EP260723a — All Circulars

K. R. Ni (CCNU), Y. J. Zhang (PKU), X. H. Tang (THU), and H. Sun (NAOC) report on behalf of the Einstein Probe (EP) team:

EP-FXT performed a second follow-up observation of EP260723a, which was detected by EP-WXT (Zhang et al., GCN 45201) and previously followed up by EP-FXT (Ni et al., GCN 45209). The observation started at 2026-07-25T11:52:47 UTC, about 40.9 hr after the WXT detection, with an exposure time of approximately 6.0 ks. On-ground analysis of the FXT data identified an uncatalogued X-ray source within the WXT error circle at R.A. (J2000) = 2.9759 deg and Dec. (J2000) = -12.1022 deg, with an uncertainty of 10 arcsec in radius (90% confidence level, including both statistical and systematic uncertainties).

The source position is consistent with the optical counterpart reported by the Liverpool Telescope (Eyles-Ferris et al., GCN 45202, 45212), the Nordic Optical Telescope (He et al., GCN 45203), COLIBRÍ (Basa et al., GCN 45204), and SVOM/VT (Ma et al., GCN 45205), and also with the weak X-ray excess reported in the first EP-FXT follow-up observation (Ni et al., GCN 45209).

The average FXT spectrum in the 0.5-10 keV band can be fitted with an absorbed power-law model, with the hydrogen column density fixed at the Galactic value of 2.0 x 10^20 cm^-2 and a photon index of 2.96 (+1.53, -1.21). The derived average unabsorbed 0.5-10 keV flux is 3.1 (+3.0, -1.4) x 10^(-14) erg cm^-2 s^-1. The uncertainties quoted above are at the 90% confidence level.

Further EP-FXT follow-up observations will be arranged. Continued multi-band follow-up observations are encouraged.

Launched on January 9, 2024, EP is a space X-ray observatory designed to monitor the dynamic soft X-ray sky and perform follow-up observations of newly discovered transients (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics).

R. A. J. Eyles-Ferris (Leicester) reports on behalf of a larger collaboration:

We observed the field of the Einstein Probe fast X-ray transient EP260723a (Zhang et al., GCN 45201; Ni et al. GCN 45209) with the 2m Liverpool Telescope on La Palma using the IO:O instrument. We obtained 12x150 s and 12x150 s exposures in each of the SDSS r’ and z’ filters starting at 2026-07-25 03:26:08 UT, approximately 32.5 hours after the X-ray detection.

We detect the optical counterpart (Eyles-Ferris et al., GCN 45202; He et al., GCN 45203; Basa et al., GCN 45204; Ma et al., GCN 45205) in both bands at AB magnitudes of r = 22.17 +/- 0.08 and z = 21.48 +/- 0.13. Our photometry is calibrated to PanSTARRS and not corrected for Galactic extinction.

We note that these magnitudes are consistent with those reported by Basa et al. (GCN 45204) approximately 19 hours earlier, suggesting an extended optical plateau or rebrightening. Further observations are planned.

K. R. Ni (CCNU), Y. J. Zhang (PKU), X. H. Tang (THU), H. Sun (NAOC) on behalf of the Einstein Probe (EP) team:   
EP-FXT performed a follow-up observation of EP260723a detected by EP-WXT (Zhang et al., GCN 45201). The observation started at 2026-07-24 08:44:22 UTC, about 13.8 hr after the WXT detection, with a total exposure time of around 3.0 ks.
On-ground analysis of the FXT data found no significant uncatalogued X-ray source in either the individual FXT-A or FXT-B image. After stacking the FXT-A and FXT-B data, a weak X-ray excess, with an S/N of approximately 2.2, was found at the position of the optical counterpart reported by the Nordic Optical Telescope (NOT; He et al., GCN 45203):
R.A. (J2000) = 2.97518 deg
Dec. (J2000) = −12.09851 deg
The optical counterpart was initially reported by the Liverpool Telescope (Eyles-Ferris et al., GCN 45202) and was subsequently confirmed by NOT (He et al., GCN 45203), COLIBRÍ (Basa et al., GCN 45204), and SVOM/VT (Ma et al., GCN 45205).
Given the low statistical significance, it is still unclear whether this weak X-ray excess is associated with EP260723a. The limited number of source counts does not allow meaningful spectral analysis.
Further EP-FXT follow-up observations will be arranged.
Launched on January 9, 2024, EP is a space X-ray observatory designed to monitor the dynamic soft X-ray sky and perform follow-up observations of newly discovered transients (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics).

Stéphane Basa (UAR Pytheas), Enrique Moreno Méndez (UNAM), Jean-Grégoire Ducoin (CPPM), 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), Massimiliano Lincetto (CPPM), Nikos Mandarakas (LAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Margarita Pereyra (UNAM), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM), Antonio de Ugarte Postigo (LAM), and Alan M. Watson (UNAM)

We imaged the field of the EP260723a (Zhang et al., GCN 45201) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-07-24T07:43:35.559 to 2026-07-24T09:27:41.815 UTC (from 12.96 to 14.68 hours after the trigger) and obtained 64 minutes of simultaneous exposure in the r and z filters.

