19.1 mag in the r-band.
The presented magnitude is calibrated using the field stars from the Pan-STARRS1 catalog and is not corrected for the expected Galactic foreground extinction of A_r = 0.096 mag in the direction of the transient (Schlafly & Finkbeiner 2011). The methodology, details on the Lulin observatory telescopes, and a compilation of our optical follow-up campaign for FXTs discovered within the first year of operation of the Einstein-Probe mission are presented in Aryan et al. 2025, ApJS, 281, 20, doi:10.3847/1538-4365/adfc69.
GCN Circular 45853
A. Bochenek and D. A. Perley (LJMU) report:
We observed the field of EP261006a (Lao et al., GCN 45826) using the IO:O optical camera on the 2m robotic Liverpool Telescope. We obtained 3x100s exposures in SDSS g filter and 4x100s in SDSS r, starting at 2026-10-07 21:33:20 UT, approximately 27.7 hours after trigger.
Image subtraction is performed against reference imaging from the Pan-STARRS survey. We detect a source in the subtracted images at the counterpart position reported by Eyles-Ferris et al. (GCN 45831) and Jiang et al. (GCN 45833), inside the galaxy Gaia DR3 2587044864659595392. The source remains blue, and has brightened significantly since the observations of Eyles-Ferris et al. (GCN 45831) and Wu et al. (GCN 45852).
| MJD (mid) | T_mid-T_0 | Filter | Mag. (AB) |
|---|---|---|---|
| 61320.90009 | 27.8 h | sdss-g | 18.53 ± 0.07 |
| 61320.90500 | 27.9 h | sdss-r | 18.73 ± 0.07 |
The observations were taken in very poor seeing (3-5 arcsec), and the values above should be treated as preliminary. Further observations are scheduled. The photometry is in the AB magnitude system, was calibrated using nearby PanSTARRS secondary standards and was not corrected for extinction.
GCN Circular 45852
Mingxin Wu (USTC), Ning Jiang (USTC), Zelin Xu (USTC), Linna Xu (USTC), Chengyi Wang (USTC), Jiazheng Zhu (USTC), Tinggui Wang (USTC), Ji-an Jiang (USTC) on behalf of the Wide Field Survey Telescope (WFST) team:
We report optical observations of the X-ray transient EP261006a (Lao et al., GCN 45826) obtained with the Wide Field Survey Telescope (WFST) as part of its Wide Field Survey program.
Difference imaging reveals a clear optical counterpart within the EP-FXT localization, with an angular separation of less than 0.1 arcsec from the position of the galaxy Gaia DR3 2587044864659595392. Forced photometry on 2026-10-04 at 20:45:24 UT (g band) and 21:40:49 UT (r band), approximately two days before the X-ray detection, yields a non-detection with 3sigma upper limits of ~ 23 and 22 mag, respectively, indicating that the brightening is temporally coincident with EP2611006a. Our detections on 2026-10-06 at 20:43:33 UT (r band) and 21:31:00 UT (u band) precede those reported by LT and NOT (Eyles-Ferris et al., GSN 45831). Combining the WFST photometry with the LT and NOT measurements indicates a rising r-band light curve. The blue u-r color and nuclear location are consistent with the tentative classification as a TDE.
We summarise our difference photometry below, where t0 denotes the EP/WXT trigger time (2026-10-06 at 17:49:57 UT).
| t_mid-t0 (hrs) | Filter | AB magnitude | Exposure time (s) |
|---|---|---|---|
| −45.08 | g | > 23.15 (3σ) | 30 |
| −44.15 | r | > 22.40 (3σ) | 30 |
| 2.89 | r | 20.42 ± 0.05 | 30 |
| 3.68 | u | 19.76 ± 0.05 | 30 |
GCN Circular 45833
S.Q. Jiang, D. Xu, J. An (NAOC), L. Lei, S.Y. Fu (HUST), L.B. He, X. Liu, Z.P. Zhu (NAOC), J.P.U. Fynbo (DAWN/NBI), E. Georgiou, A.S. Maartensson (NOT) report:
We performed the spectroscopy of the nucleus of the galaxy, Gaia DR3 2587044864659595392 (Hughes et al. 2022) with a photometric redshift of 0.184 (Duncan et al. 2022), which is within the EP-FXT error circle of EP261006a (Lao, et al., GCN 45826), using the Nordic Optical Telescope (NOT) equipped with the ALFOSC camera. Observations started at 04:39:42 UT on 2026-10-07, i.e., 10.83 hr after the EP trigger, and 2 x 1200 s exposures were obtained, covering the wavelength range of ~ 3600 - 9200 AA.
The whole spectrum shows a clear blue continuum, without any prominent absorption or emission feature. The spectrum is different from the red one of the galaxy, which is close to an elliptical E0 type.
The spectral difference shows that the optical counterpart of EP261006a is at or very close to the nucleus of the galaxy, as reported in Eyles-Ferris et al. (GCN 45831).
The blue continuum spectrum would be consistent with an early transient such as TDE, FBOT, and so on (e.g., similar to EP260806a).
Further observations are planned.
