EP261015c

G. -L. Huang, Z.-X. Li (IHEP), R.-Z. Li (YNAO), M. Nie (YNAO) and H. Sun (NAOC) on behalf of the Einstein Probe (EP) team:

The fast X-ray transient EP261015b was detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Huang et al., GCN 45959
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 ). The source exhibited a flare that began at approximately 2026-10-15T17:37:52 UTC and lasted for approximately 700 s (Huang et al., GCN 45959
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 ). A follow-up observation with the Follow-up X-ray Telescope (FXT) on board EP began at 2026-10-17T17:08:30 UTC, approximately 48 hours after the onset of the flare, with an exposure time of approximately 6000 s.

The FXT detected an uncatalogued X-ray source within the WXT error circle at R.A. = 21.9613 deg and Dec. = 66.6144 deg (l = 126.5406 deg, b = 3.9981 deg), with an uncertainty radius of 10 arcsec (90% confidence level, including both statistical and systematic uncertainties). We fitted the FXT spectrum with a simple absorbed power-law model and obtained an equivalent hydrogen column density of 0.4 (-/+0.2), a photon index of 3.9 (-0.7/+0.8), and an unabsorbed 0.5-10 keV flux of 1.1 (-0.4/+1.0) x 10^-12 erg cm^-2 s^-1.

We note that three Gaia sources are located within the FXT error circle: Gaia DR3 525425756171519616, Gaia DR3 525425756171519488, and Gaia DR3 525425756167531904, at distances of approximately 104, 105, and 2962 pc, respectively. We also fitted the FXT spectrum with an apec model and obtained kT = 1.5 (-0.2/+0.3) keV. If the event was due to a stellar flare, the flare luminosities estimated using the unabsorbed WXT flare flux would be approximately 1.9 (-0.8/+1.6) x 10^32, 2.0 (-0.8/+1.6) x 10^32, and 1.6 (-0.6/+1.3) x 10^35 erg s^-1, respectively.

Given the source’s low Galactic latitude, the relatively soft WXT and FXT spectra, and the approximately two-order-of-magnitude decline in flux between the WXT observation and the FXT observation two days later, we tend to interprete the event as a stellar flare.

Launched on January 9, 2024, EP is a space X-ray observatory designed to monitor the soft X-ray sky with rapid X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics). EP is a mission of the Chinese Academy of Sciences in collaboration with ESA, MPE, and CNES.

Tanishk Mohan (IITB), Geoffrey Mo (Caltech/Carnegie), Viraj Karambelkar (Columbia), Benjamin Schneider (LAM), Tomas Ahumada (NOIRLab), Robert Stein (UMD), Danielle Frostig (CfA), Nathan Lourie (MIT), Robert Simcoe (MIT), and Mansi Kasliwal (Caltech) report:

We observed the field of EP261015c (Huang et al., GCN 45959
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 ) in the near-infrared J band with the Palomar 1-m telescope, equipped with the 1.2-square degree WINTER camera (Lourie et al. 2020, Frostig et al. 2024).

Observations began on 2026-10-16 at 03:42:19.088 UTC in the J band (10.2 hr after the WXT observation), consisting of 20*120 sec exposures. The images were processed using the WINTER data reduction pipeline implemented with mirar (https://github.com/winter-telescope/mirar, https://doi.org/10.5281/zenodo.13352565
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 ).

We do not detect any new source in the WXT localization in our stacked image, and obtain the following 5-sigma upper limit: J = 18.35 mag (AB).

WINTER (Wide-field INfrared Transient ExploreR) is a partnership between MIT and Caltech, housed at Palomar Observatory, and funded by NSF MRI, NSF AAG, the David and Lucile Packard Foundation, and the MIT Kavli Institute for Astrophysics and Space Research.

G. -L. Huang, Z.-X. Li (IHEP), R.-Z. Li, M. Nie (YNAO) and H. Sun (NAOC) on behalf of the Einstein Probe (EP) team:

We report the detection of a fast X-ray transient by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission, designated EP261015c. The EP-WXT observation (ID: 11900946826) started at 2026-10-15 17:29:32 (UTC), with an exposure time of 1200 s. The WXT light curve of the transient began at 2026-10-15T17:37:52 UTC lasted for 700 s. The WXT position of the source is R.A. = 21.920 deg, DEC = 66.601 deg (J2000) (l=126.526 deg, b=3.983 deg) with an uncertainty of 2.695 arcmin in radius (90% C.L. statistical and systematic).

We fitted the WXT spectrum with an absorbed power-law model, fixing nH at 0.581e22 cm-2. We obtained a spectral index of 4.3 (-1.8/+1.9) and an unabsorbed flux of 1.5 (-0.6/+1.2) e-10 erg/cm2/s during the flare. Over the entire observation, the spectral index was 4.4 (-1.8/+1.9), with an unabsorbed flux of 9.0 (-3.7/+6.9) e-11 erg/cm2/s.

A follow-up observation by EP-FXT have been scheduled. Multi-wavelength observations are strongly encouraged.

Launched on January 9, 2024, EP is a space X-ray observatory designed to monitor the soft X-ray sky with rapid X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics). EP is a mission of the Chinese Academy of Sciences in collaboration with ESA, MPE, and CNES.