EP251118a, GRB 251118C
GCN Circular 43056
Subject
EP251118a / GRB 251118C: VLT/MUSE redshift confirmation
Date
2025-12-09T17:24:40Z (4 days ago)
From
Andrea Saccardi at CEA/Irfu <andrea.saccardi@cea.fr>
Via
Web form
A. Saccardi (CEA/Irfu), D. B. Malesani (DAWN/NBI and Radboud), L. Izzo (INAF/OACn & DARK/NBI), V. D’Elia (ASI/SSDC), J. An, D. Xu (NAOC), M. De Pasquale (U. of Messina) report on behalf of the Stargate collaboration:
We observed the field of EP251118a / GRB 251118C detected by EP (Jiang et al., GCN 42749; Shi et al., GCN 42754), Konus-Wind (Frederiks et al., GCN 42765), SVOM/GRM (Wang et al., GCN 42766), Swift/BAT (Delaunay et al., GCN 42770), and Glowbug (Woolf et al., GCN 42771), using the ESO VLT UT4 (Yepun) equipped with the MUSE spectrograph. Our observation started at 07:17:25 UT on 2025 November 19 (14.6 hr after the GRB trigger), and consisted of 3 exposures of 700 s each.
The optical counterpart (Malesani et al., GCN 42751; Lipunov et al., GCN 42755; Belkin et al., GCN 42758; Yadav et al., GCN 42760; Busmann et al., GCN 42763; Francile et al., GCN 42767; Urquijo-Rodríguez et al., GCN 42773; Li et al., GCN 42776; Lee et al., GCN 42780; Moskvitin and Spiridonova, GCN 42785; Gupta et al., GCN 42787; Liu et al., GCN 42793; Aryan et al., GCN 42798; Gupta et al., GCN 42810; Volnova et al., GCN 42926) is well detected in the wavelength-stacked “white light” image.
Our spectra cover the wavelength range 4750 - 9330 AA. In a preliminary reduction, we detect a continuum over the entire covered wavelength range. From the detection of several absorption features that we identify as Fe II, Mg II, Mg I, and Ca II, we infer a redshift of z = 1.217 for the GRB. Additionally, we note the presence of the [O II] 3726,3729 doublet in emission at a comparable redshift (z = 1.218).
Our result is thus fully consistent with, and improves, the value previously reported by the Nordic Optical Telescope (An et al., GCN 42756).
We acknowledge expert support from the observing staff in Paranal, in particular Cedric Ledoux, Fuyan Bian, and Juan Carlos Olivares.
GCN Circular 42926
Subject
EP251118a: Mondy, CrAO, Assy and SAO RAS/Zeiss-1000 optical observations
Date
2025-12-01T18:59:42Z (12 days ago)
From
Alina Volnova at IKI RAS <alinusss@gmail.com>
Via
Web form
A. A. Volnova (IKI), A. S. Pozanenko (IKI), A. S. Moskvitin (SAO RAS), E. Klunko (ISTP), V. Rumyantsev (CrAO), M. Krugov (FAI), O. I. Spiridonnova (SAO RAS), N. S. Pankov (IKI) report on behalf of IKI-GRB-FuN:
We observed the field of the transient EP251118a (Jiang et al., GCN 42749) detected by the Einstein Probe Wide-field X-ray Telescope (EP WXT), with the AZT-33IK 1.5m telescope of the Sayan Solar Observatory (Mondy) on Nov. 20 and 24 in R filter, with the ZTSH telescope of CrAO on Nov. 20 and 22 in B and R bands, with the AZT-20 1.5m telescope of Assy-Turgen observatory on Nov. 21 in Sloan r filter and Zeiss-1000 of SAO RAS on Nov. 30 in Rc-filter. The optical counterpart (Malesani et al., GCN 42751; Lipunov et al., GCN 42755; An et al., GCN 42756; Belkin et al., GCN 42758; Yadav et al., GCN 42760; Busmann et al., GCN 42763; Francile et al., GCN 42767; Urquijo-Rodríguez et al., GCN 42773; Li et al., GCN 42776; Lee et al., GCN 42780; Moskvitin and Spiridonova, GCN 42785; Gupta et al., GCN 42787; Liu et al., GCN 42793; Aryan et al, GCN 42798; Gupta et al., GCN 42810) is detected in the stacked frame of each epoch.
However the flux of the optical transient might be affected by the nearby red star SDSS J081239.57+105619.8 (r_SDSS = 21.911+/-0.178, g-r ~ 3 mag) located in 2.5 arcsec to the East.
Some of our preliminary PSF photometry and observational details are the following:
Date UT start t-T0 Exp. Filter OT Err. UL Site/Instrum
(mid,days) (n*s) (3sigma)
2025-11-20 18:58:01 2.12539 45*120 R 21.45 0.11 22.4 Mondy/AZT-33IK
2025-11-20 21:40:22 2.23886 42*120 R 21.29 0.10 23.2 CrAO/ZTSh
2025-11-20 23:13:01 2.31556 51*120 B 21.02 0.11 23.1 CrAO/ZTSh
2025-11-21 19:57:19 3.16164 57*60 r 22.39 0.08 23.8 Assy/AZT-20
2025-11-22 22:55:20 4.29925 48*120 R 21.62 0.17 22.5 CrAO/ZTSh
2025-11-24 19:51:39 6.15840 36*120 R 23.1 0.4 22.5 Mondy/AZT-33IK
2025-11-30 01:49:42 11.40863 12*300 Rc 22.90 0.15 24.0 SAO/Zeiss-1000
The photometry is calibrated using several nearby stars, USNO-B1.0 B2, R2 magnitudes for AZT-33IK, ZTSh and Zeiss-1000 data, and SDSS r magnitudes for AZT-20 data, all magnitudes are not corrected for the Galactic extinction.
GCN Circular 42822
Subject
EP251118A: 6 GHz VLA Detection
Date
2025-11-24T15:40:05Z (19 days ago)
From
muskan.yadav@students.uniroma2.eu
Via
Web form
Muskan Yadav, Roberto Ricci, Eleonora Troja (U Rome) report:
We observed the position of EP251118A (Jiang et al., GCN 42749; Malesani et al., GCN 42751) with the Karl G. Jansky Very Large Array (VLA) on 2025 November 20, starting at 13:52 UT (~1.88 days after the EP/WXT trigger), at a central frequency of 6 GHz.
In our preliminary analysis, we detect a radio counterpart at 6 GHz with a flux density of ∼74 μJy. The position is consistent with the optical counterpart (An et al., GCN 42756; Belkin et al., GCN 42758; Yadav et al., GCN 42760; Busmann et al., GCN 42763; Francile et al., GCN 42767; Urquijo-Rodríguez et al., GCN 42773; Lee et al., GCN 42780; Li et al., GCN 42776; Liu et al., GCN 42793; Moskvitin et al., GCN 42785; Aryan et al., GCN 42798; Gupta et al., GCN 42787) and the X-ray counterpart (Shi et al., GCN 42754).
We thank the VLA staff for rapidly executing these observations.
GCN Circular 42810
Subject
EP251118A: 3.6m DOT optical afterglow detection
Date
2025-11-23T04:45:13Z (20 days ago)
From
ANSHIKA GUPTA at ARIES <anshika05180@gmail.com>
Via
Web form
Anshika Gupta, Pankaj Pawar, Dhruv Jain, Debolina Kar, and Kuntal Misra (ARIES) report:
We observed the field of EP251118A detected by Einstein Probe( Jiang et al. 2025, GCN 42749) with the 3.6m Devasthal Optical Telescope (DOT), located at the Devasthal Observatory of the Aryabhatta Research Institute of Observational Sciences (ARIES), India. The observations were started on 2025-11-22 at 04:57:21.74 UT, i.e., ~84.24 hours after the Einstein Probe trigger. We have taken multiple frames with an exposure time of 120 s in the r filter. We stacked the images after the alignment. We detect the optical counterpart in our stacked image within the error box of NOT (Malesani et al. 2025, GCN 42751). We obtain the following preliminary magnitude in the stacked image:
Date Start_UT T_start-T0 (hour) Filter Exp time (s) Magnitude
======================================================================
2025-11-22 04:57:21.74 ~ 84.24 r 120s*9 22.33 +/- 0.18
Our detection is consistent with Malesani et al. 2025 (GCN 42751); Belkin et al. 2025 (GCN 42758); Yadav et al. 2025 (GCN 42760); Busmann et al. 2025 (GCN 42763); Francile et al. 2025 (GCN 42767); Rodríguez et al. 2025 (GCN 42773); Li et al. 2025 (GCN 42776); Lee et al. 2025 (GCN 42780); Moskvitin et al. 2025 (GCN 42785); Gupta et al. 2025 (GCN 42787); Liu et al. 2025 (GCN 42793); Aryan et al. 2025 (GCN 42798).
The magnitude is not corrected for the Galactic extinction in the direction of the burst.
Photometric calibration is performed using the standard stars from the Gaia catalog.
GCN Circular 42798
Subject
EP251118a: Continued Optical Detection with Kinder observations
Date
2025-11-22T13:04:05Z (21 days ago)
From
Amar Aryan at National Central University, Institute of Astronomy (NCUIA) <amararyan941@gmail.com>
Via
Web form
A. Aryan, T.-W. Chen, Y.-H. Lee, C.-S. Lin (all NCU), A. K. H. Kong (NTHU), J. Gillanders, S. J. Smartt (both Oxford), Y. J. Yang (NYUAD), A. Sankar.K, Y.-C. Pan, C.-C. Ngeow, M.-H. Lee, C.-H. Lai, W.-J. Hou, H.-C. Lin, H.-Y. Hsiao, J.-K. Guo (all NCU), S. Yang, Z. N. Wang, L. L. Fan, G. H. Sun (all HNAS), H.-W. Lin (UMich), H. F. Stevance, S. Srivastav, L. Rhodes (all Oxford), M. Nicholl, M. Fulton, T. Moore, K. W. Smith, C. Angus, A. Aamer (all QUB), A. Schultz and M. Huber (both IfA, Hawaii) report:
Followed by our previous detection (Lee et al., GCN 42780 ), we continued to observe the field of the fast X-ray transient EP251118a (Jiang et al., GCN 42749) using the 1m LOT at Lulin Observatory in Taiwan as part of the Kinder collaboration (Chen & Yang et al., 2025, ApJ, 983, 86, doi:10.3847/1538-4357/adb428). The first LOT epoch of observations began at 18:09 UTC on November 21, 2025 (MJD 61000.756), 73.45 hours after the EP-WXT trigger.
We utilized the astroalign (Beroiz et al. 2020, A&C, 32, 100384) and astropy (Astropy Collaboration et al. 2022, ApJ, 935, 167) packages to align and stack the individual frames. We utilized the Python-based package AutoPhOT (Brennan & Fraser, 2022, A&A, 667, A62) to perform template subtraction with the DESI Legacy Survey (Dey et al. 2019, AJ 157, 168) DR10 image using the 'hotpants' (Becker A., 2015, ascl.soft. ascl:1504.004) algorithm. In the difference image, we onece again clearly detected the optical counterpart candidate proposed by Malesani et al. (GCN 42751) at a spectroscopic redshift of z = 1.216 (An et al., GCN 42756), and confirmed by Belkin et al. (GCN 42758); Yadav et al. (GCN 42760); Busmann et al. (GCN 42763); Francile et al. (GCN 42767); Urquijo-Rodríguez et al. (GCN 42773); Moskvitin et al. (GCN 42785); Gupta et al. (GCN 42787); and Liu et al (GCN 42793)
Moreover, we further utilized AutoPhOT to perform the PSF photometry. The details of the observations and the measured magnitude(in the AB system) are as follows:
Telescope | Filter | MJD (start) | t-t0 (hr) | Exposure (s) | Magnitude | avg. Seeing | med. Airmass
LOT | r | 61000.756 | 73.45 | 300 * 6 | 22.5 +/- 0.1 | 0".92 | 1.12
The presented magnitude is calibrated using the field stars from the ATLAS-RefCat2 catalog from MAST (Tonry J. L. et al. 2018, ApJ, 867, 105) and is not corrected for the expected Galactic foreground extinction of A_r = 0.09 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 can be found in Aryan et al. 2025, ApJS, 281, 20, doi:10.3847/1538-4365/adfc69.
