EP260806a — All Circulars

R. A. J. Eyles-Ferris (Leicester), L. Rhodes (TSI/McGill), D. Williams-Baldwin (JBCA/Manchester), F. Carotenuto (INAF-OAR) report on behalf of a larger collaboration:

We observed the field of the X-ray transient (Li et al., GCN 45299; Li et al., GCN 45305) with the e-MERLIN interferometer at a central frequency of 5 GHz. Our observation started at 2026-09-18 15:15 UT, ended at 2026-1919 10:35 UT and utilised the Lovell, Mk 2, Cambridge, Knockin, Darnhall and Pickmere telescopes.

We clearly identify the radio counterpart of EP260806a (de Ugarte Postigo et al., GCN 45541; Ho et al., GCN 45547; Nohara et al., GCN 45643) at a preliminary flux density of ~3.0 mJy. Our preliminary position appears broadly consistent with that of Ho et al. (GCN 45547). Further observations are planned.

We thank the e-MERLIN staff for the time allocation and their assistance with the observations.

Shogo Nohara, Natsuki Maekawa, Hiroki Takebuchi, Kotaro Niinuma, Kenta Fujisawa (Yamaguchi University)
Takanori Sakamoto (Aoyama Gakuin University)

We observed the Einstein Probe transient EP260806a (Li et al., GCN 45299) with the Yamaguchi Interferometer (Fujisawa et al., 2022) in Yamaguchi, Japan, on August 18, September 11, and September 16, 2026 UTC. The observations were conducted simultaneously at central frequencies of 6856 and 8448 MHz, with a bandwidth of 512 MHz for each frequency band. We used 3C48 and J0043+3426 as the flux and gain calibrators. We summarize our measurements of EP260806a at each observing epoch in the table below. The upper limits are quoted at the 3-sigma noise level. 

MJD		6856 MHz flux [mJy]	8448 MHz flux [mJy]
61270.57	< 1.77			< 2.57
61294.85	Affected by RFI		7.03+/-0.86
61299.52	8.02+/-0.46		9.26+/-0.77

Our observations are consistent with the radio emission reported at 10 GHz with the VLA (Ho et al., GCN 45547). The measurements performed on September 16 suggest a flat or inverted radio spectrum between 6856 and 8448 MHz.

We will continue monitoring the radio flux of EP260806a with the Yamaguchi Interferometer. 

A. de Ugarte Postigo (LAM), M. Bremer (IRAM), C. C. Thoene (AbAO), S. Basa (LAM), S. Geier (GTC), L. Izzo (INAF/OACN and DARK/NBI), A. Martin-Carrillo (UCD), M. Michalowski (AOI-AMU), D. A. Perley (LJMU), D. B. Malesani (DAWN/NBI and Radboud), B. Schneider (LAM), N.R. Tanvir (U. Leicester) report:

We observed the field of EP260806a (Li et al. GCN 45299, Yang et al. GCN 45390) with the NOEMA interferometer, located at Plateau de Bure (France). Observations started on the 11th of August 2026 (4.9 days after the Einstein Probe detection) at 81, 96, 136 and 151 GHz. On a preliminary reduction, we detect the afterglow (He et al. GCN 45300, de Ugarte Postigo et al. GCN 45302, Eyles-Ferris GCN 45303, Ma et al. GCN 45305, Li et al. GCN 45305, de Ugarte Postigo et al. GCN 45306, Anumarlapudi et al. GCN45312, Abidkhanov et al. GCN 45354, Hua et al. GCN 45385, Maksut et al. GCN 45389, Pankov et al. GCN 45396) in all bands at a flux density of ~0.4 mJy.

We acknowledge excellent support from the IRAM staff. Based on observations carried out under project number S26DT with the IRAM NOEMA Interferometer. IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain).

N. Pankov (IKI), A. Moskvitin (SAO RAS), A. Pozanenko (IKI), O. Spiridonova 
(SAO RAS), S. Kaisin (SAO RAS) report on behalf of IKI-GRB-FuN:

We observed the field of the X-ray transient EP260806a detected by EP 
(Li et al., GCN Circ. 45299) with the Zeiss-1000 1-meter and the BTA 6-meter 
telescopes of Special Astrophysical Observatory (SAO RAS) on six epochs between 
2026-08-07T00:56:47 and 2026-08-10T23:19:45 UT. We have applied image 
subtraction using PS1 g,r images as reference and clearly detected the optical 
counterpart (He et al., GCN 45300; de Ugarte Postigo et al., GCN 45302; 
Eyles-Ferris et al.,  GCN 45303; Ma et al., GCN 45304; de Ugarte Postigo et al., 
GCN 45306; Anumarlapudi et al., GCN 45312; Abidkhanov et al., GCN 45354; 
Hua et al., GCN 45385; Maksut et al., GCN 45389) at each epoch. The observational 
summary and preliminary photometry are presented below:

Date       UTstart    t-T0    Exp.   Filter  Mag   Err.  Telescope
                    (mid, d) (n*s)                       
2026-08-07  00:56:47 0.08539   7*120  Rc     19.519 0.030 Zeiss-1000
2026-08-07  23:18:25 1.01898   4*300  Rc     18.842 0.055 Zeiss-1000
2026-08-08  23:18:25 2.02679   5*300  Rc     18.783 0.043 Zeiss-1000
2026-08-10  02:53:28 3.16181   12*20  r      18.877 0.008 BTA-6m
2026-08-10  04:25:25 3.22439   2*20   g      17.650 0.140 BTA-6m
2026-08-10  00:57:26 3.08632   3*300  Rc     18.618 0.007 Zeiss-1000
2026-08-10  23:19:45 4.02827   6*300  Rc     18.935 0.084 Zeiss-1000
2026-08-11  23:26:37 5.02719   6*300  Rc     18.618 0.007 Zeiss-1000

The photometry is based on set of nearby stars from USNO-B1.0 (for Rc) 
and PS1 (for g,r) filters, respectively. No correction for the Galactic 
extinction was applied. The plot of the light curve derived from own data 
and GCN photometry is available online at: 

https://heaiki.ru/lvc/r/GRBs/EP260806a/EP260806a_lc.png.

We observe a significant g-r color evolution from -0.24 +/- 0.13 at ~0.5 days 
(Anumarlapudi et al., GCN 45312) to -1.23 +/- 0.16 at ~3.2 days since trigger.

The observed light curve behavior is consistent with an AGN outburst, as was 
suggested by de Ugarte Postigo et al. (GCN 45302).

N. Pankov (IKI), A. Moskvitin (SAO RAS), A. Pozanenko (IKI), O. Spiridonova 
(SAO RAS), S. Kaisin (SAO RAS) report on behalf of IKI-GRB-FuN:

We observed the field of the X-ray transient EP260806a detected by EP 
(Li et al., GCN Circ. 45299) with the Zeiss-1000 1-meter and the BTA 6-meter 
telescopes of Special Astrophysical Observatory (SAO RAS) on six epochs between 
2026-08-07T00:56:47 and 2026-08-10T23:19:45 UT. We have applied image 
subtraction using PS1 g,r images as reference and clearly detected the optical 
counterpart (He et al., GCN 45300; de Ugarte Postigo et al., GCN 45302; 
Eyles-Ferris et al.,  GCN 45303; Ma et al., GCN 45304; de Ugarte Postigo et al., 
GCN 45306; Anumarlapudi et al., GCN 45312; Abidkhanov et al., GCN 45354; 
Hua et al., GCN 45385; Maksut et al., GCN 45389) at each epoch. The observational 
summary and preliminary photometry are presented below:

Date       UTstart    t-T0    Exp.   Filter  Mag   Err.  Telescope
                    (mid, d) (n*s)                       
2026-08-07  00:56:47 0.08539   7*120  Rc     19.519 0.030 Zeiss-1000
2026-08-07  23:18:25 1.01898   4*300  Rc     18.842 0.055 Zeiss-1000
2026-08-08  23:18:25 2.02679   5*300  Rc     18.783 0.043 Zeiss-1000
2026-08-10  02:53:28 3.16181   12*20  r      18.877 0.008 BTA-6m
2026-08-10  04:25:25 3.22439   2*20   g      17.650 0.140 BTA-6m
2026-08-10  00:57:26 3.08632   3*300  Rc     18.618 0.007 Zeiss-1000
2026-08-10  23:19:45 4.02827   6*300  Rc     18.935 0.084 Zeiss-1000
2026-08-11  23:26:37 5.02719   6*300  Rc     18.618 0.007 Zeiss-1000

The photometry is based on set of nearby stars from USNO-B1.0 (for Rc) 
and PS1 (for g,r) filters, respectively. No correction for the Galactic 
extinction was applied. The plot of the light curve derived from own data 
and GCN photometry is available online at: 

https://heaiki.ru/lvc/r/GRBs/EP260806a/EP260806a_lc.png.

We observe a significant g-r color evolution from -0.24 +/- 0.13 at ~0.5 days 
(Anumarlapudi et al., GCN 45312) to -1.23 +/- 0.16 at ~3.2 days since trigger.

The observed light curve behavior is consistent with an AGN outburst, as was 
suggested by de Ugarte Postigo et al. (GCN 45302).

Z. Maksut (NU), E. Abdikamalov (NU), D. Berdikhan (NU), T. Komesh (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:

We observed the field of EP260806a (Li et al., GCN 45299) with the NUTTELA-TAO 0.7 m telescope of the Assy-Turgen Observatory (ATO) starting on 2026-08-12 at 21:10:42 UTC and obtained 7x120 s exposure in Sloan g’ and i’ bands. 

After subtracting the template, we detected the optical counterpart (He et al., GCN 45300; de Ugarte Postigo et al., GCN 45302; Eyles-Ferris et al.,  GCN 45303; Ma et al., GCN 45304; de Ugarte Postigo et al., GCN 45306; Anumarlapudi et al., GCN 45312; Abidkhanov et. al., GCN 45354; Hua et. al., GCN 45385). We report the following results:

Date           UT_mid        i'(mag)  err(mag)   g’(mag)  err(mag) 
--------      ------         -------             -------
2026-08-12    21:10:42       18.45    0.03       18.25       0.04

The preliminary measurements are given in AB magnitudes and are not corrected for Galactic extinction. Observation will be continued. 
The data were reduced by a 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/).

Yan-Long Hua, Jin-Jun Geng, Tian-Rui Sun, Ye Li, Yi-Fang Liang, Yuan-Tai Yang, Ji-An Jiang, Zhi-Ping Jin and Xue-Feng Wu report on behalf of the WFST team:

Following the detection of EP260806a by Einstein Probe (Li et al., GCN 45299), we performed follow-up observations using the Wide Field Survey Telescope (WFST Collaboration; arXiv:2306.07590) at the Lenghu Astronomical Observation Base in Qinghai Province, China. Observations in the g-band began at 2026-08-14T15:55:51 UTC, approximately 184.89 hours after the trigger.

The optical candidate reported by LCO (Ducoin et al., GCN 45300) and COLIBRÍ (Antonio et al., GCN 45306) was detected as the optical counterpart.

We performed image subtraction on the stacked images using PanSTARS templates and detected the optical counterpart (He et al., GCN 45300; de Ugarte Postigo et al., GCN 45302) in both bands. No other sources are detected within the EP/FXT error region (Li et al., GCN 45299).

The preliminary measurements are given in AB magnitudes and are not corrected for Galactic extinction:

g = 18.312 ± 0.008 (AB)

We thank the WFST staff for supporting these observations.

