- GCN Circular #42387
M. E. Wortley, D. O'Neill, G. Ramsay, B. P. Gompertz, B. Godson, R. Starling, K. Ackley, M. J. Dyer, J. Lyman, K. Ulaczyk, D. Steeghs, D. K. Galloway, V. Dhillon, P. O'Brien, K. Noysena, R. Kotak, R. P. Breton, L. K. Nuttall, and J. Casares report on behalf of the GOTO collaboration:
We report on optical observations with the Gravitational-wave Optical Transient Observer (GOTO; Steeghs et al. 2022, Dyer et al. 2024) in response to Einstein Probe alert 01709247295.
Serendipitous imaging covered the position at 2025-10-23 05:25:22 (+2.9h post trigger) and targeted observations at 2025-10-23 05:36:01 UT, (+3.08h post trigger). The observations consisted of 4x45s and 4x90s exposures respectively in the GOTO L-band (400-700 nm). Images were processed immediately after acquisition using the GOTO pipeline. Difference imaging was performed using deeper template observations.
A new optical source, GOTO25ixt, is identified within the WXT 90% localisation region with coordinates:
RA,DEC (J2000) =127.134217, 20.86431 | 08:28:32.21, +20:51:51.51
The source was initially detected with magnitude L = 18.57 ± 0.06 AB mag (+2.9h), with marginal fading to L = 18.64 ± 0.04 AB mag (+3.08h). The source was not detected in ATLAS forced photometry (Shingles et al 2021) at 2025-10-22 15:04:55 (11.44 hours pre-trigger), with a 5-sigma limiting magnitude of o > 19.45 AB mag.
Magnitudes were calibrated using ATLAS-REFCAT2 (Tonry et al. 2018) and are not corrected for Galactic extinction.
GOTO (https://goto-observatory.org) is a network of telescopes that is principally funded by the STFC and operated at the Roque de los Muchachos Observatory on La Palma, Spain, and Siding Spring Observatory in NSW, Australia, on behalf of a consortium including the University of Warwick, Monash University, Armagh Observatory & Planetarium, the University of Leicester, the University of Sheffield, the National Astronomical Research Institute of Thailand (NARIT), the University of Turku, the University of Portsmouth, the University of Manchester and the Instituto de Astrofisica de Canarias (IAC).
- GCN Circular #42388
Y. Wu (NJU), D. F. Hu (PMO, CAS), H. N. Yang, T. Zhao, W. D. Zhang (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 EP251023a. The transient triggered EP-WXT (ID: 01709247295) at 2025-10-23T02:31:09 (UTC). The WXT position of the source is R.A. = 127.151 deg, Dec. = 20.871 deg (J2000) with an uncertainty of 3 arcmin in radius (90% C.L. statistical and systematic).
We performed a Target-of-Opportunity observation with the Follow-up X-ray Telescope on board EP. The observation began at 2025-10-23T07:44:08 (UTC).An uncatalogued X-ray source was detected within the WXT error circle at R.A. = 127.1368 deg, Dec. = 20.8634 deg (J2000), with a positional uncertainty of 20 arcsec in radius (90% confidence level, including both statistical and systematic uncertainties). The source is spatially consistent with the counterpart reported in optical bands (Wortley et al. GCN 42387).
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 #42389
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 optical follow-up observations of EP251023a (Wu et al. GCN 42388) obtained with a Las Cumbres Observatory (LCO) 1-m telescope at Teide Observatory, Tenerife, ~3.6 hours after the EP-WXT trigger.
We acquired 2 × 300 s exposures using the broad optical w band.
The optical counterpart reported by Wortley et al. (GCN 42387) is detected. Preliminary photometry, calibrated to the r band, yields r ≈ 18.5 mag (not corrected for Galactic extinction). No known source is found at this position in the MPC or other major catalogs.
These observations were taken as part of the Global Supernova Project (GSP).