The data were reduced, coadded, and analysed with the COLIBRÍ pipeline. 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 images, we detect the optical counterpart reported by Eyles-Ferris et al. (GCN Circ. 45202) and He et al. (GCN Circ. 45203), with the following preliminary photometry:
  r = 22.25 +/- 0.09
  z = 21.57 +/- 0.14

Further observations and analysis are ongoing.

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.

L.B. He, Z.P. Zhu, J. An, X. Liu, S.Q. Jiang, D. Xu (NAOC), S.Y. Fu (HUST), J.P.U. Fynbo (DAWN/NBI), V. Vuolteenaho (NOT) report:

We observed the field of EP260723a detected by EP (Zhang et al., GCN 45201) using the 2.56m Nordic Optical Telescope (NOT) equipped with the ALFOSC camera.

An uncatalogued source is detected within the EP-WXT error circle at coordinates:

R.A. (J2000): 00:11:54.04 (2.97518)
Dec. (J2000): -12:05:54.6 (-12.09851)

with an uncertainty of ~ 0.5 arcsec . The source had r = 22.10 +/- 0.05 at a median time of 10.18 hr post-trigger, calibrated with Pan-STARRS DR2 and not corrected for Galactic extinction.

This position is consistent with the previously reported optical conterpart (Eyles-Ferris et al., GCN 45202). We thus think this is the optical counterpart of EP260723a.

L.B. He, Z.P. Zhu, J. An, X. Liu, S.Q. Jiang, D. Xu (NAOC), S.Y. Fu (HUST), J.P.U. Fynbo (DAWN/NBI), V. Vuolteenaho (NOT) report:

We observed the field of EP260723a detected by EP (Zhang et al., GCN 45201) using the 2.56m Nordic Optical Telescope (NOT) equipped with the ALFOSC camera.

An uncatalogued source is detected within the EP-WXT error circle at coordinates:

R.A. (J2000): 00:11:54.04 (2.97518)
Dec. (J2000): -12:05:54.6 (-12.09851)

with an uncertainty of ~ 0.5 arcsec . The source had r = 22.10 +/- 0.05 at a median time of 10.18 hr post-trigger, calibrated with Pan-STARRS DR2 and not corrected for Galactic extinction.

This position is consistent with the previously reported optical conterpart (Eyles-Ferris et al., GCN 45202). We thus think this is the optical counterpart of EP260723a.

R. A. J. Eyles-Ferris (Leicester), G. Corcoran (UCD), D. B. Malesani (DAWN/NBI and Radboud) and R. L. C. Starling (Leicester) report on behalf of a larger collaboration:

We observed the field of the Einstein Probe fast X-ray transient EP260723a (Zhang et al., GCN 45201) with the 2m Liverpool Telescope on La Palma using the IO:O instrument. We obtained 4x150 s and 5 x150 s exposures in each of the SDSS r’ and g’ filters respectively starting at 2026-07-24 04:35:57 UT, approximately 10.8 hours after the X-ray detection.

We performed image subtraction on the stacked images using reference images from Pan-STARRS and also compared the stacked and reference images manually. In our r-band subtraction and in the stacked image, we identify a new source within the EP/WXT localisation at a position of:

RA 2.97513 (00:11:54.03)
Dec -12.09839 (-12:05:54.2)

We measure an AB magnitude of r = 21.86 +/- 0.11 and a g-band 5-sigma upper limit at the position of g > 22.25. Our photometry is calibrated to PanSTARRS and not corrected for Galactic extinction.

We note the presence of a source in the Legacy Survey consistent with our candidate’s position at a magnitude of r = 23.62 +/- 0.15 and a photometric redshift of 0.91 +/- 0.13. We suggest this could be the host galaxy of EP260723a.

Y.J. Zhang (PKU), K. R. Ni (CCNU), X. H. Tang (THU), H. Sun (NAOC) on behalf of the Einstein Probe (EP) team:              

We report on the detection of a fast X-ray transient, designated EP260723a (trigger ID: 11916655551), by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission. The transient started at 2026-07-23T18:56:01 (UTC) and lasted for about 100s, with a peak flux of ~ 1.4 x 10^(-9) erg cm^-2 s^-1 in 0.5-4 keV. The position of the transient is R.A. = 2.938 deg, Dec. = -12.112 (J2000), with an uncertainty radius of 3 arcmin in radius (90% confidence level, including both statistical and systematic uncertainties). 

The average WXT 0.5-4 keV spectrum can be fitted by an absorbed power-law model with a fixed Galactic hydrogen column density of 2.0×10^20 cm^-2 and a photon index of 1.4 (-0.8/+0.9). The derived average unabsorbed 0.5-4 keV flux is 4.1(-1.6/+2.7) x 10^(-10) erg cm^-2 s^-1. The uncertainties are at the 90% confidence level for the above parameters.

Follow-up observations by EP-FXT will be arranged.

Launched on 2024 January 9, Einstein Probe is a space-based X-ray observatory designed to monitor the dynamic X-ray sky and perform rapid follow-up observations of newly discovered transients (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics).