EP261015b

G. Corcoran (UCD), D. B. Malesani (DAWN/NBI and Radboud), A. Martin-Carrillo (UCD), D. Xu (NAOC),  A. L. Bouquin (NOT), A. Caballero Almagro (NOT), E. Georgiou (NOT and Turku Univ.) report on behalf of a larger collaboration,

We observed the field of the Einstein Probe fast X-ray transient EP 261015b (Li et al., GCN 45958
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 ) using the Nordic Optical Telescope (NOT) equipped with the ALFOSC imager. A total of 3x300 s were secured in the SDSS r band, with an observation mid time on 2026-10-17 at 00:00:59 UT (1.42 days after the EP trigger). We performed image subtraction against a Legacy Survey stacked image as well as a stack of archival CFHT images.

Li et al. (GCN 45971
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 ) present two X-ray sources detected by EP/FXT, for which we report the following information. All our photometry is calibrated using nearby stars in the Pan-STARRS catalog, given in the AB system and not corrected for Galactic extinction.

EPF_J000211.4+305730: The high proper-motion star UCAC4 605-000101 does not subtract cleanly due to its positional offset between our image and the template, as well as its bright flux. We measure an integrated magnitude of r = 16.06 +/- 0.02. This measurement is consistent with the Pan-STARRS DR2 catalogued value of r =  16.0673 +/- 0.0006. Within the FXT uncertainty region, we do not identify any other source in our difference images.

EPF_J000231.0+305716: In addition to the QSO SDSS J000230.78+305717.6 (already noted by Li et al., GCN 45971
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 ), located just to the west of the error circle, there exists an additional spectroscopically confirmed QSO within it (https://www.legacysurvey.org/viewer/desi-spectrum/dr1/targetid39628499002066108). The SDSS classified QSO produces a negative residual in our subtraction while the DESI QSO produces a positive residual (integrated mag r = 22.29 +/- 0.06, compared to r = 22.60 +/- 0.07 measured from the Legacy Survey image). Just outside of the uncertainty region we detect a third variable source consistent with the Pan-STARRS object PSO J000.6355+30.9555 (LS DR 11 ID 48424592024274146). This object appears point-like and while it does not have a spectroscopic QSO classification, there is tentative evidence for long-term variability in ZTF and WISE W1 data. We therefore believe that this is also likely a QSO or some form of variable star. No other sources are identified in or near this uncertainty region in our difference images.

We also examined the difference images outside of the EP-FXT regions but within the WXT localisation and after manual vetting of the residuals we did not identify any candidate counterparts down to a 3 sigma upper limit of r > 24.5.

Given the relatively late epoch of our follow-up, the lack of measured variability for UCAC4 605-000101 does not rule out a stellar flare. A stellar flare would also be in agreement with the non-detection of any candidate counterparts but does not confirm it either. The presence of multiple QSOs near or within the FXT localisation of EPF_J000231.0+305716 would point towards it being an unrelated X-ray source with X-ray emission coming from one or multiple of the mentioned QSOs. Overall, while a stellar flare origin is consistent with our data, the nature of this event is not fully confirmed, and we encourage further X-ray observations in order to test the X-ray sources for variability.

Z. -X. Li, G. -L. Huang (IHEP), R. -Z. Li (YNAO), M. Nie (YNAO) and H. Sun (NAOC) on behalf of the Einstein Probe (EP) team:
The X-ray transient EP261015b triggered the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission at 2026-10-15T13:56:38 UTC (Li et al., GCN 45958
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 ). A subsequent observation was reported by Cao et al. (GCN 45970
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 ). Follow-up observation with Follow-up X-ray Telescope (FXT) on board EP began at 2026-10-16T20:20:28 UTC, approximately 30.40 hours after the WXT trigger, with an exposure time of 5129 s.
FXT-B detected two X-ray sources within the WXT error region reported in GCN  45958
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 :
EPF_J000211.4+305730: R.A. = 0.5465 deg, Dec. = 30.9576 deg (J2000).
EPF_J000231.0+305716: R.A. = 0.6320 deg, Dec. = 30.9546 deg (J2000).
Each position has a 90% confidence error radius of approximately 10 arcsec, including statistical and systematic uncertainties.
For EPF_J000211.4+305730, 2.04 arcsec away from the position of a high proper motion star: UCAC4 605-000101, we fitted the spectra in the 0.5-10 keV band with an absorbed APEC model. The hydrogen column density was fixed at the Galactic value of 4.83 × 10^20 cm2. The best-fit plasma temperature is 1.44 (+0.77/-0.25) keV. The unabsorbed 0.5–10 keV flux is 1.19 (+0.57/-0.35) × 10^-13 erg s^-1 cm^-2.
For EPF_J000231.0+305716, 11.50 arcsec away from the position of a Quasar SDSS J000230.78+305717.6, we fitted the spectra in the 0.5-10 keV band with an absorbed power-law model, fixing NH at the Galactic value. The best-fit photon index is 2.84 (+1.96/-1.78). The average unabsorbed 0.5–10 keV flux has an upper limit of 8.43 × 10^-14 erg/s/cm2.
We tend to interpret EP261015b as a stellar flare from the high proper motion star UCAC4 605-000101. All quoted uncertainties on the spectral parameters and fluxes are at the 90% confidence level. 
Launched on January 9, 2024, EP is a space-based X-ray observatory designed to monitor the soft X-ray sky and perform X-ray follow-up observations (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics).

Jian-Hong Cao, Jia-Qi Lin, Zhong-Nan Dong, Jin-Ji Li, Pu Lin, Yun Shi, Hao-Nan Yang, Rui-Chen Gao, Jing-Kai Sun, Hao-Ran Zhang, P. H. Thomas Tam, Rong-Feng Shen, and Bin Ma (Sun Yat-sen University) report on behalf of the SYSU 80cm infrared telescope team:

We observed the field of EP261015b, detected by the Wide-field X-ray Telescope (WXT) on board Einstein Probe (Li et al., GCN 45958
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 ), using the Sun Yat-sen University (SYSU) 80cm infrared telescope. Our J-band observations were carried out on 2026 Oct 16 from 16:31 to 17:32 UTC, approximately 26.57–27.59 hours after the start of the WXT observation. We combined 150 exposures of 20 s each, yielding a total exposure time of 3000 s.

We do not detect any new source in the WXT localization in our stacked image, and obtain the following 5-sigma upper limit: J = 19.5 mag (Vega).

The SYSU 80cm infrared telescope is operated and managed by the Department of Astronomy, Sun Yat-sen University.

Z. -X. Li, G. -L. Huang (IHEP), R. -Z. Li (YNAO), M. Nie (YNAO) and H. Sun (NAOC) on behalf of the Einstein Probe (EP) team:
 
We report on the detection of an X-ray transient, designated EP261015b, by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission. The transient was detected by EP-WXT during an observation starting from 2026-10-15T13:56:38 (UTC), and identified by ground-based telemetry analysis. The WXT position of the source is R.A. = 0.581 deg, DEC = 30.973 deg (J2000) with an uncertainty of 2.83 arcmin in radius (90% C.L. statistical and systematic). No cataloged X-ray sources have been found within the WXT error circle. 

The average WXT 0.5-4 keV spectrum can be fitted by an absorbed power-law model with a Galactic hydrogen column density of 4.83e20 cm^-2 and a photon index of 2.11 (-0.81/+0.96). The derived average unabsorbed 0.5-4 keV flux is 3.50 (-1.27/+2.03) e-11 erg cm^-2 s^-1. All uncertainties are reported in 90% C.L.
 
We have arranged a follow-up observation with the Follow-up X-ray Telescope onboard EP and will update the information upon receiving the data. Multi-wavelength observations are strongly encouraged.
 
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).