EP250807a

Y. Wang (PMO, CAS), Y. Q. Zhao (USTC,PRIC), H. Q. Cheng, Z. X. Ling (NAOC) on behalf of the Einstein Probe (EP) team:

EP-FXT performed a follow-up observation of EP250807a (Wang et al. GCN 41277
Loading...
 
 ) at 2025-08-08 10:27:06 (UTC), about 11 hours after WXT trigger, with an exposure time of 2972 s. No significant X-ray source was detected by the FXT within the WXT error circle, the flux upper limit is 7.4e-14 erg/cm^2/s in the 0.5-10 keV.

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-Kislovodsk robotic telescope  (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L)  located in Russia (Lomonosov MSU, Kislovodsk Solar Station of Pulkovo observatory) was pointed to the EP250807a ( EP Team et al., GCN 41277
Loading...
 
 ) errorbox  36084 sec after notice time and 64631 sec after trigger time at 2025-08-08 17:30:13 UT, with upper limit up to  17.1 mag. Observations started at twilight.  The observations began at zenith distance = 33 deg. The sun  altitude  is -11.5 deg. 

The galactic latitude b =  7 deg., longitude l = 45 deg.


Real time updated cover map and OT discovered available here: 
https://master.sai.msu.ru/site/master2/observ.php?id=2963680

We obtain a following upper limits.  

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

   64661 | 2025-08-08 17:30:13 |   MASTER-Kislovodsk | (18h 43m 28.97s , +14d 43m 36.5s) |   C |    60 | 17.1 |        
   64831 | 2025-08-08 17:33:02 |   MASTER-Kislovodsk | (18h 43m 28.20s , +14d 42m 55.0s) |   C |    60 | 17.1 |        
   65007 | 2025-08-08 17:35:59 |   MASTER-Kislovodsk | (18h 43m 24.84s , +14d 43m 09.9s) |   C |    60 | 17.1 |        
Filter C is a clear (unfiltred) band. 


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

A.P.C. van Hoof (Radboud), J.N.D. van Dalen (Radboud), D. B. Malesani (DAWN/NBI and Radboud), R. A. J. Eyles-Ferris (U of Leicester), A. Martin-Carrillo (UCD) and P. G. Jonker (Radboud) report on behalf of a larger collaboration:

We observed the field of EP250807a (Wang et al., GCN 41277
Loading...
 
 ) at 2025-08-08 from 05:17:33 (5.7 hr after the trigger) to 05:33:33 UTC, using the ESO NTT in La Silla (Chile), equipped with the EFOSC2 camera. Observations consisted of 5x200 s exposures in the Gunn z band.

We do not identify any new sources within the EP-WXT uncertainty region, down to a 3-sigma limiting magnitude of z > 20.4 (AB system). The upper limit is calculated using nearby stars from Pan-STARRS and not corrected for galactic extinction.

We also performed image subtraction using a reference image from Pan-STARRS. There are no conclusive signs of variability compared to the reference, but we note that the crowded field has impacted our preliminary subtraction and there are artefacts across the field.

Further observations are planned to investigate if any of the known sources do show variability.

We thank the excellent support from the NTT operator, Duncan Castex.

EP250807a: EP-WXT detection of a fast X-ray transient

Y. Wang (PMO, CAS), Y. Q. Zhao (USTC,PRIC), H. Q. Cheng, Z. X. Ling (NAOC) 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 EP250807a. The transient triggered EP-WXT (ID: 06800000803) at 2025-08-07T23:33:02 (UTC). The WXT position of the source is R.A. = 281.311 deg, DEC = 14.383 deg (J2000) with an uncertainty of 3 arcmin in radius (90% C.L. statistical and systematic). The duration of the flare is about 250 seconds, with a peak flux of 2.08 x 10^-8 ergs/s/cm^2. The WXT spectrum during the flare can be fitted with an absorbed power-law model, yielding best-fit absorption column density of 0.80 (-0.26, +0.27) x 10^22 cm^-2 and photon index of 1.99 (-0.11, +0.14). The 0.5-4 keV unabsorbed flux is 7.14 (-1.59, +2.71) x 10^-9 ergs/s/cm^2.
 Follow-up observations with several optical telescopes (Globus et al., GCN 41275
Loading...
 
 ; Kumar et al., GCN 41276
Loading...
 
 ) yielded only upper limits.

Follow-up observations with EP-FXT are 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).

Noémie Globus (UNAM), Francis Fortin (IRAP), Camila Angulo (UNAM), Alan M. Watson (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Jean-Luc Atteia (IRAP), Dalya Akl (AUS), Sarah Antier (OCA), Rosa L. Becerra (U Roma), Nathaniel R. Butler (ASU) , Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Kin Ocelotl López (UNAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Alvarez (UNAM), Benjamin Schneider (LAM), and Antonio de Ugarte Postigo (LAM) report:

We imaged the field of the EP X-ray transient EP250807a using the DDRAGO wide-field imager on the COLIBRÍ telescope. We observed from 2025-08-08 04:38 to 04:57 UTC (from 5.12 to 5.44 hours after the trigger) and obtained 16 minutes of exposure in the i-band filter.

The data were reduced and coadded with the COLIBRÍ pipeline and analysed with STDWeb/STDPipe (Karpov 2025). The photometry was calibrated using nearby stars from the PanSTARRS DR1 catalog, is in the AB system, and is not corrected for Galactic extinction.

In the stacked image, we do not detect any new source in the WXT uncertainty region, either in the direct image or after subtracting a Pan-STARRS DR2 template, down to the following 5-sigma limit:

i > 21.67

We thank the staff of the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir and the COLIBRÍ and DDRAGO engineering teams.

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.