GCN Circular 42793
Subject
EP251118a: Xinglong-2.16m optical observation
Date
2025-11-22T01:34:51Z (21 days ago)
From
liuxing@nao.cas.cn
Via
Web form
X. Liu, J.J. Jin, J. Zheng, Z. Fan, D. Xu, L.B. He, J. An, S.Q. Jiang, Z.P. Zhu, J.J. Jia(NAOC), S.Y. Fu(HUST) report on behalf of a large collaboration:
We observed the field of EP251118a detected by EP/WXT (Jiang et al., GCN 42749; Shi et al., GCN 42754;) and likely GRB 251118C (Frederiks et al., GCN 42765; Wang et al., GCN 42766; Delaunay et al., GCN 42770; Woolf et al, GCN 42771) using the Xinglong-2.16m telescope equipped with the BFOSC camera. Observations started at 19:28:32 UT on 2025-11-21 (i.e., 74.77 h after the EP trigger), and 15x300 s R-band frames have been obtained.
The optical afterglow (Melasani et al., GCN 42751; An et al., GCN 42756; Belkin et al., GCN42758; Yadav et al., GCN 42760; Busmann et al., GCN 42763; Francile et al., GCN 42767; Rodríguez et al., GCN 42773; Li et al., GCN 42776; Lee et al., GCN 42780; Moskvitin et al., GCN 42785; Gupta et al., GCN42787) was cleary detected in our stacked frame with a magnitude of R = 22.1 +/- 0.1 at a mid-time of 75.28hr, calibrated with Pan-STARRS1 DR2 catalog converted using Lupton (2005) equations and not corrected for Galactic extinction.
We thank the staff at Xinglong observatory for their excellent support.
GCN Circular 42787
Subject
EP251118A: 1.3m DFOT optical afterglow detection
Date
2025-11-21T12:03:39Z (22 days ago)
From
ANSHIKA GUPTA at ARIES <anshika05180@gmail.com>
Via
Web form
Anshika Gupta, Pankaj Pawar, Debolina Kar, Dhruv Jain, and Kuntal Misra (ARIES) report:
We observed the field of EP251118A detected by the Einstein Probe (Jiang et al. 2025, GCN 42749) with the 1.3m Devasthal Fast Optical Telescope (DFOT), located at the Devasthal Observatory of the Aryabhatta Research Institute of Observational Sciences (ARIES), India. The observations were started on 2025-11-20 at 20:16:13 UT, i.e., ~ 2.14 days after the Einstein Probe trigger. We have taken multiple frames with an exposure time of 300 s in the R filter. We stacked the images after the alignment. We detect the optical counterpart in our stacked image within the error box of NOT (Malesani et al. 2025, GCN 42751). We obtain the following preliminary magnitude in the stacked image:
Date Start_UT T_start-T0 (days) Filter Exp time (s) Magnitude
======================================================================
2025-11-20 20:16:13 ~2.14 R 250s*18 21.15 +/- 0.25
Our detection is consistent with Malesani et al. 2025 (GCN 42751); Belkin et al. 2025 (GCN 42758);Yadav et al. 2025 (GCN 42760); Busmann et al. 2025 (GCN 42763); Francile et al. 2025 (GCN 42767); Rodríguez et al. 2025 (GCN 42773); Li et al. 2025 (GCN 42776); Lee et al. 2025 (GCN 42780); Moskvitin et al. 2025 (GCN 42785).
The magnitude is not corrected for the Galactic extinction in the direction of the burst.
Photometric calibration is performed using the standard stars from the Gaia catalog.
GCN Circular 42785
Subject
EP251118a / GRB 251118C: SAO RAS optical observations
Date
2025-11-20T21:15:01Z (23 days ago)
From
Alexander Moskvitin at SAO RAS <mosk@sao.ru>
Via
Web form
A. Moskvitin and O. Spiridonova (SAO RAS),
report on behalf of GRB follow-up team
We observed the field of the EP251118a discovered by Einstein Probe
(Jiang et al., GCN 42749; Shi et al., GCN 42754, also detected with
the other space observatories, Frederiks et al., GCN 42765;
Wang et al., GCN 42766; DeLaunay et al., GCN 42770; Woolf et al.,
GCN 42771) with the 1-m SAO RAS telescope Zeiss-1000 equipped
with the CCD-photometer. We obtained 9 x 300 sec. images Rc band
on 2025.11.19T23:26:31--2025.11.20T03:08:31 (t_mid - T0 = 1.35766 d).
The OT (Malesani et al., GCN 42751; An et al., GCN 42756;
Belkin et al., GCN 42758; Busmann et al., GCN 42763; Francile et al.,
GCN 42767; Urquijo-Rodríguez et al., GCN 42773; Lee et al., GCN 42780;
also detected in NIR; Yadav et al., GCN 42760) is clearly visible
in our stacked image with the brightness of R = 21.24 +/- 0.07.
Preliminary photometry is based on the nearby objects from SDSS
(magnitudes converted with Lupton 2005 equations)
and has not corrected for the Galactic extinction.
