EP261103a

G. Corcoran (UCD), A.P.C. van Hoof (Radboud), A. Martin-Carrillo (UCD), J. Sanchez-Sierras (Radboud), P. G. Jonker (Radboud), H. C. I. Wichern (DTU Space), S. de Wet (DTU Space) report on behalf of a larger collaboration:

We observed the field of the Einstein Probe fast X-ray transient EP261103a (Ren et al., GCN 46142
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 ) with the New Technology Telescope (NTT) equipped with the EFOSC2 instrument. Observations consisted of a sequence of 9 x 200s exposures in both the Gunn r band and Gunn z band. The r-band observations started on 2026-11-04 4:58:46 UT (8.67 h after the EP/WXT trigger) and the z-band observations started on 2026-11-04 5:34:36 UT (9.3 h after the EP/WXT trigger).

We detect the optical counterpart (Jiang et al., GCN 46143
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 ; Ma et al., GCN 46145
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 ) in our r-band image. We obtain the following photometry on our stacked images:

r = 24.10 +/- 0.13
z > 23 (3-sigma)

Magnitudes are in the AB system, calibrated against PanSTARRS and not corrected for Galactic extinction.

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

SVOM/VT performed a Target of Opportunity observation of EP261103a detected by EP/WXT (Ren et al., GCN 46142
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 ). SVOM/VT began observing the field at 2026-11-03T21:22:47 UTC, 1.07 hours after the trigger, in the VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously. 

With X-band data available, the NOT counterpart candidate (Jiang et al., GCN 46143
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 ) was only detected in VT_R band. The photometric measurements in AB magnitudes are:

mid time (h) | exposure time (s) | band |  mag (AB) 
-------------|-------------------|------|---------------
     2.19    |      89*50        | VT_B |  >23.6 (3-sigma)
     2.19    |      89*50        | VT_R |  23.4+/-0.3 

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.

I. Pérez-García, E. Fernández-García, M.D. Caballero-García, M. Gritsevich, S. Guziy, S.-Y. Wu, and A. J. Castro-Tirado (IAA-CSIC, Granada), P. J. Meintjes and H. J. van Heerden (UFS, South Africa), A. Martín-Carrillo and L. Hanlon (UCD, Ireland), Y.-D. Hu (GXU), D. R. Xiong (Yunnan Observatories of CAS) and C. J. Pérez del Pulgar (UMA, Malaga), on behalf of a larger collaboration, report:

Following the detection of EP261103a by EP (Ren et al., GCN 46142
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 ), the BOOTES-6/DPRT 0.6m robotic telescope at Boyden Observatory in Maselspoort (South Africa) automatically responded to this high-energy event starting on Nov 03, 20:24:58 UT (i.e. 396 sec after detection). Nothing is detected down to 15.0 mag in the first 10 s image at the position reported by Jiang et al. (GCN 46143
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 ). On a coadd image (7 x 60sec) at 20:40:23 UT (mid exposure time), nothing is detected down to 18.8 mag (clear filter) within the same region. Photometric calibration was performed against Gaia G-band magnitudes.

We thank the staff at Boyden Observatory for their excellent support.

S.Q. Jiang, D. Xu, J. An, L.B. He, X. Liu, Z.P. Zhu (NAOC), S.Y. Fu, L. Lei, A.D. Zhu (HUST), J.P.U. Fynbo (DAWN/NBI), A. Caballero Almagro, F. Mustafaj (NOT) report: 

We observed the field of EP261103a detected by EP/WXT (Ren et al., GCN 46142
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 ) using the the 2.56-m Nordic Optical Telescope (NOT) equipped with the ALFOSC camera. We obtained 9 x 120 s Sloan r-filter images.

An uncatalogued weak source, within the 20 arcsec EP/FXT error circle (Ren et al., GCN 46142
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 ), is detected in our stacked r-band image at coordinates:

R.A. (J2000)= 5:27:24.31
Dec. (J2000)= -25:19:30.74

with an uncertainty of ~0.5 arcsec. Preliminary photometry for the source gives r = 23.86 +/- 0.25 mag, calibrated against nearby Legacy Survey DR10 stars and not corrected for Galactic extinction. The source is not present in the Legacy Survey r-band image, which is deeper than our NOT r-band image.

We thus think the source is likely the optical counterpart of EP261103a.

H. Y. Ren (NAO, CAS), B. T. Wang, M. Nie (YNAO, CAS),L.-N. Xu (USTC),X. Tian, S. X. Ren(GXU) and H. W. Pan (NAO, CAS) on behalf of the Einstein Probe (EP) team:

We report on the detection of an X-ray transient by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission, designated EP261103a. The transient triggered EP-WXT (ID: 01709322209) at 2026-11-03T20:18:26 (UTC). The WXT position of the source is R.A. = 81.849 deg, DEC = -25.330 deg (J2000) with an uncertainty of 3 arcmin in radius (90% C.L. statistical and systematic).

A follow-up observation with the Follow-up X-ray Telescope (FXT) was performed automatically. Within the WXT error circle, an uncatalogued X-ray source was detected at R.A. = 81.8516 deg, DEC = -25.3269 deg (J2000) with an uncertainty of 20 arcsec in radius (90% C.L. statistical and systematic).

Further information will be updated when the telemetry data is 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).