Gamma-ray Burst 251220C = EP251220a

(All information courtesy of the instrument teams.)

Previous IAU Circulars

none

Results of Observations

GCN Circular #43162
Q. C. Liu (THU), Y. Wang (PMO), H. Y. Liu, H. W. Pan (NAO,CAS) on behalf of the Einstein Probe team:
We report on the detection of a fast X-ray transient, designated EP251220a, by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission. The transient event started at 2025-12-20T08:26:43 (UTC). The position of the source is R.A. =17.322 deg, DEC = 19.080 deg (J2000) with an uncertainty of 2.2 arcmin in radius (90% C.L. statistical and systematic).
The transient event lasts for nearly 170 seconds. The transient did not trigger the WXT onboard unit. The average 0.5-4 keV spectrum can be fitted by an absorbed power law with a photon index of 2.2 (+1.5/-1.1) with a column density of 0.2 (+0.4/-0.2) x 10^22 cm^-2. The derived average unabsorbed 0.5-4 keV flux is 1.2 (+1.2/-0.4) x 10^-9 erg/s/cm^2. The peak flux is around 4.4 x 10^-9 erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters.
The Follow-up X-ray Telescope (FXT) on board the EP performed a ToO observation for EP251220a, starting at 2025-12-21T03:16:08 (UTC). Within the WXT error circle, an uncatalogued X-ray source was detected at R.A. = 17.3160 deg, DEC = 19.0935 deg (J2000) with an uncertainty of 20 arcsec in radius (90% C.L. statistical and systematic).
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 #43163
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-Kislovodsk robotic telescope [1] located in Russia (Lomonosov MSU, Kislovodsk Solar Station of Pulkovo observatory) was pointed to the EP251220a ( EP Team et al., GCN 43162) errorbox 23634 sec after trigger time at 2025-12-20 15:00:37 UT, with upper limit up to 17.9 mag. Observations started at twilight. The observations began at zenith distance = 28 deg. The sun altitude is -14.4 deg.
MASTER-OAFA robotic telescope located in Argentina (OAFA observatory of San Juan National University) was pointed to the EP251220a errorbox 70921 sec after trigger time at 2025-12-21 04:08:44 UT, with upper limit up to 16.1 mag. The observations began at zenith distance = 81 deg. The sun altitude is -35.3 deg.
The galactic latitude b = -43 deg., longitude l = 130 deg.
Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=3079135
We obtain a following upper limits.
```
Tmid-T0  |      Date Time      |          Site       |             Coord (J2000)          |Filt.| Expt. | Limit| Comment
_________|_____________________|_____________________|____________________________________|_____|_______|_______|________

   23665 | 2025-12-20 15:00:37 |   MASTER-Kislovodsk | (01h 06m 41.00s , +20d 06m 46.9s) |   C |    60 | 14.1 |        
   24358 | 2025-12-20 15:12:11 |   MASTER-Kislovodsk | (01h 13m 48.06s , +18d 34m 54.1s) |   C |    60 | 17.8 |        
   24438 | 2025-12-20 15:13:31 |   MASTER-Kislovodsk | (01h 14m 45.05s , +20d 30m 16.8s) |   C |    60 | 17.7 |        
   24598 | 2025-12-20 15:16:11 |   MASTER-Kislovodsk | (01h 01m 10.07s , +18d 35m 48.4s) |   C |    60 | 17.9 |        
   24598 | 2025-12-20 15:16:11 |   MASTER-Kislovodsk | (01h 05m 50.48s , +18d 12m 44.0s) |   C |    60 | 14.2 |        
   70952 | 2025-12-21 04:08:44 |         MASTER-OAFA | (01h 09m 37.27s , +19d 10m 42.5s) |   C |    60 | 16.1 |        
   71017 | 2025-12-21 04:09:50 |         MASTER-OAFA | (01h 09m 37.22s , +19d 10m 42.5s) |   C |    60 | 15.9 |        
   71083 | 2025-12-21 04:10:56 |         MASTER-OAFA | (01h 09m 37.31s , +19d 10m 39.9s) |   C |    60 | 15.9 |        
   71159 | 2025-12-21 04:12:11 |         MASTER-OAFA | (01h 09m 37.27s , +19d 10m 38.5s) |   C |    60 | 15.1 |        
```
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 #43167
Fredd Sánchez Álvarez (UNAM), Ny Avo Rakotondrainibe (LAM), Leonardo García García (UNAM), Alan M. Watson (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), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Jean-Grégoire Ducoin (CPPM), Enrique Moreno Méndez (UNAM), Kin Ocelotl López (UNAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Nikos Mandarakas (LAM), Margarita Pereyra (UNAM), Benjamin Schneider (LAM), and Antonio de Ugarte Postigo (LAM).