GCN Circular 42780
Subject
EP251118a: Optical detection with Kinder observations
Date
2025-11-20T09:35:57Z (23 days ago)
From
Amar Aryan at National Central University, Institute of Astronomy (NCUIA) <amararyan941@gmail.com>
Via
Web form
Y.-H. Lee, A. Aryan, C.-S. Lin (all NCU), A. K. H. Kong (NTHU), T.-W. Chen (NCU), J. Gillanders, S. J. Smartt (both Oxford), Y. J. Yang (NYUAD), A. Sankar.K, Y.-C. Pan, C.-C. Ngeow, M.-H. Lee, C.-H. Lai, W.-J. Hou, H.-C. Lin, H.-Y. Hsiao, J.-K. Guo (all NCU), S. Yang, Z. N. Wang, L. L. Fan, G. H. Sun (all HNAS), H.-W. Lin (UMich), H. F. Stevance, S. Srivastav, L. Rhodes (all Oxford), M. Nicholl, M. Fulton, T. Moore, K. W. Smith, C. Angus, A. Aamer (all QUB), A. Schultz and M. Huber (both IfA, Hawaii) report:
We observed the field of the fast X-ray transient EP251118a (Jiang et al., GCN 42749) using the 1m LOT at Lulin Observatory in Taiwan as part of the Kinder collaboration (Chen & Yang et al., 2025, ApJ, 983, 86). The first LOT epoch of observations started at 16:21 UTC on the 19th of November 2025 (MJD 60998.681), 23.64 hr after the EP-WXT trigger.
We utilized the astroalign (Beroiz et al. 2020, A&C, 32, 100384) and astropy (Astropy Collaboration et al. 2022, ApJ, 935, 167) packages to align and stack the individual frames. We utilized the Python-based package AutoPhOT (Brennan & Fraser, 2022, A&A, 667, A62) to perform template subtraction with the DESI Legacy Survey (Dey et al. 2019, AJ 157, 168) DR10 image using the 'hotpants' (Becker A., 2015, ascl.soft. ascl:1504.004) algorithm. In the difference image, we clearly detected the optical counterpart candidate proposed by Malesani et al. (GCN 42751) at a spectroscopic redshift of z = 1.216 (An et al., GCN 42756), and confirmed by Belkin et al. (GCN 42758); Yadav et al. (GCN 42760); Busmann et al. (GCN 42763); Francile et al. (GCN 42767); and Urquijo-Rodríguez et al. (GCN 42773).
Moreover, we further utilized AutoPhOT to perform the PSF photometry. The details of the observations and the measured magnitude(in the AB system) are as follows:
Telescope | Filter | MJD (start) | t-t0 (hr) | Exposure (s) | Magnitude | avg. Seeing | med. Airmass
LOT | r | 60998.681 | 23.64 | 300 * 6 | 21.136 +/- 0.049 | 1".47 | 1.67
The presented magnitude is calibrated using the field stars from the ATLAS-RefCat2 catalog from MAST (Tonry J. L. et al. 2018, ApJ, 867, 105) and is not corrected for the expected Galactic foreground extinction of A_r = 0.034 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 can be found in Aryan et al. 2025, ApJS, 281, 20, doi:10.3847/1538-4365/adfc69.
GCN Circular 42776
Subject
EP251118a / GRB 251118C: SVOM/VT optical observation
Date
2025-11-20T06:52:54Z (23 days ago)
From
Huali Li at at NAOC, SVOM <lhl@nao.cas.cn>
Via
Web form
H. L. Li, L. P. Xin, Y. L. Qiu, C. Wu, Y. N. Ma, Z. H. Yao, Y. Xu, X. H. Han, J. Wang, P. P. Zhang, W. J. Xie, Y. J. Xiao, H. B. Cai, L. Lan, J. R. Xu, J. S. Deng, J. Y. Wei (NAOC), J. Palmerio (CEA) report on behalf of the SVOM/VT team.
SVOM/VT performed ToO observation on EP251118a/GRB 251116C (Jiang et al., GCN 42749; Shi et al., GCN 42754; Frederiks et al., GCN 42765; Wang et al., GCN 42766; Delaunay et al., GCN 42770; Woolf et al, GCN 42771). The observation started at 2025-11-19T07:02:19 UT, i.e. 14.33 hour after EP-WXT staring time in VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously.
The optical counterpart reported by Malesani et al. (GCN 42751) and other optical observations (An et al., GCN 42756; Belkin et al, GCN 42758; Yadav et al., GCN 42760; Busmann et al., GCN 42763; Francile et al., GCN 42767; Urquijo-Rodríguez et al., GCN 42773) was clearly detected by VT in both channels. The measurements in AB magnitudes were derived as below:
Mid time | Band | Exposure Time | Magnitudes (AB)
15.84 hour VT_B 20*70 sec 21.07+/-0.05 mag
15.84 hour VT_R 20*70 sec 20.64+/-0.05 mag
Our photometry was not corrected for Galactic extinction.
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 42773
Subject
EP251118a / GRB 251118C: ULL-ASTRO-MASTER detection of the optical afterglow with LCO 1-m telescope at CTIO
Date
2025-11-19T23:38:46Z (24 days ago)
From
Ismael Perez-Fournon at Instituto de Astrofisica de Canarias <ipf@iac.es>
Via
Web form
E. Urquijo-Rodríguez, 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, A. Selezneva, T. Tundidor Rodríguez (all ULL), M. Abdul-Masih (IAC and ULL), and I. Pérez-Fournon (IAC and ULL).
Following the detection of the fast X-ray transient EP251118a by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Jiang et al., GCN circ. 42749) and by the EP Follow-up X-ray Telescope (FXT) (Shi et al., GCN circ. 42754), we observed the field with one of the three Las Cumbres Observatory (LCO) 1-m telescopes equipped with Sinistro cameras located at the LCO node at Cerro Tololo Inter-American Observatory (CTIO), Chile. The observation, a single exposure of 500 sec in the SDSS r' filter, started on 2025-11-19 at 07:58:16 UT, about 15.26 hours after the EP-WXT trigger. The optical counterpart first detected by NOT ALFOSC (Malesani et al., GCN 42751), at a redshift of z = 1.216 (An et al., GCN circ. 42756) is clearly detected in our image with a magnitude of r' = 20.69 +/- 0.06 (AB), calibrated against PanSTARRS-1 DR2 stars and not corrected for Galactic extinction. Our result is consistent with other optical and near-infrared observations: Belkin et al. (GCN circ. 42758), Yadav et al. (GCN circ. 42760), Busmann et al. (GCN circ. 42763), and Francile et al. (GCN circ. 42767).
