EP260727a — All Circulars

B. Schneider (LAM), M. Garnichey (LUX-Paris Obs.), L. Xin (NAOC), D. Xu (NAOC), G. Corcoran (UCD), V. D'Elia (ASI-SSDC), L. Izzo (INAF/OACn & DARK/NBI), A. Rossi (INAF-OAS), A. Saccardi (CEA/Irfu), A. L. Thakur (INAF/IAPS), B. Topcu (University of Bath), S. D. Vergani (LUX-Paris Obs.) report on behalf of the Stargate collaboration:

We observed the optical counterpart (He et al., GCN 45234; Maksut et al., GCN 45238; Eyles-Ferris et al., GCN 45239; Adami et al., GCN 45243; Guziy et al., GCN 45245) of the EP260727a (Hu et al., GCN 45235; Wu et al., GCN 45244) using the ESO VLT UT3 (Melipal) equipped with the X-shooter spectrograph. Our spectra cover the wavelength range 3000-25000 AA, and consist of 4 exposures of 1200 s each. The observation mid time was 09:00:34 UT on 2026 July 28 (12.38 hr after the EP/WXT trigger).

In a 3x60s acquisition image (taken 11.46 hr after the EP/WXT trigger), the optical counterpart is detected with a magnitude r = 22.51 +/- 0.06 AB mag, calibrated against Pan-STARRS objects, and not corrected for Galactic extinction.

In a preliminary reduction of the spectra, we detect a faint continuum down to ~3800 AA. We identify a trough around 4639 AA, likely due to H I Lyman-alpha. Multiple absorption lines are also detected, which we interpret as due to Si II, O I, C II, Si IV, Mg II, all at a common redshift of z = 2.817. Additionally, at a consistent redshift (z = 2.817) we note the presence of several emission lines due to the [O II] doublet, [O III] doublet, and Hgamma from the faint host galaxy visible in Legacy Survey DR10 as noted by He et al. (GCN 45234).

We acknowledge the excellent support from the observing staff in Paranal, in particular Florian Rodler and Marcela Espinoza. The analysis of this spectrum was carried out with the help of the zHunter tool (https://doi.org/10.5281/zenodo.15189495).

A. Moskvitin (SAO RAS), A. Pozanenko (IKI), N. Pankov (IKI)
report on behalf of GRB follow-up team
We observed the field of EP260727a (Hu et al., GCN 45235) 
with the Zeiss-1000 1-m telescope of SAO RAS equipped 
with the CCD photometer. We obtained 9*20 + 4*300 sec. images 
with Rc filter from 2026-07-27T23:31:14 to 2026-07-28T00:07:51
under mediocre weather conditions and seeing of 3".3.
The OT (He et al., GCN 45234; Eyles-Ferris et al., GCN 45239;
Adami et al., GCN 45243; Guziy  et al., GCN 45245; also consistent 
with the EP-FXT position, Wu et al., GCN 45244) is clearly detected 
in the stacked image with the following brightness.
Date        UTstart  t-T0     Exp.   Filter Mag    Err.    UL
                    (mid, h) (n*s)                      (3-sigma)
                   
2026-07-27  23:31:14  3.1932  1380   Rc     21.64 +/- 0.17  22.5    
Near the position of the second candidate proposed by Adami et al., 
(GCN 45243) we note the presence of an extended source 
with the coordinates (J2000.0) R. A. = 23:05:48.72; Dec. = +03:15:23.5
and brightness of R = 21.55 +/- 0.11.
This preliminary photometry was given in the Vega system, calibrated 
against nearby SDSS stars (magnitudes converted with Lupton 2005 
equations) and not corrected for Galactic extinction.

S. Guziy, J. Rodrigo, E. Fernandez-Garcia, A. J. Castro-Tirado, S.-Y. Wu, I. Perez-Garcia, M.D. Caballero-Garcia  and M. Gritsevich (IAA-CSIC), G. Garcia-Segura (UNAM) and Y.-D. Hu (GXI), on behalf of a larger collaboration, report:

Following the detection of EP260727a by Einstein Probe (Hu et al. GCN 45235), we triggered the 1.5m telescope at Sierra Nevada Observatory (OSN) in Granada (Spain) starting on July 27, 23:51 UT (~3.24 h post trigger) in the Ic-band. The optical counterpart in the r-band proposed by He et al. (GCNC 45234) and also detected in z-band by Eyles-Ferris (GCNC 45239), and Adami et al. (GCNC 45243), is recorded in our images as well. We measure Ic=21.76+-0.26 on a  12x300s coadded image (July 28, 00:16 UT, mid exposure time).

The additional candidate reported by Adami et al. (GCGC 45243) is not detected on our images (taken ~1 h earlier) down to Ic=21.9 limiting magnitude. Further optical monitoring is ongoing.

We thank the staff at OSN for their excellent support.

T. Wu, H. C. Ding (AHNU), J. W. Hu, Z. X. Ling (NAO, CAS) on behalf of the Einstein Probe (EP) team:

The fast X-ray transient EP260727a triggered the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Hu et al., GCN 45235) and followed by several optical telescopes (He et al., GCN 45234; Maksut et al., GCN 45238; Eyles-Ferris et al., GCN 45239; Adami et al., GCN 45243) with z_phot = 1.03 +/- 0.24 (He et al., GCN 45234). The refined analysis of the WXT data shows that the event started at T0=2026-07-27T20:36:47Z (UTC) and lasted for about 70 s before the observation was interrupted by the autonomous follow-up observation. The average WXT 0.5–4 keV spectrum can be fitted with an absorbed power-law model (tbabs*ztbabs*powerlaw) with a fixed Galactic hydrogen column density of 4.76 × 10^20 cm^-2 and a fixed redshift of z = 1.03. The best-fit photon index is 0.5 (-0.6/+1.2), and the intrinsic hydrogen column density is 7.1 (-7.1/+240.0) × 10^20 cm^-2. The derived average unabsorbed 0.5–4 keV flux is 1.8 (-0.5/+0.8) × 10^-9 erg/s/cm^2, corresponding to a luminosity of 3.0 (-0.8/+1.3) × 10^48 erg/s.

The autonomous observation by the Follow-up X-ray Telescope (FXT) was performed at 2026-07-27T21:26:27 (UTC), about 50 minutes after T0. The exposure time of this observation is 2568 s. The on-ground analysis shows that an uncatalogued source was detected at R.A., Dec. = 346.4612, 3.1939 deg (J2000) with an uncertainty of about 10 arcsec (radius, 90% C.L. statistical and systematic). The average FXT 0.5-10 keV spectrum can be fitted with an absorbed power law with a fixed Galactic equivalent hydrogen column density of 4.76 × 10^20 cm^-2 and a photon index of 2.0 (-0.6/+1.1), and the intrinsic hydrogen column density is 0.15 (-0.15/+0.87)×10^22 cm^-2. The derived average unabsorbed 0.5-10 keV flux is 4.4 (-2.7/+0.2)×10^-13 erg/s/cm^2,  corresponding to a luminosity of 3.0 (-1.8/+0.11) × 10^45 erg/s. The uncertainties are at the 90% confidence level for the above parameters.

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).

C. Adami (LAM/Pytheas/AMU), E. Le Floc'h (CEA/Irfu), B. Schneider (LAM),  A. Saccardi (CEA/Irfu)  report on behalf of a larger collaboration:

We observed the field of EP260727a (Hu et al., GCN45235) using the T193cm telescope at Observatoire de Haute-Provence (France) with the MISTRAL spectro-imager. We obtained 14x4 + 1x5 + 1x2 minutes of exposure in the r-band under a 2.1arcsec seeing, starting at 20260728_005726UT (4.3 hr after the Hu et al. trigger) and ending at 20260728_024425UT (6.1 hr after the Hu et al. trigger).

In the preliminary stacked images, we detect two potential optical counterparts. 

The first one is barely detected and consistent with the position of He et al. (GCN 45234), and Eyles-Ferris et al. (GCN 45239). See also Maksut et al. (GCN 45238). We detect it at a preliminar magnitude of: 
r = 22.82 +/- 0.28 mag

We also detect another candidate not present in PanStarrs images at:
RA (J2000): 23:05:48.7
DEC (J2000): +03:15:27.4

with preliminary magnitude of:

r = 20.49 +/- 0.06 mag

The photometric calibration was performed using nearby stars from the PanSTARRS catalog and the STDWeb/STDPipe tools (Karpov 2025), is in the AB system, and is not corrected for Galactic extinction.

We acknowledge the excellent support from Observatoire de Haute-Provence and in particular Yoann Degot-Longhi Larralde Erasun. We also thank the SOPHIE observer:  Deepanshi Singh.

R. A. J. Eyles-Ferris (Leicester), G. Corcoran (UCD), P. G. Jonker (Radboud), D. B. Malesani (DAWN/NBIA and Radboud) and P. T. O’Brien (Leicester) report on behalf of a larger collaboration:

We observed the field of the Einstein Probe fast X-ray transient EP260727a (Hu et al., GCN 45235) with the 2m Liverpool Telescope on La Palma using the IO:O instrument. We obtained 6x150 s and 6x150 s exposures in each of the SDSS r’ and z’ filters starting at 2026-07-28 02:34:17 UT, approximately 5.9 hours after the X-ray detection.