We imaged the field of the fast X-ray transient EP251220a (Liu et al., GCN Circ. 43162) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2025-12-21 04:08 UTC to 04:35 UTC (from 19.70 to 20.13 hr after the trigger) and obtained 18 minutes of simultaneous exposure in the r and z filters.
The data were reduced, coadded and analyzed with the COLIBRÍ pipeline. The photometry was calibrated using nearby stars from the PanSTARRS DR1 catalog, is in the AB system, and was not corrected for Galactic extinction.
In the stacked image, we detect an uncataloged source at the FXT uncertainty region (Liu et al., GCN Circ. 43162) at
```
RA (J2000) = 01:09:16.60 = 17.31915 deg
Dec (J2000) = +19:05:32.3 = 19.09231 deg
```
with an uncertainty of about 0.5 arcsec and with preliminary magnitudes of
```
r = 21.15 +/- 0.02
z = 20.72 +/- 0.04
```
We suggest that this is the counterpart of the fast X-ray transient. Further observations will be necessary to detect fading.
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 #43169
A. Kumar, G. Ramsay, M. Kennedy, D. O'Neill, B. P. Gompertz, R. Starling, K. Ackley, M. J. Dyer, J. Lyman, K. Ulaczyk, B. Godson, 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 observations with the Gravitational-wave Optical Transient Observer (GOTO; Steeghs et al. 2022, Dyer et al. 2024) that serendipitously covered the field of EP 251220a (Liu et al. GCN 43162). The observations were conducted with GOTO-North at 2025-12-20 19:51:42 UT (11.4 hours post-trigger), consisting of 4x45s exposures in the GOTO L-band (400-700 nm). Difference imaging was performed using deeper template observations of the same pointings.
We detect the optical counterpart of EP 251220a (Álvarez et al., GCN 43167) in the GOTO L-band at 3σ, with a magnitude of 20.7 ± 0.3 (AB).
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 #43170
Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), Alan M. Watson (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), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Enrique Moreno Méndez (UNAM), Kin Ocelotl López (UNAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Nikos Mandarakas (LAM), Margarita Pereyra (UNAM), Benjamin Schneider (LAM), and Antonio de Ugarte Postigo (LAM).
We reobserved the field of EP251220a (Liu et al., GCN Circ. 43162) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2025-12-21 04:35 to 07:13 UTC (from 20.13 to 22:51 hours after the trigger) and obtained 8, 32, 8, 32, and 5 minutes of exposure, respectively, in the g, r, i, z, and y filters.
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.
Compared to our first epoch of observations (Sánchez Álvarez et al., GCN Circ. 43167), the afterglow has faded with a temporal index of -1.38 +/- 0.25. This confirms the association with the X-ray transient.
We detect the afterglow in all filters. After correcting for the Galactic foreground extinction corresponding to a reddening of E(B-V) = 0.037 mag (Schlafly & Finkbeiner 2011) and fitting a power-law with a SMC dust extinction model to the grizy-bands, we estimate a photometric redshift upper limit of z < 3.4.
We therefore encourage follow-up observations.
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 #43172
Q. C. Liu (THU), Y. Wang (PMO), H.Y. Liu, H. W. Pan (NAO,CAS) on behalf of the Einstein Probe team:
EP-FXT performed a follow-up observation of EP251220a (detected by EP-WXT, GCN 43162; and followed by MASTER-Net observations, COLIBRI telescope and GOTO; GCN 43163, GCN 43167, GCN 41369, GCN 43170) about 19 hours after the WXT detection, with an exposure time of 2.6 ks. The FXT telemetry data show that an uncatalogued source was detected within the WXT error circle at R.A. = 17.3189, DEC = 19.0923 (J2000) with an uncertainty of 10 arcsec (radius, 90% C.L. statistical and systematic). The FXT X-ray spectrum of this uncatalogued source can be fitted with an absorbed powerlaw with a hydrogen column density fixed at the Galactic value of 4.8x 10^20 cm^-2 and a photon index of 1.7 (+0.5/-0.5). The derived unabsorbed 0.5-10 keV flux is approximately 3.6 (+2.4/-1.2) x 10^-13 erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters.