EPP251118a is likely associated with the long-duration GRB 251118C detected by Konus-Wind (Frederiks et al., GCN circ. 42765), SVOM/GRM (Wang et al., GCN circ. 42766), Swift/BAT (DeLaunay et al., GCN circ. 42770),and Glowbug (Woolf et al., GCN circ. 42771).
This work makes use of observations from the Las Cumbres Observatory global telescope network (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 42771
Subject
GRB 251118C: Glowbug gamma-ray detection of the fast X-ray transient EP251118A
Date
2025-11-19T20:54:02Z (24 days ago)
From
richard.s.woolf.civ@us.navy.mil
Via
Web form
R. Woolf, C.C. Cheung, M. Kerr, J.E. Grove (NRL), A. Goldstein (USRA), C.A. Wilson-Hodge, D. Kocevski (MSFC), and M.S. Briggs (UAH) report:
The Glowbug gamma-ray telescope [1,2,3], operating on the International Space Station, reports the detection of GRB 251118C, a possible gamma-ray counterpart of EP251118A (GCN 42749), also detected by Konus-Wind (GCN 42765), SVOM/GRM (GCN 42766), and Swift/BAT (GCN 42770).
Using an adaptive window with a resolution of 32-ms, the burst onset is determined to be 2025-11-18 16:44:45.264 with a duration of 8.2 s and a total significance of about 23.8 sigma. The light curve comprises a single primary peak. Note that data from ~T0+8s to ~T0+12s suffered from deadtime in various detectors.
The best-fit localization is RA, Decl. (J2000, deg) = 125.0, 14.2 with a radius of 10.5 deg (95% confidence), with a highly uncertain systematic uncertainty. The Glowbug position for GRB 251118C is consistent with that reported for the fast X-ray transient EP251118A (GCN 42749).
The analysis results presented here are preliminary and use a response function that lacks a detailed characterization of the surrounding passive structure of the ISS.
Glowbug is a NASA-funded technology demonstrator for sensitive, low-cost gamma-ray transient telescopes developed, built, and operated by the U.S. Naval Research Laboratory (NRL) with support from the University of Alabama in Huntsville, USRA, and NASA MSFC. It was launched on 2023 March 15 aboard the Department of Defense Space Test Program’s STP-H9 to the ISS and operated until 2024 April when it was put in safe storage on orbit. Glowbug was removed from storage and resumed operation on 2025 September 12.
[1] Grove, J.E. et al. 2020, Proc. Yamada Conf. LXXI, arXiv:2009.11959
[2] Woolf, R.S. et al. 2022, Proc. SPIE, 12181, id. 121811O
[3] Woolf, R.S. et al. 2024, Proc. SPIE, 13151, id. 1315108
Distribution Statement A: Approved for public release. Distribution is unlimited.
GCN Circular 42770
Subject
GRB 251118C: Swift/BAT rates detection of a burst with a possible association with EP 251118A
Date
2025-11-19T20:06:57Z (24 days ago)
From
Jimmy DeLaunay at Penn State <delauj2@gmail.com>
Via
Web form
James DeLaunay (PSU), Samuele Ronchini (GSSI), Aaron Tohuvavohu (Cosmic Frontier), Jamie A. Kennea (PSU), Tyler Parsotan (NASA GSFC), Maia Williams (Northwestern) report:
Swift/BAT did not localize GRB 251118C onboard (T0: 2025-11-18T16:44:49 UTC, Konus-Wind GCN 42765, SVOM/GRM GCN 42766)
Due to the lack of prompt trigger, the Gamma-ray Urgent Archiver for Novel Opportunities (GUANO; Tohuvavohu et al. 2020, ApJ, 900, 1) was unable to save time-tagged event data around this burst.
The GRB is clearly detected in the BAT rates data with a S/N ~ 13. The emission is seen starting at ~T0 - 12 s and lasts ~28 s.
There are minimal counts seen at energies less than 50 keV, which is consistent with a GRB that is outside the coded field of view of BAT, especially with the soft spectrum observed by Konus-Wind and SVOM/GRM. There is also a 10 s time tagged event data file during the burst due to an onboard rate trigger that failed to recover an image position. We used the 10 s of event data to create an image (15 - 350 keV) and find no new sources in it.
Due to the lack of sources in the image and the observed energy spectrum, this GRB is likely to be outside the 10% coded field of view of Swift/BAT. Also, we can confirm that the GRB is not Earth Occulted for Swift. This rules out a circular region of the sky centered at ra, dec = 21.69, 12.65 deg (J2000) with a radius of 69.5 deg.
This rules out only ~half of the sky, and does not rule out the position of EP 251118A (GCN 42749).
There are no NITRATES results due to the lack of GUANO data, but the rates light-curve and position of the Earth and BAT coded FoV at the time can be seen on the results summary page
https://guano.swift.psu.edu/trigger_report?id=785177125
GCN Circular 42767
Subject
EP251118A / GRB 251118C : MASTER optical counterpart observations
Date
2025-11-19T17:40:22Z (24 days ago)
From
Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov@xray.sai.msu.ru>
Via
email
Carlos Francile, Ricardo Podesta, Federico Podesta (OAFA),
V. Lipunov, A.Kuznetsov, E.Gorbovskoy, G.Antipov, P.Balanutsa,
I. Panchenko, N. Tiurina, K.Zhirkov, Ya.Kechin, A.Chasovnikov, V.Topolev,
D. Vlasenko, Yu. Tselik, V.Senik (Lomonosov MSU, SAI, Moscow),
E. Gonzalez (OAFA, San Juan Uni., Argentina),
D. Buckley (SAAO, South Africa),
O. Gress, N.Budnev, O.Ershova (Irkutsk State University),
A. Sosnovskij (CrAO RAS),
A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory),
A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity),
V.M.Pillet, R.Rebolo Lopez (The Instituto de Astrofisica de Canarias,Spain),
L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino, J.Martinez,A.R.Corella,L.H.Rodriguez (INAOE, Guillermo Haro Astrophysic Observatory, Mexico)
MASTER-OAFA robotic telescope (Global MASTER Net [1-4],http://observ.pereplet.ru)
located in Argentina started EP251118A (EP:Jiang et al. GCN 42749, Shi et al. GCN 42754, Konus-Wind: Frederiks et al. GCN 42765 as GRB 251118C)
at 2025-11-19 07:57:04 UT (Lipunov et al. GCN 42755)
There is optical transient MASTER OT J081239.45+105619.7 with m_OT=19.8 at 2025-11-19 07:57:04UT (calibrated by Gaia G).
