EP260405a

J. Sanchez-Sierras (Radboud), A. J. Levan (Radboud), A. Martin-Carrillo (UCD), J. Quirola-Vasquez (Radboud), D. B. Malesani (DAWN/NBI and Radboud), P. G. Jonker (Radboud), F. E. Bauer (SSI and UTA), J. Chácon (PUC) report on behalf of a larger collaboration:

We observed the field of the Einstein Probe trigger EP260405a (Li et al. 2026, GCN 44222
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 ) using the GMOS-N instrument mounted on the Gemini-North telescope. Two observation epochs were obtained, on 2026-04-05 12:13:28 (~8.45 hours after T0) and 2026-04-06 07:15:39 (~27.49 hours after T0), in the z band (4x90 s) and r band (4x90 s).

We carried out image subtraction in the z band using the second GMOS epoch as a template. The subtractions were performed using the ZOGY algorithm (Zackay et al. 2016), as implemented in PyZOGY (Guevel et al. 2021). We do not find any new, uncatalogued sources within the EP-WXT uncertainty region. From the stacked images we obtain the following 3-sigma upper limit:

r > 24.98
z > 24.96

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

H. L. Li, C. Wu, Y. L. Qiu, Y. N. Ma, Z. H. Yao, J. R. Xu, L. P. Xin, X. H. Han, J. Wang, Y. Xu, P. P. Zhang, W. J. Xie, Y. J. Xiao, H. B. Cai, J. S. Deng, J. Y. Wei (NAOC), J. Palmerio (CEA/Irfu) report on behalf of the SVOM/VT team.

SVOM/VT performed ToO observations to the fast X-ray transient EP260405a triggered by Einstein Probe (Li et al., GCN 44222
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 ). The observation started at 2026-04-05T11:28:35 UTC, approximately 7.70 hours post trigger in the VT_B (400-650 nm) and VT_R (650-1000 nm) channels simultaneously. 

No uncatalogued sources were detected within the error box of EP-WXT (Li et al., GCN 44222
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 ), compared to the Legacy Survey. The measurements in AB magnitude are as follows:

Mid time  |  Band   | Exposure Time | 3 sigma upper limit
8.86 hr      VT_B       107*50 sec         > 23.7 mag    
8.86 hr      VT_R       105*50 sec         > 23.6 mag

Our photometry was not corrected for Galactic extinction.

The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA, China), National Center for Space Studies (CNES, France) and the Chinese Academy of Sciences (CAS, China), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. VT was jointly developed by Xi'an Institute of Optics and Precision Mechanics (XIOPM), CAS and National astronomical observatories (NAOC), CAS.

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  [1]  located in Argentina (OAFA observatory of San Juan National University) was pointed to the EP260405a ( EP Team et al., GCN 44222
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 ) errorbox 21911 sec after trigger time at 2026-04-05 09:51:38 UT, with upper limit up to  17.1 mag. Observations started at twilight.  The observations began at zenith distance = 78 deg. The sun  altitude  is -12.9 deg. 

The galactic latitude b = 42 deg., longitude l = 293 deg.


Real time updated cover map and OT discovered available here: 
https://master.sai.msu.ru/site/master2/observ.php?id=3235319

We obtain a following upper limits.  

