EP260518a
GCN Circular 44695
Subject
EP260518a (AT2026ndp): LCO observations and Gemini-South Redshift z = 2.28
Date
2026-05-22T14:46:39Z (8 days ago)
Edited On
2026-05-22T22:14:37Z (7 days ago)
From
Gregory Corcoran at University College Dublin <gregory.corcoran@ucdconnect.ie>
Edited By
Vidushi Sharma at NASA GSFC/UMBC <vidushi.sharma@nasa.gov> on behalf of Gregory Corcoran at University College Dublin <gregory.corcoran@ucdconnect.ie>
Via
Web form
G. Corcoran (UCD), D. B. Malesani (DAWN/NBI and Radboud), R. A. J. Eyles-Ferris (Leicester), A. J. Levan (Radboud & Warwick), J. Chacón (PUC), F. E. Bauer (UTA/SSI), P. G. Jonker (Radboud), C. Rodriguez (Gemini), L. Magil (Gemini), report on behalf of a larger collaboration:
We observed the optical transient AT2026ndp associated with the Einstein Probe sub-threshold event EP260518a (Liang et al., GCN 44690) using the LCO 1m telescopes located at the Cerro Tololo Inter-American Observatory (Chile) equipped with SINISTRO instruments. A series of 6x300s exposures were taken in both SDSS-r and SDSS-z bands. In both sequences the optical transient was clearly detected blended with the nearby Legacy Survey object.
We carried out image subtraction against Legacy Survey images (DR 10) in both r and z band using PyZOGY (Guevel et al. 2021, Zackay et al. 2016) and obtained the following AB magnitudes, calibrated using nearby stars in the SkyMapper catalog and not corrected for Galactic extinction:
r = 20.78 +/- 0.04 (midtime: 2026-05-22 09:54:14 UT, 3.84 days after trigger);
z = 20.31 +/- 0.10 (midtime: 2026-05-22 09:15:40 UT, 3.81 days after trigger).
We then observed the transient using the GMOS spectrograph equipped on Gemini–South. Our observations consisted of 2 x 600 s spectra taken using a 1" slit and the B480 grating, covering the wavelength range 4470-8500 Å. Observations started at 2026-05-22 09:54:13 UT (3.84 days after trigger).
Continuum is detected over the entire wavelength range and a number of absorption features can be seen. We identify lines consistent with Fe II (1608, 2344, 2374, 2383, 2587), Si IV (1394, 1403), Si II 1527, C IV (1548, 1550), Al II (1670), Al III (1855, 1863), Mg I / Zn II 2026 at a redshift of z=2.28. Given the lack of detected fine structure lines or lyman alpha absorption, this redshift is strictly speaking a lower limit however we identify no other higher redshift systems and the lack of a Lyman alpha break in our spectrum sets an upper limit to the redshift of z < 2.75. We therefore conclude that z = 2.28 is likely the redshift for this event.
The point-like source in the Legacy Survey noted in Liang et al. (GCN 44690) was also placed on the slit. Its spectrum is consistent with that of a Galactic M-dwarf.
GCN Circular 44690
Subject
EP260518a: Einstein Probe detection of an sub-threshold X-ray transient with counterpart AT 2026ndp
Date
2026-05-22T08:38:05Z (8 days ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
R. D. Liang, W. X. Li, D. Y. Li, C. C. Jin (NAO, CAS) on behalf of the Einstein Probe (EP) team:
We report the detection of a sub-threshold X-ray transient, EP260518a, associated with the optical transient AT 2026ndp.
EP260518a was detected by the Wide-field X-ray Telescope (WXT) aboard the Einstein Probe (EP) mission at R.A. = 299.034 deg, Dec. = -48.618 deg (J2000), with an uncertainty radius of 3 arcmin, at 2026-05-18 13:44:21 UTC. The measured 0.5–4 keV flux was approximately 2.5 x 10^-11 erg cm^-2 s^-1. Due to the low signal-to-noise ratio, no significant flare-like feature is apparent in the X-ray light curve.
A candidate optical counterpart, AT 2026ndp, was identified through real-time cross-matching. The transient was discovered by ATLAS at 2026-05-19 07:44:02.400 UTC, approximately 17 hours after the WXT detection, with a c-band magnitude of 18.73 +/- 0.10. There is a faint point-like source in Legacy Survey dr10, which is 1.8 arcsec away.
Follow-up observations were performed with the Follow-up X-ray Telescope (FXT) aboard EP, starting at 2026-05-21 08:23:08 UTC (~66 hr after the WXT detection). An uncatalogued X-ray source was detected within the WXT error region at R.A. = 299.0407 deg, Dec. = -48.6362 deg (J2000), with an uncertainty of 10 arcsec. The FXT position is offset by 4.7 arcsec from the optical counterpart.
The FXT spectrum can be fitted with an absorbed power-law model with a photon index of 2.5 (+0.8/-0.7) and hydrogen column density of 2.3 (+2.0/-1.7) x 10^21 cm^-2. The estimated average 0.3–10 keV flux is around 4.6 (+1.9/-1.2) x 10^-13 erg cm^-2 s^-1.
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).