This optical counterpart of GRB 251118C was discovered by NOT (Malesani et al. GCN 42751) and also observed by VLT (Yadav et al. GCN 42760) and FTW (Busmann et al. GCN 42763)
MASTER observations started at 45 degrees altitude (Sun_alt=-17deg)
GCN Circular 42766
Subject
GRB 251118C: SVOM/GRM observation of a possible gamma-ray counterpart of EP251118a
Date
2025-11-19T17:18:22Z (24 days ago)
From
Chenwei Wang at IHEP <cwwang@ihep.ac.cn>
Via
Web form
SVOM/GRM team: Chen-Wei Wang, Zheng-Hang Yu, Chao Zheng, Yue Huang, Shi-Jie Zheng, Shao-Lin Xiong, Shuang-Nan Zhang (IHEP)
SVOM/ECLAIRs team: Maria-Grazia Bernardini (INAF-OAB)
Report on behalf of the SVOM team:
SVOM/GRM was triggered on-ground by a soft burst GRB 251118C at 2025-11-18T16:44:48.500 UTC (T0).
With the event-by-event data downloaded through the X-band ground station, the GRM light curve shows that this burst consists of a single pulse with a T90 of 29 +22/-12 s in the 15-5000 keV band, which is also detected by Kouns-Wind (D. Frederiks et al., GCN#42765).
The SVOM/GRM light curve can be found here:
https://www.bursthub.cn//admin/static/svgrb251118C.png
We note that the time coincidence between EP251118a (S. Q. Jiang et al., GCN#42749) and GRB 251118C suggest the association of these two events.
In addition, the position of this burst, if is associated with EP251118a (RA= 123.160, DEC= 10.952, GCN#42749), is located at about 136 degrees from the SVOM optical axis, which is outside the ECLAIRs field of view.
With this localization, the time-averaged spectrum from T0-6 to T0+17 s is best fitted by a power law function with an exponential high-energy cutoff. The power law index is -1.86 +0.17/-0.09 and the cutoff energy, parameterized as Epeak, is 44 +37/-21 keV. The event fluence (10-1000 keV) in this time interval is (7.82 +0.84/-0.83)E-06 erg/cm^2.
The time-averaged spectrum is also well fitted by a power law function with a power law index of -2.4 +0.15/-0.16, corresponding to a fluence of (1.00 +0.12/-0.11)E-05 erg/cm^2.
The 1s peak spectrum, measured from T0-0.5 to T0+0.5 s, if fitted by a power law function with an exponential high-energy cutoff, the power law index is -1.83 +0.25/-0.12 and the cutoff energy, parameterized as Epeak, is 64 +55/-33 keV. The flux (10-1000 keV) in this time interval is (9.47 +1.30/-1.29)E-06 erg/cm^2/s.
Assuming the redshift z= 1.216 (J. An et al., GCN#42756), the localization of GRB 251118C in the 'Amati' relation diagram is shown at:
https://www.bursthub.cn//admin/static/svgrb251118C_amati.png
And the localization in the 'Yonetoku' relation diagram is shown at:
https://www.bursthub.cn//admin/static/svgrb251118C_yonetoku.png
We caution that this analysis based on the BREAKFAST framework is very preliminary [1]. Refined analysis will be reported later.
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. GRM is developed by the Institute of High Energy Physics (IHEP) of CAS.
The SVOM point of contact for this burst is: Chen-Wei Wang (IHEP)(cwwang@ihep.ac.cn)
[1] Chen-Wei Wang et al. arXiv: 2510.15816 [astro-ph.HE] https://arxiv.org/abs/2510.15816
GCN Circular 42765
Subject
Konus-Wind detection of GRB 251118C (EP251118a)
Date
2025-11-19T16:58:51Z (24 days ago)
Edited On
2025-11-20T21:41:04Z (23 days ago)
From
Dmitry Frederiks at Ioffe Institute <fred@mail.ioffe.ru>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov>
Via
email
D. Frederiks, A.Lysenko, A. Ridnaia, D. Svinkin,
A. Tsvetkova, M. Ulanov, and T. Cline,
on behalf of the Konus-Wind team, report:
A long-duration GRB 251118C was detected by Konus-Wind (KW)
in the waiting mode at T0=T0(KW)~16:44:49 UT.
A Bayesian block analysis of the KW data in the 20-1500 keV
band reveals a ~17 sigma count rate increase in the interval
from ~T0-0.5 s to ~T0+23 s.
Corrected for the propagation time, the burst started ~140 s
after the start time of the fast X-ray transient EP251118a
(T0(EP)=16:42:29 UT; Jiang et al., GCN 42749).
The burst source is located in the southern ecliptic hemisphere
at a low ecliptic latitude, which is consistent with the EP251118a localization.
The positional and temporal coincidence of GRB 251118C with
the EP251118a supports the conclusion that both events have
a common origin.
The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB251118C/
The time-integrated spectrum of the burst, measured from T0-0.5 s to T0+23 s,
can be described by a simple power function with the photon index (2.29 ± 0.11).
The total burst fluence is (5.08 ± 0.40)x10^-6 erg/cm^2,
and the 2.944 s peak energy flux, measured from T0-1.96 s,
is (4.84 ± 0.38)x10^-7 erg/cm^2(both in the 20 - 1500 keV energy range).
Assuming the redshift z=1.216 (An et al., GCN 42756)
and a standard cosmology with H_0 = 67.3 km/s/Mpc, Omega_M = 0.315,
and Omega_Lambda = 0.685 (Planck Collaboration, 2014),
we estimate the burst isotropic energy release E_iso to (2.06 ± 0.16)x10^52 erg,
the isotropic peak luminosity L_iso to (4.34 ± 0.34)x10^51 erg/s.