We detect the optical counterpart (identified by He et al. (GCN 45234) in the r band) in the z band at an AB magnitude of z = 21.66 +/- 0.17. This is significantly brighter than the historical magnitude for the underlying source of z = 23.46 +/- 0.08 derived from archival HSC imaging. This underlying source was also identified by He et al. in the r band.

In the r band, we obtain an upper limit of r > 22.85. As this upper limit is less deep than the detection reported in He et al. (GCN 45234), we cannot assess if the source is fading. 

Our photometry is calibrated to PanSTARRS and not corrected for Galactic extinction.

Z. Maksut (NU), T. Komesh (NU), D. Berdikhan (NU), E. Abdikamalov (NU), M. Krugov (FAI), K. Baigarin (NU), M. Coughlin (UMN), P. Hello (IJCLAB), S. Antier (IJCLAB), M. Pillas (IAP), and S. Karpov (FZU) on behalf of the GRANDMA/NUTTelA-TAO:

The Nazarbayev University Transient Telescope at Assy-Turgen Astrophysical Observatory (NUTTELA-TAO) pointed at EP 260727a on receipt of an automated GCN / EP position alert, observing in Sloan g' and i' bands.
We started observations at 21:40:12 UT on 2026-07-27, 1.05 hour after the EP/WXT trigger (J. Hu et al., GCN 45235). No optical source consistent with the NOT optical counterpart (L. He et al., GCN 45234) was detected. We report the following results:

start time  t-t0(s) UL g'   UL i'  exposure_time (im x s)
--------------------------------------------------------------
21:40:12    3879    20.3   19.75   10x30 sec

Where t is the middle time, t0 is the EP/WXT trigger time, t-t0 is given in seconds. UL gives the 5 sigma upper limit sensitivity in magnitudes, for images of the given exposure time. No color or other corrections were applied to the values above. The data were reduced by a single data processing pipeline, STDWeb (Karpov et al., 2022). Images obtained with the Sloan filters were calibrated using the PanSTARRS DR1 catalog.
----------------------------------
This research has been funded by the Science Committee of the Ministry of Science and Higher Education of the Republic of Kazakhstan (Grant No. AP26103591). The NUTTelA-TAO Team acknowledges the support of the staff of the Assy-Turgen Astrophysical Observatory, Almaty, Kazakhstan, and the Fesenkov Astrophysical Institute, Almaty, Kazakhstan. GRANDMA is a worldwide telescope network (grandma.ijclab.in2p3.fr) devoted to the observation of transients in the context of multi-messenger astrophysics (Antier et al. 2020 MNRAS 497, 5518). Kilonova-Catcher (KNC) is the citizen science program of GRANDMA (http://kilonovacatcher.in2p3.fr/).

J. W. Hu (NAO, CAS) , H. C. Ding, T. Wu (AHNU), S. Q. Jiang, Z. X. Ling (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 EP260727a. The transient triggered EP-WXT (ID: 01709276927) at 2026-07-27T20:37:57 (UTC). The WXT position of the source is R.A. = 346.455 deg, DEC = 3.240 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 by the onboard software at R.A. = 346.4623 deg, DEC = 3.1953 deg (J2000) with an uncertainty of 20 arcsec in radius (90% C.L. statistical and systematic), which is consistent with the position of NOT likely optical counterpart (GCN 45234).

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).

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

We observed the field of EP-WXT trigger ID 01709276927 detected by EP using the 2.56m Nordic Optical Telescope (NOT) equipped with the ALFOSC camera. Observation started at 2026-07-28T00:19:16.548 UT, i.e., 3.69 hrs post-burst, and a series of Sloan r- and z-band frames were obtained.

A brightened source was detected within the EP/WXT error circle at coordinates
R.A. (J2000) = 23:05:50.65 (346.46104)
Dec. (J2000) = +03:11:38.65 (3.19407)
with an uncertainty of ~ 0.5 arcsec. The source had r = 21.98 +/- 0.08 mag at a median time of 4.18 hrs post-trigger, calibrated with Pan-STARRS DR2 and not corrected for Galactic extinction.

We note that there is a faint source at that position in the Legacy Survey DR10 images with r = 23.73 +/- 0.23 and z_phot = 1.03 +/- 0.24.

No NEO was at the above position at the observational time according to MPC.

We thus think the source is likely the optical counterpart.