B.Abidkhanov, Y. Rajabov, O. Burkhonov, S. Ehgamberdiev, Y. Tillayev (UBAI), A. Shaymanov, T. Boyqobilov (Maidanak Observatory/UBAI) M. Coughlin (UMN), P. Hello (IJCLAB), S. Antier (IJCLAB), M. Pillas (IAP), and S. Karpov (FZU) report on behalf of UBAI team and GRANDMA collaboration.

We observed the field of EP260806a (Li et al., GCN Circ. 45299) with the AZT-22 1.5m telescope of the Maidanak Observatory (MAO) starting on 2026-08-10 at 22:51:39 UTC and obtained 8x300 s exposure in the R-band using 4kx4k CCD SNUCAM camera (Im et al., 2010). 

All the data have been reduced by a single data processing pipeline, STDWeb (Karpov 2025). Images obtained in Johnson Cousins filters were calibrated using the Gaia DR3 Synphot catalog. For template subtraction were used Pans-STARRS DR2 catalog. Our measurements in Vega magnitude system and the data has not corrected for the Galactic extinction.

After subtracting the template, we clearly detected the optical counterpart (He et al., GCN 45300; de Ugarte Postigo et al., GCN 45302; Eyles-Ferris et al.,  GCN 45303; Ma et al., GCN 45304; de Ugarte Postigo et al., GCN 45306; Anumarlapudi et al., GCN 45312)
 
Preliminary photometry is as follows:
Date       UTstart  Exptime       Filter OT    Err.    UL    SNR	Site/Telescope
                    (nxs)                     
2026-08-10 22:51:39 8x300           R   18.16   0.01    23.17	132	MAO/AZT-22

Maidanak astronomical observatory (MAO) is an observational facility of the Ulugh Beg Astronomical Institute (UBAI), Uzbekistan Academy of Sciences (http://maidanak.uz/).

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/). Astrophysics Center for Multimessenger studies in Europe is supported Skyportal/ICARE (contact camille.douzet@ijclab.in2p3.fr, skyportal.io).

Akash Anumarlapudi (UNC CH), Robert Stein (UMD), Tomas Ahumada (NOIRLab), Aleksandra Bochenek (LJMU), Mansi Kasliwal, Lin Yan (Caltech), Michael W. Coughlin, Michael C. Davis (UMN),  D. Y. Li, C.C.Jin, W. Yuan (NAO, CAS) report, on behalf of the ZTF collaboration.

We report the serendipitous detection of ZTF26abmjjnm/AT 2026yeh by the Zwicky Transient Facility (Bellm et al. 2019, Graham et al. 2019). ZTF26abmjjnm is the optical counterpart to the X-ray transient EP260806a (GCN #45299), previously detected by GCNs #45300, #45302, #45303, #45304, #45306. It was first observed by ZTF at T0+12 hrs, resulting in detections in g,r bands. With 

g = 18.69 +/- 0.09 AB and 
r = 18.93 +/- 0.09 AB

the source appears blue. Previous observations, roughly 4 days prior, resulted in non-detection at the location of ZTF26abmjjnm with g, r > 19.9 AB.

Based on observations obtained with the Samuel Oschin Telescope 48-inch and the 60-inch Telescope at the Palomar Observatory as part of the Zwicky Transient Facility project. ZTF is supported by the National Science Foundation under Award #2407588 and a partnership including Caltech, USA; Caltech/IPAC, USA; University of Maryland, USA; University of California, Berkeley, USA; University of Wisconsin at Milwaukee, USA; Cornell University, USA; Drexel University, USA; University of North Carolina at Chapel Hill, USA; Institute of Science and Technology, Austria; National Central University, Taiwan; Operations are conducted by Caltech's Optical Observatory (COO) and Caltech/IPAC. 

Antonio de Ugarte Postigo (LAM), Enrique Moreno Méndez (UNAM), Stéphane Basa (UAR Pytheas), 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), Jean-Grégoire Ducoin (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), Alan M. Watson (UNAM) report:

We imaged the field of the EP260806a (Li et al., GCN Circ. 45299) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-08-07 05:27:53 to 06:26:13 UTC (from 6.423 to 7.396 hours after the trigger) and obtained 16 minutes of exposure in the z and y filters.