- GCN Circular #42391
X. Liu, Z.P. Zhu, L.B. He, J. An, S.Q. Jiang, D. Xu (NAOC), J.P.U. Fynbo (DAWN/NBI), Niko Pyykkinen (NOT) report on behalf of a large collaboration:
We observed the field of the EP251023a (Wu et al., GCN 42388) using the 2.56m Nordic Optical Telescope (NOT) equipped with the ALFOSC camera. Observation started at 05:55:11 UTC on 2025-10-23, i.e., 3.4 hours after the EP trigger, and a series of frames were obtained in the Sloan r- and z- bands.
Within the EP/FXT localization error region (Wu et al., GCN 42388), the optical counterpart (Wortley et al., GCN 42387; Li et al., GCN 42389) is also clearly detected in both r- and z- bands with magnitudes as follows:
T-mid (hr) | Filter | Exposure | Mag
3.49 | r | 3x200s | 18.57 +- 0.01
3.72 | z | 5x150s | 18.36 +- 0.01
calibrated against nearby Pan-STARRS stars and not corrected for Galactic extinction.
- GCN Circular #42398
R. Chornock (UC Berkeley) reports:
We observed the optical afterglow (Wortley et al., GCN 42387; Li et al., GCN 42389; Liu et al. GCN 42391) of the X-ray transient EP251023a (Wu et al., GCN 42388) using the Low-Resolution Imaging Spectrometer on the Keck-I telescope at a mean time of 13:26UT on 2025 October 23 (~10.9 hours after the EP trigger). Observations covered the range 3100-10300 Angstroms.
The afterglow remains bright and the continuum is well detected across the full spectral range. Multiple absorption systems are present, but the highest redshift absorber has many absorption lines (including Lyman-alpha, Si II, Si II*, O I, O I*, C II, Si IV, C IV, Fe II, Mg I and Mg II) present at a common redshift of z=2.232+/-0.001.
- GCN Circular #42400
Nikos Mandarakas (LAM), Ramandeep Gill (UNAM), Jean-Grégoire Ducoin (CPPM), Alan M. Watson (UNAM), Noémie Globus (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Jean-Luc Atteia (IRAP), Camila Angulo (UNAM), Dalya Akl (NYUAD), Sarah Antier (IJCLAB), Rosa L. Becerra (UNAM), Nathaniel R. Butler (ASU) , Damien Dornic (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Kin Ocelotl López (UNAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM) and Antonio de Ugarte Postigo (LAM):
We imaged the field of the EP251023a (Wu et al., GCN Circ. 42388) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2025-10-23 09:29 to 10:57 UTC (from 7.0 to 8.4 hours after the trigger) and obtained 32 minutes of simultaneous exposure in the r and z filters, alternating with other observations.
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.
We detected the optical counterpart firstly identified by GOTO (Wortley et al., GCN Circ. 42387) and subsequently reported by Las Cumbres Observatory (Li et al., GCN Circ. 42389) and NOT (Liu et al., GCN Circ. 42391) at preliminary magnitudes of:
r = 18.82 +/- 0.01
z = 18.52 +/- 0.01
Further observations are planned.
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 #42405
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, I. Gorbunov (Lomonosov MSU),
O.Gress, N.Budnev (ISU),
C.Francile, F. Podesta, R.Podesta, E. Gonzalez (Observatorio Astronomico Felix Aguilar (OAFA),
A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory),
A.Sosnovskij (CrAO),
A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity),
D.Buckley (SAAO),
R.Rebolo (The Instituto de Astrofisica de Canarias),
L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez,
A.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory)
MASTER-Tunka robotic telescope (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in Russia (Applied Physics Institute, Irkutsk State University) was pointed to the EP251023a ( EP Team et al., GCN 42388) errorbox 27379 sec after notice time and 51442 sec after trigger time at 2025-10-23 16:48:31 UT, with upper limit up to 18.2 mag. The observations began at zenith distance = 80 deg. The sun altitude is -50.5 deg.
The galactic latitude b = 31 deg., longitude l = 204 deg.
