EP250831a

S.Y. Fu (HUST), J. Sanchez-Sierras (Radboud), J. An, L.B. He, S.Q. Jiang, X. Liu, Z.P. Zhu, D. Xu (NAOC), J.P.U. Fynbo (DAWN/NBI), G. Corcoran (UCD), P. G. Jonker (Radboud), A. J. Levan (Radboud and Warwick), D. B. Malesani (DAWN/NBI and Radboud), A. Martin-Carrillo (UCD), J. A. Quirola-Vasquez (Radboud), A. Henderson (NOT) report:

We observed the field of EP250831a (Yang et al., GCN 41619
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 ) using the Nordic Optical Telescope (NOT) located in La Palma (Canary Islands, Spain). We obtained 6 x 300 s r-band and 9 x 200 s z-band images.

Within the EP-FXT error circle (Yang et al., GCN 41625
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 ), a new optical source is marginally visible in the stacked r-band image, at coordinates

R.A. (J2000) = 02:51:55.49
Dec. (J2000) = -36:11:14.9

with an uncertainty of ~ 0.5 arcsec. We measure an AB magnitude r ~ 23.8 mag at an epoch of 18.23 hr after the EP trigger, calibrated against the Legacy Survey DR10 and not corrected for Galactic extinction. This source is not detected in the stacked z-band image, with a limiting magnitude of z > 22.0 mag (AB).

There is a source in the Legacy Survey about 1.7” SE of our candidate, with a tabulated magnitude r = 23.83.

We caution about the low S/N of our detection so we only report this object as a tentative optical counterpart to EP250831a. Further observations are planned in order to secure a template imaging and assess fading.

J. Yang (ZZU), X. Tian (GXU), D. Y. Li, Y. Liu (NAO, CAS) report on behalf of the Einstein Probe (EP) team:

EP-FXT performed a follow-up observation of EP250831a (GCN 41619
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 ) at 2025-08-31T15:30:41 (UTC), about 6 hours after the WXT detection, with an exposure time of 2.9 ks. The source was also followed by Swift-XRT (GCN 41624
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 ). FXT detected an uncatalogued source within the WXT error circle at R.A. = 42.9816, DEC = -36.1872 (J2000) with an uncertainty of 10 arcsec (radius, 90% C.L. statistical and systematic).  The FXT X-ray spectrum of this uncatalogued source can be fitted with an absorbed powerlaw with the absorption fixed at the Galactic value of 1.9x10^20 cm^-2, and a photon index of 2.3(+/-0.6). The derived unabsorbed 0.5-10 keV flux is approximately 8.15x10^-14 erg/s/cm2. The uncertainties are at the 90% confidence level for the above parameters.

We noted that EP-FXT also detected the uncatalogued X-ray source outside the WXT error circle detected by Swift-XRT.

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).

P.A. Evans (U. Leicester), J.A. Kennea (PSU), E. Ambrosi  (INAF-IASFPA) , A.P.
Beardmore (U. Leicester), M.G. Bernardini (INAF-OAB), R. Brivio (INAF-OAB), D.N.
Burrows (PSU), S. Campana (INAF-OAB), M. Capalbi (INAF-OAR), A. D'Ai (INAF-IASFPA),
P. D'Avanzo (INAF-OAB), V. D'Elia (SSDC & INAF-OAR), S. Dichiara (PSU), M. Ferro
(INAF-OAB), A. Melandri (INAF-OAR), J.P. Osborne (U. Leicester), K.L. Page (U.
Leicester), M. Perri (SSDC & INAF-OAR), C. Salvaggio (INAF-OAB), T. Sbarrato
(INAF-OAB), B. Sbarufatti (INAF-OAB), M.A. Williams (PSU) report on behalf of the
Swift-XRT team:

Swift-XRT has performed follow-up observations of the Einstein Probe/WXT-detected
source EP250831a, collecting 1.8 ks of Photon Counting (PC) mode data between T0+24
ks and T0+30 ks after the trigger. We have detected 1 source 4.6 arcmin from the WXT
position, somewhat outside the 90% error region of 2.9 arcmin (GCN 41619
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 ).  This has
been automatically classified as an uncatalogued X-ray source.

