EP251221a

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 fast x-ray transient EP251221a detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Dai et al. GCN 43178
Loading...
 
 ) in J and H filters with PRIME. Observations started ~24h 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>21 AB (3-sigma) not corrected for galactic extinction in the WXT error region. 

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.

A. Bochenek and D. A. Perley (LJMU) report:
 
We observed the field of the fast X-ray transient EP251221a (Dai et al., GCN 43178
Loading...
 
 ) using the IO:O optical camera on the 2m robotic Liverpool Telescope. We obtained 3x120s exposures in SDSS r and i filters starting at 2025-12-22 23:12:54 UT, approximately 25.6 hours after the trigger.
 
We perform image subtraction of the stacked images against Pan-STARRS using PSF matching with the help of PSFEx. We do not detect any new objects within the EP/WXT error region of the transient (Dai et al., GCN 43178
Loading...
 
 ). We note the i-band Pan-STARRS reference image is affected by a small number of artefacts, covering in total less than 1% of the error region. The 3-sigma limiting magnitudes on the stacked images are r > 22.6 and i > 22.5.

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  [1]  located in Russia (Applied Physics Institute, Irkutsk State University) was pointed to the EP251221a ( EP Team et al., GCN 43178
Loading...
 
 ) errorbox  25137 sec after notice time and 48513 sec after trigger time at 2025-12-22 11:07:39 UT, with upper limit up to  14.1 mag. The observations began at zenith distance = 80 deg. The sun  altitude  is -18.4 deg. 

MASTER-Kislovodsk robotic telescope  located in Russia (Lomonosov MSU, Kislovodsk Solar Station of Pulkovo observatory) was pointed to the EP251221a errorbox  77221 sec after notice time and 1 days 14196 sec after trigger time at 2025-12-23 01:35:42 UT, with upper limit up to  18.3 mag. The observations began at zenith distance = 48 deg. The sun  altitude  is -32.6 deg. 

The galactic latitude b = 12 deg., longitude l = 193 deg.


Real time updated cover map and OT discovered available here: 
https://master.sai.msu.ru/site/master2/observ.php?id=3080482

We obtain a following upper limits.  

Tmid-T0  |      Date Time      |          Site       |             Coord (J2000)          |Filt.| Expt. | Limit| Comment
_________|_____________________|_____________________|____________________________________|_____|_______|_______|________

   48543 | 2025-12-22 11:07:39 |        MASTER-Tunka | (06h 57m 20.46s , +23d 03m 38.7s) |   C |    60 | 14.1 |        
   48543 | 2025-12-22 11:07:39 |        MASTER-Tunka | (06h 57m 34.89s , +22d 56m 36.5s) |   C |    60 | 13.3 |        
   52254 | 2025-12-22 12:08:30 |             MASTER- | (07h 00m 06.39s , +23d 40m 37.8s) |   C |   180 | 18.9 |        
   52254 | 2025-12-22 12:08:30 |             MASTER- | (07h 00m 04.96s , +24d 36m 38.8s) |   C |   180 | 18.4 |        
  100687 | 2025-12-23 01:35:42 |   MASTER-Kislovodsk | (06h 57m 08.12s , +23d 26m 58.0s) |   C |   180 | 18.2 |        
  100687 | 2025-12-23 01:35:42 |   MASTER-Kislovodsk | (06h 56m 59.41s , +23d 49m 39.5s) |   C |   180 | 18.3 |        
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

C. Y. Dai (NJU), J. H. Wu (GU), W. F. Wen (SZTU), H. Q. Cheng, W. D. Zhang (NAO,CAS) on behalf of the Einstein Probe (EP) team: 

We report on the detection of a fast X-ray transient by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission, designated EP251221a. The transient triggered EP-WXT (ID: 01709250307) at 2025-12-21T21:39:06 (UTC). The WXT position of the source is R.A. = 104.531 deg, DEC = 23.374 deg (J2000) with an uncertainty of 3 arcmin in radius (90% C.L. statistical and systematic). The analysis of the WXT data shows that the event lasted for ~100 s. The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a fixed Galactic hydrogen column density of 8.1 x 10^20 cm^-2 and a photon index of 0.45 (-0.34/+0.36). The derived average unabsorbed 0.5-4 keV flux is 7.28 (-2.23/+3.30) x 10^(-10) erg/s/cm^2 (All errors are 1 sigma uncertainties.). 

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).