EP260920b
GCN Circular 45683
Subject
EP260920b: 2.2m CAHA optical afterglow detection and 10.4m GTC redshift confirmation
Date
2026-09-21T18:29:38Z (6 hours ago)
Edited On
2026-09-21T18:31:20Z (6 hours ago)
From
I. Perez-Garcia at Instituto de Astrofísica de Andalucía <ipg@iaa.es>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of I. Perez-Garcia at Instituto de Astrofísica de Andalucía <ipg@iaa.es>
Via
Web form
I. Perez-Garcia, A.J. Castro-Tirado (IAA-CSIC), Y.-D. Hu (GXU), M.D. Caballero-Garcia, M. Gritsevich, S. Guziy (IAA-CSIC), D. Xu (NAOC), J. F. Agui Fernandez, A. Fernandez Martin (CAHA), S. Geier, I. Yanes Rizo (IAC, GRANTECAN), on behalf of a larger collaboration, report:
Following the detection of EP260920b (Ni et al., GCN 45664; Yang et al., GCN 45670), we observed the optical counterpart (Liu et al., GCN 45668; García-García et al., GCN 45669; Li et al., GCN 45671; Eyles-Ferris et al., GCN 45673) with the 2.2m telescope at Calar Alto Observatory in Almeria (Spain), equipped with CAFOS.
At the position of the optical counterpart we measure r = 20.96 +/- 0.21 (AB) on Sep 20 at 21:54 UT (7.8 h after the EP trigger) on a 900s coadded image. The object is also detected in the i-band, but not in the z-band (z > 20.5, 3-sigma).
We also obtained a nearly simultaneous spectrum with the 10.4m GTC equipped with the OSIRIS+ spectrograph, starting on 20 Sep 2026 at 22:00 UT (7.9h after the EP trigger), covering the wavelength range 3800-7750 AA, with a total exposure time of 3x500 s in the R1000B grism.
Superimposed on a strong continuum emission, we detect Si II, C II, Si IV and C IV absorption lines as well as the Lyman-alpha absorption trough and the Lyman forest with Lyman-beta, -gamma, and Lyman-limit features at slightly lower S/N, all under a common redshift of z = 3.580. This confirms the redshift of EP260920b reported by the VLT (Schneider et al. GCN 45675). We also detect the intervening system at z = 1.625.
We thank the staff at both CAHA and GTC for their excellent support.
GCN Circular 45675
Subject
EP260920b: VLT/X-shooter spectroscopic redshift z = 3.582
Date
2026-09-21T09:47:13Z (15 hours ago)
From
Valerio D'Elia at ASDC <valerio.delia@ssdc.asi.it>
Via
Web form
B. Schneider (LAM), A. Saccardi (CEA/Irfu), J. An (NAOC), D. Xu (NAOC), V. D’Elia (ASI/SSDC), M. Garnichey (LUX-Paris Obs.), G. Corcoran (UCD), N. R. Tanvir (U. Leicester), report on behalf of the Stargate collaboration:
We observed the optical counterpart (Liu et al., GCN 45668; García-García et al., GCN 45669; Li et al., GCN 45671; Eyles-Ferris et al., GCN 45673) of the EP260920b (Ni et al., GCN 45664; Yang et al., GCN 45670) using the ESO VLT UT3 (Melipal) equipped with the X-shooter spectrograph.
The acquisition images, which started at 03:51:03 UT of 21 Sep 2026 (13h44m after the EP trigger) show the counterpart with the following photometry (AB magnitudes), calibrated against Pan-STARRS and not corrected for Galactic extinction:
g = 22.6 +/- 0.1
r = 21.6 +/- 0.1
z = 21.3 +/- 0.2
A spectrum covering the wavelength range 3000-21000 AA was secured, with a total exposure time of 4x600 s in each of the UVB, VIS, and NIR arms. Our observation started at 04:00:42 UT on 2026 September 21, i.e. about 13h53m after the EP trigger.
We detect afterglow continuum in each arm. We can see Si II, C IV, Al III, Fe II, and likely Mg II, C II and C II*, the Lyman-alpha absorption trough (near the edge of the VIS arm), and the Lyman forest with Lyman-beta, -gamma, -delta, and Lyman-limit features at slightly lower S/N, all under a common redshift of z = 3.582. Based on this, we infer that this is the redshift of EP260920b.
We also detect an intervening system at z = 1.625.
We acknowledge excellent support from the ESO staff in Paranal.
GCN Circular 45673
Subject
EP260920b: LCO observations
Date
2026-09-21T09:28:18Z (15 hours ago)
From
Rob Eyles-Ferris at U of Leicester <raje1@leicester.ac.uk>
Via
Web form
R. A. J. Eyles-Ferris (Leicester), P. G. Jonker (Radboud), J. Chácon (PUC), G. Corcoran (UCD), J. A. Quirola-Vasquez (Radboud) report on behalf of a larger collaboration:
We observed the field of the fast X-ray transient EP260920b (Ni et al., GCN 45664; Yang et al., GCN 45670) using using an LCO 1m telescope located at the Cerro Tololo Interamerican Observatory (Chile) equipped with the SINISTRO instrument. We obtained 11x300 s exposures in the SDSS r-band starting at 2026-09-21 01:13:47 (~11.1 hours after the trigger).
The counterpart (Liu et al., GCN 45668; García-García et al., GCN 45669; Li et al., GCN 45671) is detected in our images at a magnitude of r = 21.38 +/- 0.13 at t_mid - t0 = 11.57 hours. Our photometry is calibrated using PanSTARRS and is not corrected for Galactic extinction.
GCN Circular 45671
Subject
EP260920b: SVOM/VT optical observation
Date
2026-09-21T07:46:45Z (17 hours ago)
Edited On
2026-09-21T15:09:17Z (9 hours ago)
From
Huali Li at at NAOC, SVOM <lhl@nao.cas.cn>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Huali Li at at NAOC, SVOM <lhl@nao.cas.cn>
Via
Web form
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 EP260920b (Ni et al., GCN 45664; Yang et al., GCN 45670). The observation started at 2026-09-21T01:24:03 UTC, about 11.3 hr after trigger in VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously.
The optical counterpart (Liu et al., GCN 45668; García-García et al., GCN 45669) was detected in VT stacked images of both channels. The following measurements are in the AB magnitude and are not corrected for Galactic extinction:
Mid time | Band | Exposure Time | Magnitude (AB)
12.14 hr VT_B 55*50 sec 22.3 +/- 0.2
12.14 hr VT_R 48*50 sec 21.5 +/- 0.1
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 45670
Subject
EP260920b: refined analysis of the EP-WXT and EP-FXT observations
Date
2026-09-21T06:10:52Z (18 hours ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
G. J. Yang (NAO, CAS), K. R. Ni (CCNU), S. Q. Zheng, Jin-Rui Liu (BNU), Tian-Yu Liu, B. B. Zhang and Z.-X. Ling (NAO, CAS), on behalf of the Einstein Probe (EP) team:
The fast X-ray transient EP260920b triggered the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission at 2026-09-20T14:07:19 (UTC) (Ni et al., GCN 45664). Subsequent follow-up observations were reported by Buckley et al. (GCN 45665), Liu et al. (GCN 45668) and García-García et al. (GCN 45669). The refined analysis of the WXT data shows that the event started at T0 = 2026-09-20T14:06:32 (UTC). The WXT light curve shows a main outburst lasting approximately 500 s. The outburst comprises two broad pulses, with the second brighter than the first, and ends with a rapid decay.
The WXT spectrum extracted over the burst interval in the 0.5–4 keV band is well fitted by an absorbed power-law model. The best-fit hydrogen column density is N_H = 2.5 (+0.7/-0.7)×10^21 cm^-2, and the photon index is 1.72 (+0.24/-0.23). The corresponding average unabsorbed flux in the 0.5–4 keV band is 2.1 (+0.2/-0.2)×10^-10 erg s^-1 cm^-2.