Real time updated cover map and OT discovered available here:
https://master.sai.msu.ru/site/master2/observ.php?id=3021715
We obtain a following upper limits.
Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment
_________|_____________________|_____________________|____________________________________|_____|_______|_______|________
51472 | 2025-10-23 16:48:31 | MASTER-Tunka | (08h 28m 06.31s , +20d 38m 18.3s) | C | 60 | 18.2 |
51472 | 2025-10-23 16:48:31 | MASTER- | (08h 28m 21.58s , +20d 31m 55.6s) | C | 60 | 18.2 |
Filter C is a clear (unfiltred) band.
The observation and reduction will continue.
The message may be cited.
- GCN Circular #42406
A. Selezneva, J. Basurto Merino, P.G. Berdayes, A. Caballero-Almagro, A. Cerón, M. Contreras, F. Díaz-Segado, T. Ferrer-Laviña, B. Gandolfi, V. Ghiraldo, J. Hernández Fung, L. Juliá-Maroto,
E. Lekaroz-Urriza, M. Manzano García, E. Mejía-Martínez, J. Prieto Polo, M. Pulido-Torres, M. Quintana-Ansaldo, A. Schenone-Zanuzzi, T. Tundidor Rodríguez, E. Urquijo-Rodríguez (all ULL), M. Abdul-Masih (IAC and ULL), and I. Pérez-Fournon (IAC and ULL).
Following the detection of the X-ray transient EP251023a by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (ID: 01709247295, Wu et al., GCN circ. 42388), we observed the field with one of the two Las Cumbres Observatory (LCO) 40-cm telescopes (Planewave Delta Rho 350 telescopes equipped with QHY600 CMOS cameras) located at the LCO node at Teide Observatory (Tenerife, Canary Islands, Spain). The observation, a single exposure of 300 sec in the SDSS r filter, started on 2025-10-23 at 06:09:24 UT, about 3.64 hours after the EP-WXT trigger. The optical counterpart detected by GOTO (GOTO25ixt, Wortley et al., GCN circ. 42387) is clearly detected in our image with a magnitude of r = 18.69 +/- 0.10 (AB), calibrated against PanSTARRS-1 DR2 stars and not corrected for galactic extinction, that is consistent with other optical detections at similar epochs (Li et al., GCN circ. 42389; Liu et al., GCN circ. 42391) and brighter than the COLIBRÍ detection at a later epoch (Mandarakas et al., GCN circ. 42400).
A redshift of z = 2.232 has been measured by Chornock (GCN circ. 42398) using Keck/LRIS.
Based on observations made with the Las Cumbres Observatory’s education network telescopes that were upgraded through generous support from the Gordon and Betty Moore Foundation (LCO program IAC2025B-010). These observations are part of a course in Astrophysical Techniques of the Master in Astrophysics of the Astrophysics Department of the University of La Laguna in collaboration with the Instituto de Astrofísica de Canarias (La Laguna, Tenerife, Canary Islands, Spain).
This work made use of the Astro-COLIBRI platform (P. Reichherzer et al. 2021, ApJS, 256, 5).
- GCN Circular #42411
T. Mohan (IITB), D. Eappachen (IIA), V. Swain (IITB), A.P. Saikia (IITB), V. Bhalerao (IITB), G.C. Anupama (IIA), S. Barway (IIA) and K. Angail (IAO) report on behalf of the GIT team:
We observed the field of EP transient EP251023a (Wu et al., GCN 42388), with 0.7m GROWTH-India Telescope (GIT). We started the observation at 2025-10-23 20:05:36 UT, i.e., 17.57 hours after the EP trigger. We obtained multiple exposures in g', r', and i' filters. We detect the optical afterglow in our stacked images, at the position reported by GOTO (Wortley et al., GCN 42387). The photometry result follows as:
| MJD (mid) | tmid - t0 (hours) | Filter | Exposure (s) | Magnitude (AB) |
| --------- | ----------------- | ------ | ------------ | -------------- |
|60971.87998| 18.60 | r' | 5 x 360 | 20.39+-0.09 |
|60971.90145| 19.12 | i' | 5 x 360 | 20.24+-0.11 |
|60971.92296| 19.63 | g' | 5 x 360 | 20.88+-0.10 |
The measurement is calibrated against PanSTARRS DR1 (Chambers et al., 2016) and not corrected for Galactic extinction.