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 #43181
Malte Busmann (LMU), Xander J. Hall (CMU), Julius Gassert (LMU/CMU), Brendan O'Connor (CMU), Daniel Gruen (LMU), and Antonella Palmese (CMU) report:
We observed the counterpart of EP251220a (Liu et al. GCN 43162, Sánchez Álvarez et al. GCN 43167, Liu et al. GCN 43172) with the Three Channel Imager (3KK) at the Fraunhofer Telescope at Wendelstein Observatory (FTW) in the r, i, and J-band simultaneously for 5 x 180 s starting at 2025-12-21T18:45:36 UT (1.43 days after the trigger). We detect the counterpart in the r and i-band and measure an r-band magnitude of
r = 21.94 +/- 0.12 AB mag.
Our measurements are consistent with the reports by Lipunov et al. (GCN 43163), Sánchez Álvarez et al. (GCN 43167), Kumar et al. (GCN 43169), and Rakotondrainibe et al. (GCN 43170).
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 #43182
A. P. C. van Hoof (Radboud), G. Corcoran (UCD), J. An (NAOC), R. A. J. Eyles-Ferris (U. Leicester), D. B. Malesani (DAWN/NBI and Radboud), D. Xu (NAOC), J. Sanchez-Sierras (Radboud), P. G. Jonker (Radboud), A. Martin-Carrillo (UCD), A. Kadela (NOT and NBI), report on behalf of a larger collaboration:
We observed the optical counterpart (Álvarez et al., GCN 43167; Kumar et al., GCN 43169; Rakotondrainibe et al., GCN 43170; Busmann et al., GCN 43181) of EP251220a (Liu et al., GCNs 43162, 43172) with the Nordic Optical Telescope (NOT) equipped with the ALFOSC camera. A sequence of 4x300 s exposures were taken in SDSS r-band starting on 2025-12-21 at 19:37:19.906 UT (~35.18 hr since the EP trigger).
The source is well detected in the stacked image with a preliminary photometry of
r = 21.79 +/- 0.05 (AB),
calibrated against nearby stars from the Pan-STARRS catalog and not corrected for Galactic extinction. Our photometry is consistent with the temporal index found by Rakotondrainibe et al. (GCN 43170).
GCN Circular #43184
J. An (NAOC), A. Saccardi (CEA/Irfu), R. A. J. Eyles-Ferris (U. Leicester), D. B. Malesani (DAWN/NBI and Radboud), M. Ferro (INAF-OABr), A. P. C. van Hoof (Radboud), A. L. Thakur (INAF-IAPS), D. Xu (NAOC), report on behalf of the Stargate collaboration:
We observed the optical counterpart of EP251220a (Sánchez Álvarez et al., GCN 43167; Rakotondrainibe et al., GCN 43170; Busmann et al., GCN 43181; van Hoof et al., GCN 43182) detected by the Einstein Probe (Liu et al., GCN 43162) using the ESO VLT UT1 (Antu) equipped with the FORS2 spectrograph. Our observation started at 01:02:04 UT on 2025 December 22 (40.6 hr after the EP trigger). A series of spectra was obtained with the 300V grism covering the range 3500-6500 AA and consisted of 4 exposures of 600 s each.
In the acquisition image (40.46 hr after trigger), we measure for the transient a magnitude r = 21.80 +- 0.05 (AB), calibrated against nearby stars from the Pan-STARRS catalogue, and not corrected for Galactic extinction. This value indicates little decay compared to the other reported optical observations.
In a preliminary reduction of the data, we identify multiple absorption features, including Lyα broad trough likely due to H I, as well as Si II, C II, and Al II, at a common redshift of 2.375, which we thus suggest to be the redshift for this transient. The COLIBRÍ redshift upper limit (Sánchez Álvarez et al., GCN 43167) is thus fully consistent with our spectroscopic measurement.