The Follow-up X-ray Telescope (FXT) on board EP observed the source field at 2026-09-20T20:14:12, approximately 6 hours after the T0. The refined analysis shows that an uncatalogued X-ray source was detected at R.A. = 325.5875 deg, Dec. = -28.0402 deg (J2000), with an uncertainty of 10 arcsec in radius (90% C.L. statistical and systematic). The FXT light curve shows a decline 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. The best-fit hydrogen column density is NH = 8.5 (-7.6/+8.0) x 10^20 cm^-2, and the photon index is 1.96 (-0.24/+0.25). The derived average unabsorbed 0.5-10 keV flux is 1.3 (+0.1/-0.1) × 10^-12 erg s^-1 cm^-2. The uncertainties are at the 1sigma confidence level for the above spectral parameters.
More observations with EP-FXT have been scheduled. Further multi-wavelength follow-up observations are encouraged.
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 45669
Subject
EP260920b: COLIBRÍ optical observations
Date
2026-09-21T04:32:10Z (20 hours ago)
Edited On
2026-09-21T15:09:47Z (9 hours ago)
From
Leonardo Enrique García García <lgarcia@astro.unam.mx>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Leonardo Enrique García García <lgarcia@astro.unam.mx>
Via
Web form
Leonardo García-García (UNAM), Marion Guelfand (CPPM), Rosa L. Becerra (UNAM), Stéphane Basa (UAR Pytheas), Edilberto Aguilar-Ruiz (UNAM), William H. Lee (UNAM), Jean-Luc Atteia (IRAP), Camila Angulo (UNAM), Dalya Akl (NYUAD), Sarah Antier (IJCLAB), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Asuka Kuwata (UNAM), Massimiliano Lincetto (CPPM), Nikos Mandarakas (LAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM), Antonio de Ugarte Postigo (LAM), Alan M. Watson (UNAM) report:
We imaged the field of the EP260920b (Ni et al., GCN 45664) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-09-21 02:56 to 03:34 (from 12.81 to 13.47 hours after the trigger) and obtained 31 and 29 minutes of exposure in the r and z filters, respectively, under very poor conditions of transparency.
The data were reduced and coadded with the ASU pipeline. 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 the stacked images, we marginally detect the optical counterpart reported by Liu et al. (GCN Circ. 45668), at a preliminary magnitude of:
r = 20.9 +/- 0.3
Comparing our observations with those reported by Liu et al. (GCN Circ. 45668), the transient has faded, and we estimate a temporal decay index alpha ≈ 0.6.
We note the presence of a bright spiral galaxy 2MASX J21422474-2802228 about 52 arcsec to the east of the counterpart. This galaxy is seen in the LS DR11 catalog (Dey et al. 2019) with a photometry of g = 16.822 +/- 0.001, r = 16.005 +/- 0.001, i = 15.630 +/- 0.001 and z = 15.344 +/- 0.001. We estimate a Pcc ≈ 0.01, making it a plausible candidate host galaxy.
This galaxy has a spectroscopic redshift of z = 0.0519 from the 2dF Galaxy Redshift Survey (2dFGRS; Colless et al. 2001), corresponding to a projected offset of approximately 55 kpc if associated with EP260920b.
We encourage spectroscopic follow-up of the optical counterpart to determine its redshift and test this possible association.
We thank the staff of the Observatorio Astronómico Nacional at Sierra de San Pedro Mártir, as well as the technical and engineering teams at CEA, CPPM, IRAP, LAM, OHP, OSU Pytheas, and UNAM.
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 45668
Subject
EP260920b: LCO optical counterpart detection
Date
2026-09-21T02:13:34Z (a day ago)
From
liuxing@nao.cas.cn
Via
Web form
X. Liu, D. Xu, J. An, Z. Fan, J.F. Gou, L.B. He, S.Q. Jiang, W.X. Li, R.D. Liang, Q. Zhao, Z.P. Zhu (NAOC), S.Y. Fu, L. Lei (HUST), Y.L. Hua (PMO) report:
We observed the field of EP260920b detected by EP (Ni et al., GCN 45664), using an LCO 1m telescope network. We obtained 3*200s exposures in the SDSS r' filter.