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

   21941 | 2026-04-05 09:51:38 |         MASTER-OAFA | (12h 18m 08.46s , -19d 48m 02.5s) |   C |    60 | 16.7 |        
   22001 | 2026-04-05 09:51:38 |         MASTER-OAFA | (12h 18m 08.45s , -19d 48m 02.5s) |   C |   180 | 17.1 |  Coadd 
   22007 | 2026-04-05 09:52:43 |         MASTER-OAFA | (12h 18m 08.49s , -19d 48m 02.4s) |   C |    60 | 16.6 |        
   22072 | 2026-04-05 09:53:49 |         MASTER-OAFA | (12h 18m 08.48s , -19d 48m 02.4s) |   C |    60 | 16.5 |        
   22138 | 2026-04-05 09:54:55 |         MASTER-OAFA | (12h 18m 08.55s , -19d 48m 02.6s) |   C |    60 | 16.5 |        
   22198 | 2026-04-05 09:54:55 |         MASTER-OAFA | (12h 18m 08.55s , -19d 48m 02.7s) |   C |   180 | 16.6 |  Coadd 
   22204 | 2026-04-05 09:56:00 |         MASTER-OAFA | (12h 18m 08.56s , -19d 48m 02.5s) |   C |    60 | 16.4 |        
   22270 | 2026-04-05 09:57:06 |         MASTER-OAFA | (12h 18m 08.63s , -19d 48m 02.7s) |   C |    60 | 16.2 |        
   22337 | 2026-04-05 09:58:13 |         MASTER-OAFA | (12h 18m 08.72s , -19d 48m 03.5s) |   C |    60 | 16.0 |        
   22397 | 2026-04-05 09:58:13 |         MASTER-OAFA | (12h 18m 08.72s , -19d 48m 03.6s) |   C |   180 | 16.3 |  Coadd 
   22402 | 2026-04-05 09:59:18 |         MASTER-OAFA | (12h 18m 08.73s , -19d 48m 03.4s) |   C |    60 | 16.0 |        
   22467 | 2026-04-05 10:00:24 |         MASTER-OAFA | (12h 18m 08.80s , -19d 48m 03.7s) |   C |    60 | 15.8 |        
   22533 | 2026-04-05 10:01:30 |         MASTER-OAFA | (12h 18m 08.89s , -19d 48m 03.9s) |   C |    60 | 15.7 |        
   22599 | 2026-04-05 10:02:35 |         MASTER-OAFA | (12h 18m 08.96s , -19d 48m 04.3s) |   C |    60 | 15.5 |        
   22665 | 2026-04-05 10:03:41 |         MASTER-OAFA | (12h 18m 09.09s , -19d 48m 04.8s) |   C |    60 | 15.3 |        
   50605 | 2026-04-05 17:48:21 |             MASTER- | (12h 17m 45.04s , -19d 54m 07.3s) |   C |   180 | 16.3 |        
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

R.-Z. Li (YNAO), Y. J. Yi (BNU), H.Q. Cheng and Z.-X. Ling (NAOC) on behalf of the Einstein Probe (EP) team:

We report on the detection of a fast X-ray transient, designated EP260405a by the Wide-field X-ray Telescope (WXT) on the Einstein Probe (EP) mission (EP/WXT trigger ID: 11900672904), and followed by COLIBRI (Lincetto et al. GCN #44220
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 ). The X-ray flare started at T0 = 2026-04-05T03:46:27 (UTC) with a duration of about 150s. The WXT position of the source is R.A. = 184.423 deg, DEC = -20.156 deg (J2000) with an uncertainty of 3 arcmin in radius (90% C.L. statistical and systematic). The WXT spectrum can be fitted by an absorbed powerlaw model with a fixed Galactic column density of 4.83 x 10^20 cm^-2, and a photon index of 2.0 (-0.4, +0.4). The 0.5-4 keV unabsorbed flux is 4.2 (-0.9, +1.2) x 10^-10 erg/cm^2/s.
 
A follow-up observation with the Follow-up X-ray Telescope (FXT) was conducted, beginning at 2026-04-05T16:23:25 (UTC) with an exposure time of 3ks. Within the EP/WXT error circle, no significant X-ray source was detected in the EP/FXT data, yielding an upper limit of 5 x 10^-14 erg/cm^2/s (90% C.L.). 

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).

Massimiliano Lincetto (CPPM), Alan M. Watson (UNAM), Jean-Grégoire Ducoin (CPPM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Edilberto Aguilar-Ruiz (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), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Marion Guelfand (CPPM), Asuka Kuwata (UNAM), Nikos Mandarakas (LAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM) and Antonio de Ugarte Postigo (LAM) report:

We imaged the field of the EP-WXT transient EP260405.149 using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. Observation started within 30 seconds from the alert reception. We observed on 2026-04-05 from 09:51:17 to 10:30:32 UTC (from 6.25 to 7 hours after the trigger T0) and obtained 1920 seconds of simultaneous exposure in the r and z filters.

The data were reduced and co-added with the COLIBRÍ pipeline and analysed with STDWeb/STDPipe (Karpov 2025). The photometry was calibrated using nearby stars from the PanSTARRS DR2 catalog, is in the AB system, and is not corrected for Galactic extinction.

In the stacked image, we do not detect any new source at the WXT source position down to the following 3-sigma limit:

r > 21.11
z > 20.57

Further observations may be planned.

We thank the staff of the Observatorio Astronómico Nacional at Sierra de San Pedro Mártir, as well as the technical and engineering teams at CEA, CPPM, IRAP, LAM, OHP, OSU Pytheas, and UNAM.

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.