Uncatalogued X-ray sources
--------------------------

  Source 1 (SWIFT J025130.9-360728):
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    RA (J2000.0):   42.8789  =	02 51 30.94
    Dec (J2000.0):  -36.1245  =  -36 07 28.2
    Error:	    5.0 (arcsec, radius, 90% confidence).
    Detect flag:    GOOD
    Distance:	    4.6 arcmin from the Einstein Probe/WXT position.
    Mean rate:	    (7.6 [+2.8, -2.3])e-3 ct s^-1
    Mean flux:	    (1.97 [+0.73, -0.59])e-13 erg cm^-2 s^-1
    Peak rate:	    (7.6 [+2.8, -2.3])e-3 ct s^-1
    Peak flux:	    (1.97 [+0.73, -0.59])e-13 erg cm^-2 s^-1
    ECF:	    2.58e-11 erg cm^-2 ct^-1
		      assuming NH=3.05e+20 cm^-2, gamma=2.54
		      determined from a spectral fit.
    LSXPS UL:	    1.7e-02 ct/sec, (0.3-10 keV)
		      so the source is not above this 3-sigma upper limit.
    There is no evidence for fading.

All fluxes are 0.3-10 keV, observed. For all flux conversions and comparisons with
catalogues and upper limits  from other missions, we assumed a power-law spectrum
with NH=3x10^20 cm^-2 and photon index (Gamma)=1.7 unless otherwise stated.

The results of the XRT-team automatic analysis of the XRT observations, including a
position-specific upper limit calculator, are available at
https://www.swift.ac.uk/EP/EP_FIELD00066.

This circular is an official product of the Swift-XRT team.

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-SAAO robotic telescope  (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L)  located in South Africa (South African Astronomical Observatory) was pointed to the EP250831a ( EP Team et al., GCN 41619
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 ) errorbox  21491 sec after notice time and 40170 sec after trigger time at 2025-08-31 20:58:42 UT, with upper limit up to  18.8 mag. The observations began at zenith distance = 70 deg. The sun  altitude  is -56.9 deg. 

The galactic latitude b = -63 deg., longitude l = 239 deg.


Real time updated cover map and OT discovered available here: 
https://master.sai.msu.ru/site/master2/observ.php?id=2980777

We obtain a following upper limits.  

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

   40201 | 2025-08-31 20:58:42 |         MASTER-SAAO | (02h 50m 10.03s , -36d 31m 25.6s) |   C |    60 | 18.8 |        
   40201 | 2025-08-31 20:58:42 |         MASTER-SAAO | (02h 48m 03.66s , -36d 15m 35.6s) |   C |    60 | 18.8 |        
Filter C is a clear (unfiltred) band. 


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

J.Yang (ZZU), X. Tian (GXU), D. Y. Li, Y. Liu (NAO, CAS) report 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 EP250831a (trigger ID: 11916651030). The WXT position of the source is R.A. = 42.951 deg, DEC = -36.175 deg (J2000) with an uncertainty of 2.9 arcmin in radius (90% C.L. statistical and systematic). The transient started at 2025-08-31T09:49:12 (UTC) and lasted for about 300 seconds seen from the WXT light curve. The 0.5-4 keV spectrum can be fitted with an absorbed powerlaw model with Galactic hydrogen column density fixed to 1.9x10^20 cm^-2 and a photon index of 1.07(-0.49, +0.52). The average unabsorbed 0.5-4 keV flux is approximately 1.4 x 10^-10 erg/s/cm^2. The peak flux is estimated to be 6.8 x 10^-10 erg/s/cm^2.

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).