GCN Circular 45831
R. A. J. Eyles-Ferris (Leicester), D. B. Malesani (DAWN/NBI and Radboud), G. Corcoran (UCD), J. A. Quirola-Vasquez (Radboud), P. G. Jonker (Radboud), and A. J. Levan (Radboud and Warwick) report on behalf of a larger collaboration:
We observed the field of the fast X-ray transient EP261006a (Lao et al., GCN 45826) with the Liverpool Telescope (LT) and the Nordic Optical Telescope (NOT) equipped with the IO:O and ALFOSC imagers, respectively. Observations started at 2026-10-06 23:26:24 UT (LT) and 2026-10-07 00:26:37 (NOT).
In image subtractions using Pan-STARRS templates, we identify a clear residual spatially consistent with the nucleus of the galaxy Gaia DR3 2587044864659595392. The location is also consistent with the EP-FXT localisation of the transient (GCN 45826). This residual is seen in all our images and we summarise our host-subtracted photometry below.
| t_mid - t0 (hrs) | Telescope | Filter | Exposure time (s) | AB magnitude |
|---|---|---|---|---|
| 5.56 | LT | r | 6x150 | 20.22 +/- 0.03 |
| 5.85 | LT | i | 6x150 | 20.78 +/- 0.05 |
| 6.11 | LT | z | 5x150 | 20.59 +/- 0.07 |
| 6.75 | NOT | r | 5x180 | 19.83 +/- 0.03 |
We note the rise in r-band brightness. Our magnitudes are calibrated to Pan-STARRS and not corrected for Galactic extinction. The blue r - i colour and nuclear location of EP261006a are consistent with the tentative classification as a TDE.
GCN Circular 45827
H. L. Li, Y. N. Ma, C. Wu, L. P. Xin, Y. L. Qiu, Z. H. Yao, J. R. Xu, L. J. Chen, 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. T. Palmerio (CEA/Irfu) report on behalf of the SVOM/VT team.
SVOM/VT performed ToO observation on the field of EP261006a (EP/WXT trigger 01709320985, Lao et al., GCN 45826). The observation started at 2026-10-06T19:05:40 UTC, about 1.27 hr after trigger in VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously.
No credible uncatalogued candidates were identified in our stacked images within the error box of EP-FXT (Lao et al., GCN 45826), 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
1.66 hr VT_B 56*50 s > 23.5 mag
1.66 hr VT_R 56*50 s > 23.5 mag
Our results are consistent with the non-detections from Ondrejov D50 (Novotny et al., GCN 45821) and SVOM/C-GFT (Shi et al., GCN 45823) .
We note that no apparent variability was detected during our observations for the galaxy Gaia DR3 2587044864659595392, which lies within the EP‑FXT error box (Lao et al., GCN 45826).
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 45826
Q.H. Lao (NAO, CAS), L.N. Xu (USTC), H.Y. Liu (NAO, CAS), W.F. Wen (SZTU), 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 EP261006a. The transient triggered EP-WXT (ID: 01709320985) at 2026-10-06T17:49:57 (UTC) . The WXT position of the source is R.A. = 21.806 deg, DEC = 12.9 deg (J2000) with an uncertainty of 3 arcmin in radius (90% C.L. statistical and systematic). The exposure time of WXT observation (ID: 01709320985) is 384 s, and no significant variability was detected during the observation.
The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power-law model, with the hydrogen column density fixed at Galactic NH = 3.84 x 10^20 cm^-2 at the source position. The best-fit photon index is 1.73 (-0.66/+0.77), and the derived average unabsorbed 0.5-4 keV flux is 1.46 (-0.53/+0.78) x 10^-10 erg s^-1 cm^-2.
The Follow-up X-ray Telescope (FXT) on board EP observed the source autonomously following the WXT trigger at 2026-10-06T17:53:48(UTC), 231s after the trigger. With the telemetry data received, the refined on-ground analysis gives an X-ray source position of RA=21.7913 deg, DEC=12.8907 deg (J2000) , with an uncertainty of about 10 arcsec in radius (90% C.L. statistical and systematic). The FXT light curve shows an overall declining trend during the observation.
We jointly fitted the FXT-A and FXT-B spectra in the 0.5-10 keV band with an absorbed power-law model, with the hydrogen column density fixed at Galactic NH = 3.84 x 10^20 cm^-2. The best-fit photon index is 1.64 (-0.01/+0.01). The derived average unabsorbed 0.5-10 keV flux is 1.17 (-0.02/+0.02) x 10^-10 erg s^-1 cm^-2.
Within the FXT error circle, there is a galaxy, Gaia DR3 2587044864659595392 (Hughes et al. 2022) with a photometric redshift of 0.184 (Duncan et al. 2022). If this transient is located within this galaxy, its estimated 0.5-10 keV luminosity is 1.12×10^46 erg s^-1.
The uncertainties are at the 90% confidence level for the above spectral parameters.
Subsequent follow-up observations were reported by Novotny et al. (GCN 45821) and Shi et al. (GCN 45823). Further multi-wavelength follow-up observations are encouraged to explore the nature of EP261006a.
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).