The data were reduced and coadded with the COLIBRÍ pipeline and analysed with STDWeb (Karpov 2025). The photometry was calibrated using nearby stars from the PanSTARRS catalog, is in the AB system, and is not corrected for Galactic extinction.

After subtracting the template, we clearly detected the optical counterpart (He et al., GCN Circ. 45300; de Ugarte Postigo et al. GCN Circ. 45302; Eyles-Ferris GCN Circ. 45303; Ma et al. GCN Circ. 45304) at a preliminary magnitude of:

z = 19.44 +/- 0.04

Further observations 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.

R.-Z. Li (YNAO, CAS), Y.-H. Cheng (YNU), G.-J. Yang and W.-D. Zhang (NAOC) on behalf of the Einstein Probe (EP) team:

The X-ray transient EP260806a was detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Li et al., GCN 45299), and followed up by LCO (He et al., GCN 45300), GTC (de Ugarte Postigo et al., GCN 45302), Liverpool Telescope (Eyles-Ferris et al., GCN 45303) and SVOM/VT (Ma et al., GCN 45304). We report the refined analysis of the EP-WXT observation during which the source was detected, and the autonomous EP-FXT follow-up observation.

The WXT observation in which the source was detected started at 2026-08-06 22:57:37 (UTC) and lasted for ~290 seconds. No obvious variability or flaring activity of the source was seen in its lightcurve during the WXT observation. The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a fixed Galactic hydrogen column density of 4.64 x 10^20 cm^-2, an intrinsic column density of 1.17 (+/- 0.51) x 10^18 cm^-2 and a photon index of 1.94 (-0.81/+2.7). The derived average unabsorbed 0.5-4 keV flux is 1.65 (-0.58/+5.7) x 10^-10 erg s^-1 cm^-2.

The Follow-up X-ray Telescope (FXT) on board EP observed this source autonomously starting at 2026-08-06T23:03:51 (UTC, 374 seconds after the first WXT observation). The exposure time of this observation was 3950 s. On-ground analysis of the FXT data found an uncatalogued X-ray source at R.A. = 12.3066  deg, DEC = 35.5603 deg (J2000), with an uncertainty of 10 arcsec (radius, 90% C.L. statistical and systematic), which is consistent with the WXT position.

The average FXT 0.5-10 keV spectrum can be fitted with an absorbed power law with a fixed Galactic hydrogen column density of 4.64 x 10^20 cm^-2, an  intrinsic column density of 1.92 (+/-0.13) x 10^21 cm^-2 and a photon index of 2.02 (+/-0.04). The derived average unabsorbed 0.5-10 keV flux is 2.13 (+/-0.03) x 10^-10 erg s^-1 cm^-2. The peak luminosity is estimated to be 3.8 × 10^45 erg s^-1 in the 0.5-10 keV energy band, adopting a redshift of z = 0.089 (de Ugarte Postigo et al., GCN 45302). The source flux had been declining during the FXT observation.

The uncertainties are at the 90% confidence level for the above spectral parameters.

Further follow-up observations with EP/FXT are scheduled.

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).

R. A. J. Eyles-Ferris (Leicester), G. Corcoran (UCD), P. G. Jonker (Radboud), J. N. D. van Dalen (Radboud) and  A. Martin-Carrillo (UCD) report on behalf of a larger collaboration:

We observed the field of the Einstein Probe fast X-ray transient EP260806a (Li et al., GCN 45299) 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 2026-08-07 01:24:49 UT, approximately 2.38 hours after the X-ray detection.

We performed image subtraction on the stacked images using PanSTARRS templates and detect the optical counterpart (He at al., GCN 45300; de Ugarte Postigo et al., GCN 45302) in both bands. No other sources are detected within the EP/FXT error region (Li et al., GCN 45299).