Our magnitude is consistent with other optical observations (Wortley et al., GCN 42387; Li et al., GCN 42389; Liu et al., GCN 42391; Mandarakas et al., GCN 42400; Lipunov et al., GCN 42405; Selezneva et al., GCN 42406).
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 #42420
N.A. Rakotondrainibe (LAM), C. Adami (LAM/Pytheas/AMU), E. Le Floc'h (CEA/Irfu), report on behalf of the MISTRAL GRB collaboration:
We carried out observations of EP251023a (Mohan et al., GCN 42411; Selezneva et al., GCN 42406; Lipunov et al., GCN 42405; Mandarakas et al., GCN 42400; Liu et al., GCN 42391; Li et al., GCN 42389; Wu et al., GCN 42388; Wortley et al., GCN 42387) using the T193cm telescope at Observatoire de Haute-Provence (France) equipped with the MISTRAL spectro-imager. We obtained 4 exposures of 15 min in the r-band at a midtime of 2025 10 24 03:17:52UT corresponding to T-T0 = 24.78 hours.
The afterglow is well detected in r’ and we measured the following preliminary magnitude:
r’ = 21.55+/-0.10 mag (AB)
The photometric calibration was performed using nearby stars from the PanSTARRS catalog and the magnitudes are not corrected for Galactic extinction. We used the STDWeb/STDPipe tools (Karpov 2025).
We acknowledge the excellent support from the Observatoire de Haute-Provence and in particular Jean Balcaen.
- GCN Circular #42422
Y. Rajabov, O. Burkhonov, B.Abidkhanov, S. Ehgamberdiev, Y. Tillayev (UBAI), T. Boyqobilov, A. Shaymanov (Maidanak Observatory/UBAI) report on behalf of UBAI team.
We observed the field of EP transient EP251023a (Wu et al., GCN 42388), with the AZT-22 1.5m telescope of the Maidanak Observatory (MAO) starting on 2025-10-23 at 22:52:18 UT, i.e. ~10.37 hours since trigger. In total we obtained 6x300 s exposures in the R-band using 4kx4k CCD SNUCAM camera (Im et al., 2010).
The optical afterglow initially detected by GOTO (Wortley et al., GCN 42387) and also observed (Li et al., GCN 42389; Liu et al., GCN 42391; Mandarakas et al., GCN 42400; Lipunov et al., GCN 42405; Selezneva et al., GCN 42406; Mohan et al., GCN 42411; Rakotondrainibe et al., GCN 42420) is detected in our individual and stacked frames.
Preliminary photometry is as follows:
Date UTstart Exptime t-T0 Filter OT Err. UL Site/Telescope
(nxs) (mid, days)
2025-10-23 23:08:31 6x300 0.85928 R 20.56 0.03 22.99 MAO/AZT-22
All the data have been reduced by a single data processing pipeline, STDPipe (Karpov et al., 2022).
Images obtained in Johnson Cousins filters were calibrated using the Gaia DR3 Synphot catalog.
The data has not corrected for the Galactic extinction.
Maidanak astronomical observatory (MAO) is an observational facility of the Ulugh Beg Astronomical
Institute (UBAI), Uzbekistan Academy of Sciences (http://maidanak.uz/).
