EP260927a

H. Yang, M. Brunet, O. Godet (IRAP)
report on behalf of the SVOM team: 

We performed a targeted search to the X-ray transient EP260927a, detected by EP-WXT at 2026-09-27T14:02:35 UTC (T0) lasting for about 800 s (EP team, GCN 45779
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 ), with the SVOM/ECLAIRs event-by-event data downloaded through the X-band ground station. 

At T0, the transient position was within the ECLAIRs field of view. No statistically significant signal was identified in the [T0 - 3000 s, T0 + 1000 s] interval by the ECLAIRs offline trigger. 

For the interval from T0 to T0 + 800 s, we derive a 3-sigma upper limit of 5e-10 erg/cm^2/s in the 4-120 keV energy band, assuming a power-law spectrum with a photon index of -2.28, as measured by EP-FXT for the uncatalogued source (GCN 45779
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 ). Alternatively, assuming a “soft” spectral template (a Band function with Epeak = 25 keV, alpha = -1.5, beta = -3.0), we derive a 3-sigma upper limit of 5.4e-10 erg/cm^2/s in the 4-120 keV energy band. We note that the EP/WXT flux of 2.91e-11 erg/cm^2/s in the 0.5-4 keV (GCN 45779
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 ) is more than an order of magnitude lower than our 3-sigma upper limit. 

The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA), French Space Agency (CNES), and the Chinese Academy of Sciences (CAS), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. ECLAIRs was developed jointly by APC, CEA, CNES, and IRAP.

The SVOM/ECLAIRs point of contact for this event is : Hui Yang (hui.yang@cnrs.fr) 

M. Segura,  F. Podesta, R.Podesta, E. Gonzalez (Observatorio Astronomico Felix Aguilar (OAFA)

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 (Lomonosov MSU)

D.Buckley (SAAO),

A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity)
A.Sosnovskij (Lomonosov MSU)

A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory)

L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez,
A.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory)

R.Rebolo (The Instituto de Astrofisica de Canarias)

O.Gress, N.Budnev(ISU)


MASTER-OAFA robotic telescope  [1]  located in Argentina (OAFA observatory of San Juan National University) was pointed to the EP260927a ( EP Team at NAOC/CAS <ep_ta@bao.ac.cn> et al., GCN 45779
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 ) errorbox  174 sec after notice time and 1 days 50700 sec after trigger time at 2026-09-29 04:07:35 UT, with upper limit up to  19.3 mag. The observations began at zenith distance = 27 deg. The sun  altitude  is -55.7 deg. 

The galactic latitude b = -73 deg., longitude l = 217 deg.


Real time updated cover map and OT discovered available here: 
https://master.sai.msu.ru/site/master2/observ.php?id=3513970

We obtain a following upper limits.  

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

  137131 | 2026-09-29 04:07:35 |         MASTER-OAFA | (02h 06m 25.91s , -27d 12m 13.8s) |   C |    60 | 19.3 |        
  137570 | 2026-09-29 04:14:55 |         MASTER-OAFA | (02h 06m 25.91s , -27d 18m 24.7s) |   C |    60 | 19.3 |        
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

C. L. Guo, B. B. Zhang, K. Ao, S. Q. Jiang, H. Sun (NAO, CAS), D. F. Hu (PMO, CAS), A. D. Zhu(HUST), J. Y. Liu(PMO,CAS), on behalf of the Einstein Probe (EP) team: 

We report the discovery of a new fast X-ray transient EP260927a, identified in the EP-WXT telemetry data at RA = 31.833 deg, Dec =-27.169 deg (J2000), with an uncertainty of 2.6 arcmin in radius (90%, C.L statistical and systematic). The transient started about 2026-09-27T14:02:35 (UTC) and lasted for about 800 s. The 0.5-4 keV flux estimated from the WXT data is approximately 2.91x10^-11 erg/s/cm^2. We note the presence of a historical X-ray source 1eRASS J020729.2-271052, with flux density 7.79e-13 erg/s/cm^2, located 2.2 arcmin away from the center of the WXT error circle.This source is likely associated with Gaia DR3 5117556925932530176 and may be a QSO candidate according to GAIA classification.

We performed a follow-up observation of EP260927a with the Follow-up X-ray Telescope on board EP to confirm the transient nature of the WXT detection. The observation started at 2026-09-28T03:53:50 (UTC). The effective exposure time is around 6 ks. On-ground analysis of the FXT data found two sources within the WXT error circle. The first source is an uncatalogued source, detected at RA = 31.8105 deg, Dec =-27.1855 deg (J2000), with an uncertainty of 10 arcsec in radius (90%, C.L statistical and systematic). The average 0.5-10 keV spectrum can be fitted with an absorbed power law with a fixed Galactic hydrogen column density of 1.47 × 10^20 cm^-2, and a photon index of 2.28(-0.78/+0.87). The derived average unabsorbed 0.5-10 keV flux is 5.6(+4.9/-2.0)× 10^(-14) erg/s/cm^2. 

The position of the second source is consistent with the historical X-ray source 1eRASS J020729.2-271052. The average 0.5-10 keV spectrum can be fitted with an absorbed power law with a fixed Galactic hydrogen column density of 1.47 × 10^20 cm^-2, and a photon index of 2.7(-0.3/+0.3). The derived average unabsorbed 0.5-10 keV flux is 2.5(+0.4/-0.4)× 10^(-13) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above spectral parameters.

We tend to suggest that the first FXT source is the X-ray counterpart of EP260927a. Multi-wavelength follow-up observations are encouraged.

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).