All the quoted errors are estimated at the 68% confidence level.
All the presented results are preliminary.
GCN Circular 42763
Subject
EP251118a: FTW optical and NIR observations
Date
2025-11-19T15:14:34Z (24 days ago)
From
Malte Busmann at Ludwig-Maximilians-Universität München <m.busmann@physik.lmu.de>
Via
Web form
Malte Busmann (LMU), Xander J. Hall (CMU), Brendan O'Connor (CMU), Daniel Gruen (LMU), and Antonella Palmese (CMU) report:
We observed the counterpart of EP251118a (Jiang et al., GCN 42749; Malesani et al., GCN 42751; Shi et al., GCN 42754; An et al., GCN 42756) with the Three Channel Imager (3KK) at the Fraunhofer Telescope at Wendelstein Observatory (FTW) in the r, i, and J bands simultaneously for 5 x 180 s starting at 2025-11-19T04:06:56 UT (0.48 days after the trigger). The counterpart is detected in all bands. Using Legacy Survey templates for difference imaging, we measure an r-band magnitude of
r = 20.56 +/- 0.04 AB mag.
Our measurements are consistent with the reports by Malesani et al. (GCN 42751), Lipunov et al. (GCN 42755), Belkin et al. (GCN 42758), and Yadav et al. (GCN 42760).
The magnitude is calibrated against the PS1 catalog and not corrected for Galactic extinction.
We thank Michael Schmidt from the Wendelstein Observatory for obtaining these observations.
GCN Circular 42760
Subject
EP251118a: VLT near-infrared observation
Date
2025-11-19T13:22:26Z (24 days ago)
From
muskan.yadav@students.uniroma2.eu
Via
Web form
Muskan Yadav, Yu-Han Yang, Massine El Kabir, Niccolò Passaleva, Nicolás Monsalves (U Rome) on behalf of the ERC BHianca team:
We observed the field of EP251118a (Jiang et al., GCN 42749) with the HAWKI imager on the ESO VLT UT4 (Yepun). Observations began at 15.5 hr after the EP/WXT trigger and were carried out at an average airmass of about 1.2 in the J filter.
We detect the optical counterpart (Malesani et al., GCN 42751; Belkin et al., GCN 42758) at a preliminary magnitude J ~20.35 AB mag calibrated using nearby stars in the 2MASS Catalogue (Skrutskie et al. 2006) and not corrected for Galactic extinction.
We thank the staff at the VLT, for the rapid execution of these observations.
GCN Circular 42758
Subject
EP251118a: GOTO optical afterglow detection
Date
2025-11-19T11:15:29Z (24 days ago)
From
Sergey Belkin at Monash University <sergey.belkin@monash.edu>
Via
Web form
S. Belkin, M. E. Wortley, D. O'Neill, B. P. Gompertz, G. Ramsay, R. L. C. Starling, K. Ackley, M. Dyer, J. Lyman, K. Ulaczyk, D. Steeghs, D. Galloway, V. Dhillon, P. O'Brien, K. Noysena, R. Kotak, R. Breton, J. Casares, L. Nuttall, B. Godson, T. Killestein, A. Kumar, M. Pursiainen report on behalf of 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 the EP/WXT alert WXT11900480769 (Jiang et al., GCN 42749).
Observations covering the position began on 2025-11-19 at 02:58:22 UT (+10.26 hr post trigger) and continued through to 2025-11-19 at 04:30:49 UT (+11.81 hr post trigger). Three epochs were obtained: the first through serendipitous survey coverage (4x45 s) and the second and third through targeted response (4x90 s), all 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.
We detect the optical counterpart (Malesani et al., GCN 42751), consistent with the X-ray counterpart (Shi et al., GCN 42754), with L = 20.08 ± 0.21 (AB) on 2025-11-19 at 02:58:22 UT (+10.26 hr). Follow-up L-band images at 03:22:37 UT (+10.67 hr) and 04:30:49 (+11.81 hr) yield L = 20.24 ± 0.17 and 20.17 ± 0.16 (AB), respectively.
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, the University of Birmingham, and the Instituto de Astrofisica de Canarias (IAC).
GCN Circular 42756
Subject
EP251118a: NOT spectroscopic redshift of z = 1.216
Date
2025-11-19T09:17:33Z (24 days ago)
From
J. An <jiean0813@foxmail.com>
Via
Web form
J. An (NAOC), D. B. Malesani (DAWN/NBI and Radboud), L.B. He, D. Xu, X.Liu (NAOC), G. Corcoran (UCD), L. Cotter (UCD), A. J. Levan (Radboud and Warwick), P. G. Jonker (Radboud), L. Fuglsang (NOT), report on behalf of a large collaboration:
We observed the optical afterglow (Malesani et al., GCN 42751) of EP251118a (Jiang et al., GCN 42749) with the Nordic Optical Telescope (NOT) equipped with the ALFOSC camera. A spectrum using grism #4 was secured starting on 2025 Nov 19.19 UT (11.9 h after the GRB). A total of 4 spectra of 1200 s each were obtained, covering the wavelength range 3500-9600 AA.
Preliminary reduction of the stacked spectrum shows a clear continuum down to at least 3700 AA. The lack of a Ly-alpha feature in the spectrum sets a redshift limit z < 2. A few narrow absorption features are detected, which we attribute to the Mg II doublet and several Fe II lines (2600, 2586, 2344), all at a common redshift of z = 1.216. We suggest this is the redshift of EP251118a.
GCN Circular 42755
Subject
EP251118a: Global MASTER-Net observations report
Date
2025-11-19T08:27:23Z (24 days ago)
From
Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov@xray.sai.msu.ru>
Via
email
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-OAFA robotic telescope [1] located in Argentina (OAFA observatory of San Juan National University) was pointed to the EP251118a ( EP Team et al., GCN 42749) errorbox 12817 sec after notice time and 54875 sec after trigger time at 2025-11-19 07:57:04 UT, with upper limit up to 20.2 mag. The observations began at zenith distance = 44 deg. The sun altitude is -17.5 deg.