In our subtracted images, we measure

r = 19.22 +/- 0.03
z = 19.53 +/- 0.04

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

The blue colour is broadly consistent with an AGN outburst as suggested by de Ugarte Postigo et al. (GCN 45302) but may also be evidence of a fast rising tidal disruption event origin. Further observations are encouraged.

A. de Ugarte Postigo (LAM), A. Martin-Carrillo (UCD),  L. Izzo (INAF/OACN and DARK/NBI), G. Corcoran (UCD), D. B. Malesani (DAWN/NBI and Radboud), S. Geier (GTC), C. C. Thoene (AbAO), M. A. Aloy (UV), J. P. U. Fynbo (DAWN/NBI), L. Galbany (IEEC-CSIC),G. Lombardi (GTC), N. A. Rakotondrainibe (LAM), B. Schneider (LAM), N. R. Tanvir (Univ. Leicester), G. Gómez Velarde (GTC) and A. Tejero Caro (GTC) report:

We observed the optical counterpart candidate (He et al. GCN 45300) of EP260806a (Li et al. GCN 45299) using the Gran Telescopio Canarias (GTC) equipped with the OSIRIS+ instrument.

The 30-s acquisition r-band image (beginning on 2026-08-07 at 03:21:44 UT, that is 4.32 hr after trigger), shows the underlying galaxy mentioned by He et al. We performed image subtraction and photometry with respect to a PanSTARRS reference image using STDWeb (Karpov 2025). The residuals show a clear excess at r = 19.05 +/- 0.01 mag consistent with the core of the galaxy. This implies a 0.27+/-0.04 mag brightening with respect to the observation by He et al. The location of the counterpart and the galaxy core are consistent with a catalogued LOTSS and VLASS radio source, which could be indicative of an active galactic nucleus.

A total of 3 spectra of 900 s were secured, starting on 2026-08-07 at 03:26:22 UT (4.40 hr after trigger), using grism R1000B. Continuum is visible over the wavelength range 3620 - 7800 AA. We detect strong emission lines due to [OII], [OIII], HeI, HII, [NII] and [SII], as well as CaII absorptions, all at a common redshift z = 0.089, which we suggest to be the redshift of the EP260806a event.

We also note that this galaxy is part of an interacting system with another, larger galaxy, located 22” at the West-southwest of this one. This second galaxy also falls in the slit and we confirm a consistent redshift.

This work has used the GRBspec database at http://grbspec.eu (de Ugarte Postigo et al. 2014, doi:10.1117/12.2055774).

L.B. He, J. An, S.Q. Jiang, X. Liu, Z.P. Zhu, Z. Fan, W.X. Li, D. Xu(NAOC), S.Y. Fu (HUST), Y.L. Hua (PMO) report:

We observed the field of EP260806a detected by EP (Li et al., GCN 45299), using an LCO 1m telescope located at the Teide Observatory (Spain) equipped with the SINISTRO instrument. We obtained 6x200 s exposures in the SDSS-r filter at the mid time on 2026-08-07 at 01:06:30.678 UT (2.04 hr after the trigger).

After subtracting the archival Pan-STARRS image, we found a point source lie in a brightened known galaxy, which has a redshift of ~0.136 in REGALADE (Hugo et al. 2026), within the EP/FXT error circle. The coordinates of the optical counterpart are as follows:

R.A. (J2000) = 00:49:13.554 (12.30647 deg)
Dec. (J2000) = +35:33:39.72 (35.56103 deg)

with an uncertainty of ~ 0.5 arcsec. The source had r = 19.32 +/- 0.03 mag (background subtracted), calibrated with Pan-STARRS DR2 and not corrected for Galactic extinction.

We thus think the source is likely the optical counterpart.

R.-Z. Li (YNAO, CAS), Y.-H. Cheng (YNU), G.-J. Yang and W.-D. Zhang (NAOC) 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 EP260806a. The transient triggered EP-WXT (ID: 01709277716) at 2026-08-06T23:02:28 (UTC). The WXT position of the source is R.A. = 12.278 deg, DEC = 35.588 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. = 12.3098 deg, DEC = 35.5614 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).