EP250905b

X. Mao (NAO,CAS), Q. C. Liu (THU), Y. H. Cheng (SWIFAR,YNU), G. J. Yang, Y. Liu (NAO,CAS) on behalf of the Einstein Probe team:

EP-FXT performed a follow-up observation of EP250905b (GCN 41735
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 ) at 2025-09-06T05:05:21 (UTC), about 10 hours after the WXT detection, with an exposure time of 3.2 ks. FXT detected a weak uncatalogued source within the WXT error circle at R.A. = 90.7297, DEC = -54.1582 (J2000) with an uncertainty of 10 arcsec (radius, 90% C.L. statistical and systematic). Assuming a powerlaw spectrum with the absorption fixed at the Galactic value of 7.64x10^20 cm^-2 and the photon index of 2, the flux in 0.5-10 keV is derived to be approximately 2x10^-14 erg/s/cm2 from the FXT count rate.

Further follow-up observations are encouraged to identify the nature of this source.

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).

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  (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L)  located in Argentina (OAFA observatory of San Juan National University) was pointed to the EP250905b ( EP Team et al., GCN 41735
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 ) errorbox  2238 sec after notice time and 46512 sec after trigger time at 2025-09-06 08:13:40 UT, with upper limit up to  19.5 mag. The observations began at zenith distance = 43 deg. The sun  altitude  is -33.0 deg. 

The galactic latitude b = -28 deg., longitude l = 262 deg.


Real time updated cover map and OT discovered available here: 
https://master.sai.msu.ru/site/master2/observ.php?id=2985909

We obtain a following upper limits.  

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

   46542 | 2025-09-06 08:13:40 |         MASTER-OAFA | (06h 03m 11.78s , -54d 19m 04.9s) |   C |    60 | 19.5 |        
   46618 | 2025-09-06 08:14:56 |         MASTER-OAFA | (06h 03m 19.42s , -54d 18m 45.3s) |   C |    60 | 19.5 |        
   46697 | 2025-09-06 08:16:14 |         MASTER-OAFA | (06h 03m 15.86s , -54d 17m 05.6s) |   C |    60 | 19.5 |        
   46771 | 2025-09-06 08:17:29 |         MASTER-OAFA | (06h 03m 15.93s , -54d 18m 30.1s) |   C |    60 | 19.5 |        
   46837 | 2025-09-06 08:18:35 |         MASTER-OAFA | (06h 03m 15.94s , -54d 18m 27.8s) |   C |    60 | 19.5 |        
   46903 | 2025-09-06 08:19:41 |         MASTER-OAFA | (06h 03m 15.97s , -54d 18m 25.3s) |   C |    60 | 19.5 |        
   46981 | 2025-09-06 08:20:59 |         MASTER-OAFA | (06h 03m 19.09s , -54d 17m 19.1s) |   C |    60 | 19.5 |        
   47047 | 2025-09-06 08:22:05 |         MASTER-OAFA | (06h 03m 19.11s , -54d 17m 16.9s) |   C |    60 | 19.5 |        
Filter C is a clear (unfiltred) band. 


The observation and reduction will continue. 
The message may be cited.

Q. C. Liu (THU), Y. H. Cheng (SWIFAR,YNU), X. Mao, G. J. Yang, Y. Liu (NAO,CAS) on behalf of the Einstein Probe team:

We report on the detection of a fast X-ray transient, designated EP250905b, by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission. The transient event started at 2025-09-05T19:18:28 (UTC). The position of the source is R.A. = 90.724 deg, DEC = -54.167 deg (J2000) with an uncertainty of 2.4 arcmin in radius (90% C.L. statistical and systematic).

The transient event lasts for nearly 200 seconds. No automated X-ray follow-up observation was performed due to the solar angle constraint. The average 0.5-4 keV spectrum can be fitted by an absorbed power law with a photon index of 1.4 (+0.6/-0.6). The column density is fixed at the Galactic value of 7.64 x 10^20 cm^-2. The derived average unabsorbed 0.5-4 keV flux is 7 (+4/-2) x 10^-10 erg/s/cm^2. The peak flux is around 2.9 x 10^-9 erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters. 

Follow-up observations with the Follow-up X-ray Telescope (FXT) on board EP have been scheduled, and further information will be updated when the telemetry data 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).