EP260603a

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),
M. Segura,  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-OAGH robotic telescope  [1]  located in Mexico (OAGH National Institute for Astrophysics, Optics and Electronics) was pointed to the EP260603a ( EP Team at NAOC/CAS <ep_ta@bao.ac.cn> et al., GCN 44814
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 ) errorbox  436 sec after notice time and 43770 sec after trigger time at 2026-06-04 06:00:42 UT, with upper limit up to  17.9 mag. The observations began at zenith distance = 58 deg. The sun  altitude  is -33.4 deg. 

The galactic latitude b = 25 deg., longitude l = 340 deg.


Real time updated cover map and OT discovered available here: 
https://master.sai.msu.ru/site/master2/observ.php?id=3359661

We obtain a following upper limits.  

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

   43860 | 2026-06-04 06:00:42 |         MASTER-OAGH | (15h 18m 40.19s , -26d 59m 12.9s) |   C |   180 | 17.9 |        
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

Y. F. Liang (PMO), J. W. Hu, Y. L. Wang, Z. X. Ling(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 EP260603a. The ground analysis of the WXT data shows that the event started at 2026-06-03T17:51:12 (UTC) and lasted for about 350 s. The WXT position of the source is R.A. = 229.721 deg, DEC = -26.940 deg (J2000) with an uncertainty of 2.8 arcmin in radius (90% C.L. statistical and systematic).

The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a fixed Galactic hydrogen column density of 1.56 × 10^21 cm^-2, and a photon index of 1.11 (-0.77, +0.80). The derived average unabsorbed 0.5-4 keV flux is 1.2 (-0.4, +0.6)×10^(-10) erg/s/cm^2. The peak flux is about 9.5×10^(-10) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters.

A Target-of-Opportunity observation with the Follow-up X-ray Telescope (FXT) has been scheduled. Further information will be updated when the telemetry data is received.
Further multi-wavelength observations are encouraged to determine 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).