- GCN Circular #42468
A. Volnova (IKI), A. Pozanenko (IKI) , N. Pankov (HSE, IKI), E. Klunko (ISTP) report on behalf of IKI-GRB-FuN:
We observed the field of the EP transient EP251023a (Wu et al., GCN 42388) in the R-filter with the AZT-33IK 1.5m telescope of the Sayan Solar Observatory (Mondy). We observed the field on Oct. 23 and 25, 2025, taking several frames with 120 s exposures. The optical couterpart detected by GOTO (Wortley et al., GCN 42387) and also observed by Li et al., GCN 42389; Liu et al., GCN 42391; Mandarakas et al., GCN 42400; Lipunov et al., GCN 42405; Selezneva et al., GCN 42406; Mohan et al., GCN 42411; Rakotondrainibe et al., GCN 42420; Rajabov et al., 42422, is clearly detected in the stacked frame of the first epoch and is not detected in that of the second one. The preliminary photometry is the following:
Date UT start t-T0 Exp. Filter OT Err. UL
(mid,days) (n*s) (3sigma)
2025-10-23 20:15:41 0.75316 20*120 R 20.64 0.12 22.0
2025-10-25 20:07:45 2.75807 35*120 R n/d n/d 22.7
The photometry is based on several nearby PS1 stars (R-magnitudes Lupton's transformation) and is not corrected for the Galactic extinction.
- GCN Circular #42716
A. Volnova (IKI), A. Pozanenko (IKI), A. Kochergin (UAFO), R. Ya. Inasaridze (AbAO) report on behalf of IKI-GRB-FuN:
We observed the field of the EP transient EP251023a (Wu et al., GCN 42388) with the RC-500 (0.5m) telescope of UAFO/ISON-Ussuriysk observatory on Oct., 23, 2025 starting at (UT) 17:51:51, and the AS-32 0.7m telescope of the Abastumani Observatory (AbAO) on Oct., 24, 2025, starting at (UT) 22:53:36. Several 60-second exposures in R-filter were obtained during each observational epoch. The optical counterpart (Wortley et al., GCN 42387; Li et al., GCN 42389; Liu et al., GCN 42391; Mandarakas et al., GCN 42400; Lipunov et al., GCN 42405; Selezneva et al., GCN 42406; Mohan et al., GCN 42411; Rakotondrainibe et al., GCN 42420; Rajabov et al., GCN 42422; Volnova et al., GCN 42468) is clearly detected in the stacked UAFO image and is not detected in that of the AbAO. The preliminary photometry is the following:
Date UT start t-T0 Exp. Filter OT Err. UL Site/Tel.
(mid,days) (n*s) (3sigma)
2025-10-23 17:51:51 0.67889 100*60 R 19.97 0.22 20.9 UAFO/RC-500
2025-10-24 22:53:36 1.88138 40*60 R n/d n/d 20.5 AbAO/AS-32
The photometry is based on several nearby PS1 stars (Lupton's R-band transformation, Vega system) and is not corrected for the Galactic extinction.
- GCN Circular #42717
A. Volnova (IKI), A. Pozanenko (IKI), M. Krugov (FAI) report on behalf of IKI-GRB-FuN:
We observed the field of the EP transient EP251023a (Wu et al., GCN 42388) in r' filter with the AZT-20 1.5m telescope of Assy-Turgen observatory starting on Oct., 23, 2025 (UT) 20:25:35, taking several exposures of 60 seconds. The optical counterpart (Wortley et al., GCN 42387; Li et al., GCN 42389; Liu et al., GCN 42391; Mandarakas et al., GCN 42400; Lipunov et al., GCN 42405; Selezneva et al., GCN 42406; Mohan et al., GCN 42411; Rakotondrainibe et al., GCN 42420; Rajabov et al., GCN 42422; Volnova et al., GCN 42468; Volnova et al., GCN 42716) is clearly detected in the stacked frame. The preliminary photometry is the following:
Date UT start t-T0 Exp. Filter OT Err. UL
(mid,days) (n*s) (3sigma)
2025-10-23 20:25:35 0.76378 30*60 r' 20.22 0.06 22.2
The photometry is based on several nearby PS1 stars (AB-system) and is not corrected for the Galactic extinction.
Previous IAU Circulars 