EP251124b

B. Zhang, J. P. Feng (USTC), D. F. Hu (PMO, CAS), H. Sun, H. N. Yang (NAO, CAS), Z. C. Zou (NJU) on behalf of the Einstein Probe (EP) team:

We performed a Target-of-Opportunity observation with the Follow-up X-ray Telescope on board the Einstein Probe (EP) for EP251124b (Feng et al, GCN 42834
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 ), which was also followed by Lipunov et al. (GCN #42835
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 ) and van Hoof et al. (GCN #42839
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 ). The observation began at 2025-11-25T22:12:00 (UTC) with an exposure time of about 1790 seconds. An uncatalogued X-ray source was detected within the WXT error circle at R.A. = 93.4802 deg, DEC = 52.7313 deg (J2000) with an uncertainty of 10 arcsec in radius (90% C.L. including both statistical and systematic uncertainties). 
 
The average FXT 0.5-10 keV spectrum can be fitted with an absorbed power law with a fixed Galactic hydrogen column density of 1.5 x 10^21 cm^-2 and a photon index of 0.97 (-0.84, +0.93). The derived average unabsorbed 0.5-10 keV flux is 9.02 (-5.33, +13.9) x 10^(-13) erg/s/cm^2. 
 
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).

A. P. C. van Hoof (Radboud), G. Corcoran (UCD), J. N. D. van Dalen, A. Martin-Carrillo (UCD), D. Xu (NAOC), S.Q. Jiang (NAO, CAS), T. Pursimo (NOT, Aarhus University), J.P.U. Fynbo (DAWN/NBI), P. G. Jonker (Radboud) report on behalf of a larger collaboration:

We observed the field of the fast X-ray transient EP251124b (Feng et al., GCN 42834
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 ) 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 2025-11-25 at 22:19:58.38 UTC (1.2 days after the WXT trigger). 

No new source is detected within the EP/WXT uncertainty region (Feng et al., GCN 42834
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 ). From the stacked images we obtain the following 3-sigma upper limits:

r > 24.4
z > 23.2

These upper limits are in AB magnitudes and were calibrated using nearby stars from the Pan-STARRS catalog and are not corrected for Galactic extinction.

We also performed image subtraction in r band using a reference image from the Legacy Survey. No new source is revealed compared to the reference. However, we note that there are artefacts across the field.

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 EP251124b ( EP Team et al., GCN 42834
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 ) errorbox  186 sec after notice time and 80214 sec after trigger time at 2025-11-25 15:39:56 UT, with upper limit up to  15.3 mag. The observations began at zenith distance = 68 deg. The sun  altitude  is -21.6 deg. 

The galactic latitude b = 17 deg., longitude l = 162 deg.


Real time updated cover map and OT discovered available here: 
https://master.sai.msu.ru/site/master2/observ.php?id=3051627

We obtain a following upper limits.  

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

   80245 | 2025-11-25 15:39:56 |   MASTER-Kislovodsk | (06h 13m 18.49s , +52d 39m 59.0s) |   C |    60 | 15.3 |        
   80245 | 2025-11-25 15:39:56 |   MASTER-Kislovodsk | (06h 13m 25.72s , +52d 17m 24.8s) |   C |    60 | 13.5 |        
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

J. P. Feng, B. Zhang (USTC), X. X. Sun (NAO, CAS), Q. C. Liu (THU), D. F. Hu, Y. L. Hua (PMO, CAS), C. C. Jin (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 EP251124b. The transient was detected by EP-WXT at 2025-11-24T17:23:02 (UTC). The WXT position of the source is R.A. = 93.493 deg, DEC = 52.694 deg (J2000) with an uncertainty of 2.6 arcmin in radius (90% C.L. statistical and systematic).
The transient event lasts for ～240 seconds, the average 0.5-4 keV spectrum can be fitted with an absorbed power-law model, with fixed galactic nH of 1.51 x 10^21 cm^-2 and a photon index of 1.72 (-0.78, +0.83). The unabsorbed average 0.5-4 keV flux is estimated to be 2.97 (-0.95, +1.51) x 10^-10 erg/s/cm2. The peak flux reaches around 4.8e-9 erg/s/cm2. The uncertainties are at the 90% confidence level for the above parameters.

The transient was discovered by the ground analysis of the delayed telemetry data. The follow-up observation of EP-FXT has been 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).