An uncatalogued and decaying source is detected within the EP-FXT error circle (Ni et al., GCN 45664) at coordinates
R.A. (J2000) = 21:42:20.79
Dec. (J2000) = -28:02:24.4
with an uncertainty of ~ 0.5 arcsec. The source had r = 18.97 +/- 0.04 mag at a median time of 0.68 hr post-trigger, calibrated with Pan-STARRS DR2 and not corrected for Galactic extinction.
We thus think the source is the optical counterpart of EP260920b.
GCN Circular 45665
Subject
EP260920b: Global MASTER-Net observations report
Date
2026-09-21T01:09:59Z (a day ago)
From
Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov@xray.sai.msu.ru>
Via
email
D.Buckley (SAAO),
V.Lipunov, E.Gorbovskoy, A.Kuznetsov, K.Zhirkov, I.Panchenko, N.Tiurina, P.Balanutsa, V.Topolev, D.Vlasenko,
G.Antipov, A.Sankovich, Yu.Tselik, Ya..Kechin, V.Senik, A.Chasovnikov, K.Labsina (Lomonosov MSU)
A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity)
A.Sosnovskij (Lomonosov MSU)
A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory)
L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez,
A.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory)
R.Rebolo (The Instituto de Astrofisica de Canarias)
O.Gress, N.Budnev(ISU)
M. Segura, F. Podesta, R.Podesta, E. Gonzalez (Observatorio Astronomico Felix Aguilar (OAFA)
MASTER-SAAO robotic telescope [1] located in South Africa (South African Astronomical Observatory) was pointed to the EP260920b ( EP Team at NAOC/CAS <ep_ta@bao.ac.cn> et al., GCN 45664) errorbox 755 sec after notice time and 38750 sec after trigger time at 2026-09-21 00:53:09 UT, with upper limit up to 18.0 mag. The observations began at zenith distance = 59 deg. The sun altitude is -43.7 deg.
The galactic latitude b = -49 deg., longitude l = 21 deg.
Real time updated cover map and OT discovered available here:
https://master.sai.msu.ru/site/master2/observ.php?id=3508203
We obtain a following upper limits.
Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment
_________|_____________________|_____________________|____________________________________|_____|_______|_______|________
38780 | 2026-09-21 00:53:09 | MASTER-SAAO | (21h 39m 01.99s , -27d 59m 58.7s) | C | 60 | 18.0 |
38874 | 2026-09-21 00:54:44 | MASTER-SAAO | (21h 39m 05.58s , -27d 58m 18.4s) | C | 60 | 18.0 |
Filter C is a clear (unfiltred) band.
The observation and reduction will continue.
The message may be cited.
[1] - V.M. Lipunov, V.G. Kornilov, E.S. Gorbovskoy, N.A. Tiurina & A.S.Kuznetsov, 2023, Astronomical Robotic Networks and Operative Multichanel Astrophysics, Lomonosov MSU PRESS, 591pp.
http : // www.pereplet.ru/lipunov/625.html
GCN Circular 45664
Subject
EP260920b: Einstein Probe detection of an X-ray transient
Date
2026-09-21T00:42:10Z (a day ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
K.R. Ni (CCNU), S. Q. Zheng, Jin-Rui Liu (BNU), Tian-Yu Liu, G. J. Yang, B. B. Zhang and Z.-X. Ling (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 EP260920b. The transient triggered EP-WXT (ID: 01709318522) at 2026-09-20T14:07:19 (UTC). The WXT position of the source is R.A. = 325.575 deg, DEC = -28.046 deg (J2000) with an uncertainty of 3 arcmin in radius (90% C.L. statistical and systematic).
As the autonomous FXT follow-up was not triggered, a follow-up observation with the Follow-up X-ray Telescope (FXT) was subsequently performed. Within the WXT error circle, an uncatalogued X-ray source was detected at R.A. = 325.5844 deg, DEC = -28.0406 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 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).