The galactic latitude b = 23 deg., longitude l = 213 deg.
Real time updated cover map and OT discovered available here:
https://master.sai.msu.ru/site/master2/observ.php?id=3046754
We obtain a following upper limits.
Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment
_________|_____________________|_____________________|____________________________________|_____|_______|_______|________
54905 | 2025-11-19 07:57:04 | MASTER-OAFA | (08h 12m 31.85s , +10d 55m 21.3s) | C | 60 | 20.2 |
54982 | 2025-11-19 07:58:20 | MASTER-OAFA | (08h 12m 38.49s , +10d 54m 18.7s) | C | 60 | 20.0 |
55059 | 2025-11-19 07:59:38 | MASTER-OAFA | (08h 12m 32.20s , +10d 53m 14.4s) | C | 60 | 20.0 |
55137 | 2025-11-19 08:00:55 | MASTER-OAFA | (08h 12m 37.33s , +10d 53m 29.6s) | C | 60 | 20.1 |
55223 | 2025-11-19 08:02:21 | MASTER-OAFA | (08h 12m 33.38s , +10d 54m 50.5s) | C | 60 | 20.2 |
55301 | 2025-11-19 08:03:39 | MASTER-OAFA | (08h 12m 32.94s , +10d 53m 07.3s) | C | 60 | 20.1 |
55408 | 2025-11-19 08:04:56 | MASTER-OAFA | (08h 12m 34.44s , +10d 54m 29.2s) | C | 120 | 20.0 |
55544 | 2025-11-19 08:07:13 | MASTER-OAFA | (08h 12m 28.30s , +10d 53m 45.7s) | C | 120 | 20.1 |
55681 | 2025-11-19 08:09:30 | MASTER-OAFA | (08h 12m 28.13s , +10d 54m 37.0s) | C | 120 | 20.0 |
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 42754
Subject
EP251118a: preliminary result of EP-FXT follow-up observation
Date
2025-11-19T07:36:46Z (24 days ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
R. Shi, Q. J. Huang, D. F. Hu (PMO, CAS), F. F. Song (YNAO, CAS), S. Q. Jiang (NAO, CAS), Q. C. Liu (THU), C. C. Jin (NAO, CAS) report on behalf of the Einstein Probe team:
The Follow-up X-ray Telescope (FXT) on board the Einstein Probe (EP) performed one ToO observation for EP251118a, starting at 2025-11-19T04:02:14 (UTC). Within the WXT error circle, an uncatalogued X-ray source was detected at R.A. = 123.1677 deg, DEC = 10.9364 deg (J2000) with an uncertainty of 20 arcsec in radius (90% C.L. statistical and systematic). The position is consistent with the optical counterpart reported before (Malesani et al. GCN 42751) with a seperation of 14.75 arcsec.
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 42751
Subject
EP251118a: NOT candidate optical counterpart
Date
2025-11-19T05:20:57Z (24 days ago)
From
Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani@nbi.ku.dk>
Via
Web form
D. B. Malesani (DAWN/NBI and Radboud), G. Corcoran (UCD), L. Cotter (UCD), J. An, Z.P. Zhu, D. Xu (NAOC), P. G. Jonker (Radboud), L. Fuglsang (NOT), report on behalf of a larger collaboration:
We observed the field of the transient event EP251118a (Jiang et al., GCN 42749) detected by the Einstein Probe Wide-field X-ray Telescope (EP WXT), using the Nordic Optical Telescope (NOT) equipped with the ALFOSC camera. Observations were obtained in the r (3x300 s) and z (5x200 s) bands, starting on 2025-11-19 at 03:55:38 UT (11.22 hr after the EP/WXT trigger).
In our first r-band exposure, within the WXT uncertainty region, we detect a source significantly brighter than in the archival images from the Legacy Survey. This object has coordinates:
RA(J2000): 08:12:39.4
Dec(J2000): +10:56:18.88
with an uncertainty of 0.5" in both RA and Dec. In the NOT image, we measure an AB magnitude of r = 20.52+/-0.04, calibrated against the Pan-STARRS catalog and not corrected for Galactic extinction. At this location, a source can be marginally seen in the deeper Legacy Source images, with an AB magnitude of r = 24.18 +/- 0.26.
The source seen in the NOT images is a candidate optical afterglow of EP251118a, and the archival counterpart in the Legacy Survey is its candidate host galaxy. Further multi-wavelength observations are encouraged to confirm the nature of this object.
GCN Circular 42749
Subject
EP251118a: Einstein Probe detected of a fast X-ray transient
Date
2025-11-19T04:22:00Z (24 days ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
S. Q. Jiang (NAO, CAS), Q. C. Liu (THU), D. F. Hu, R. Shi, Q. J. Huang (PMO, CAS), F. F. Song (YNAO, CAS), C. C. Jin (NAO, CAS) report on behalf of the Einstein Probe team:
We report on the detection of a fast X-ray transient EP251118a by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission. The transient did not trigger the WXT onboard unit. A GCN Notice was sent manually (trigger ID: 11900480769). The WXT light curve started at 2025-11-18T16:42:29 (UTC).
The position of the source is R.A. = 123.160 deg, DEC = 10.952 deg (J2000) with an uncertainty of 1.87 arcmin (radius, 90% C.L. statistical and systematic). The transient event has a precursor-like signature lasted about 42 seconds followed by a main burst lasting at least 79 seconds. After that, the observation ended, resulting in no further data.
The average 0.5-4.0 keV spectrum can be fitted with an absorbed powerlaw model, with free nH of 2.26 (-0.26/+0.27) x 10^21 cm^-2 and a photon index of 1.32 (-0.08/+0.08). The unabsorbed average 0.5-4.0 keV flux is estimated to be 2.50 (-0.08/+0.08) x 10^-8 erg/s/cm2. The peak flux reaches around 1 x 10^-7 erg/s/cm2. The uncertainties are at the 90% confidence level for the above parameters.
Further FXT observations are scheduled.
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).