We acknowledge expert support from the ESO observing staff in Paranal, particularly Juan Carlos Olivares and Maria Jose Rain. The analysis of this spectrum was carried out with the help of the zHunter tool (https://doi.org/10.5281/zenodo.15189495).
GCN Circular #43185
N. Passaleva (U Rome), O. Guiffreda (UMD), M. El Kabir (U Rome), J. Durbak (UMD), E. Troja (U Rome), A. S. Kutyrev (NASA/GSFC), S. B. Cenko (NASA/GSFC)
We observed the field of the optical counterpart (Sánchez Álvarez et al. GCN 43167) of the fast X-ray transient EP251220a (Liu et al., GCN 43162) in J filter with PRIME. Observations started ~35h after the trigger.
Using Two Micron All Sky Survey (2MASS) nearby stars for preliminary calibration we do not detect any uncatalogued source down to J>20.5 AB (3-sigma) not corrected for galactic extinction at the position of the candidate reported by Sánchez Álvarez et al. (GCN 43167), Kumar et al. (GCN 43169), Busmann et al. (GCN 43181), van Hoof et al. (GCN 43182), An et al. (GCN 43184).
PRIME is a 1.8m telescope with 1.56 square degree FOV (0.5 arcsec/pixel) located in Sutherland, South Africa at the South African Astronomical Observatory (SAAO) (Kutyrev et al. 2023, Yama et al. 2023, Durbak et al. 2024).
We thank the Osaka University observers at PRIME and the staff at SAAO for their support with these observations.
GCN Circular #43221
A. A. Volnova (IKI), V. V. Rumyantsev (CrAO), A. S. Pozanenko (IKI), N. S. Pankov (HSE, IKI) report on behalf of IKI-GRB-FuN:
We observed the field of the fast X-ray transient EP251220a detected by EP/WXT (Liu et al., GCN 43162) with the ZTSh 2.6m telescope of CrAO taking several 120-second frames in R-band. The observations started on Dec. 22 at UT 20:32:36, i.e., ~ 2.5 days after the EP/WXT trigger. In the stacked image we detect the optical counterpart reported previously (Sánchez Álvarez et al., GCN 43167; Kumar et al., GCN 43169; Rakotondrainibe et al., GCN 43170; Busmann et al., GCN 43181; van Hoof et al., GCN 43182; An et al., GCN 43184; Passaleva et al., GCN 43185). Preliminary photometry and observational details are the following:
```
Date        UT start  t-T0       Exp.    Filter Obj.   Err.   UL       Site/Telescope
                     (mid,days)  (n*s)                       (3sigma)
2025-12-22  20:32:36  2.51642    17*120  R      22.55  0.20   22.9     CrAO/ZTSh
```
The photometry is based on several nearby stars from the USNO-B1 catalogue (R2 magnitudes) and is not corrected for the Galactic extinction.
GCN Circular #43223
Anshika Gupta, Kuntal Misra, Divyanshu Janghel, Debalina Kar, Dhruv Jain, and Pankaj Pawar (ARIES) report:
We observed the field of EP251220a detected by EP/WXT (Liu et al. 2025; GCN 43162) 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 2025-12-21 at 17:13:18 UT, i.e., ~1.36 days after the EP/WXT trigger. We have taken multiple frames with an exposure time of 300s in the r filter. We stacked the images after the alignment. We detect a optical counterpart in our stacked image within the error box of COLIBRÍ telescope (Sánchez Álvarez et al. 2025; GCN 43167). We obtain the following preliminary magnitude in the stacked image:
```
Date Start_UT T_start-T0 (days) Filter  Exp time (s)  Magnitude
==================================================================
2025-12-21  17:13:18   ~1.36   r     300s*8    21.64 +/-0.02
```
Our detection is consistent with Lipunov et al. 2025 (GCN 43163); Sánchez Álvarez et al. 2025 (GCN 43167); Kumar et al. 2025 (GCN 43169); Rakotondrainibe et al. 2025 (GCN 43170); Busmann et al. 2025 (GCN 43181); van Hoof et al. 2025 (GCN 43182); An et al. 2025 (GCN 43184); Passaleva et al. 2025 (GCN 43185); Volnova et al. 2025 (GCN 43221).
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 PanSTARRS DR1 catalog.

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