EP260623a

General information

EP260623a was detected by EP-WXT at 2026-06-23T02:54:05 UTC, with refined start time T0=2026-06-23T02:52:27 UTC, and EP-FXT rapidly found an uncatalogued X-ray source at RA=328.2698 deg, Dec=12.7938 deg with about 10 arcsec uncertainty. A rapidly evolving optical/NIR counterpart was detected by many facilities, consistent with an afterglow-like transient.

Wavelength coverage

• X-ray: EP-WXT detected the trigger and refined analysis found a 103 s event with absorbed power-law spectrum; EP-FXT began follow-up about 3.7 min after T0, detected the source, and reported a two-pulse light curve lasting around 210 s.
• Optical: Las Cumbres, GTC/OSIRIS+, COLIBRÍ, REM, TRT, FRAM-ORM, Kinder/LOT, Liverpool Telescope, and FTW/3KK detected the counterpart in multiple filters from minutes to 1.85 days after trigger.
• Near-infrared: REM detected K band at 1.1 hr, WINTER detected J band at 4.1 hr, FTW/3KK detected J band at 1.85 days, and TNG/NICS detected K band at 1.96 days.
• Spectroscopy: GTC/OSIRIS+ obtained two 600 s spectra from 3615-7800 Å starting 1.93 hr after trigger. No radio, gamma-ray, neutrino, gravitational-wave, or Solar System observations are mentioned.

Lightcurve and spectrum

• Las Cumbres measured w-band magnitudes 16.54±0.01 and 15.62±0.01, showing brightening by about 0.9 mag in 6 minutes.
• FRAM-ORM observed the optical transient rise, peak, and decay; it peaked at about T0+24 min at r15.4, then decayed with power-law index alpha1.7 with a few rebrightenings, extrapolating to r~22.0 at T0+24 hr.
• GTC/OSIRIS+ detected the optical afterglow at AB magnitude 17.01±0.05 at 1.83 hr after trigger.
• COLIBRÍ measured g=18.73±0.04, r=18.30±0.01, i=18.00±0.01, z=17.78±0.03, and y=17.55±0.03 at 3.49-3.69 hr after trigger.
• REM measured r=16.35±0.04 AB and K=13.51±0.17 Vega at a mid-time of 1.1 hr after trigger.
• TRT measured r=17.13±0.02 at 1.767 hr post-burst.
• Kinder/LOT measured r=20.74±0.07 AB at 12.53 hr after detection.
• WINTER measured J=17.9±0.1 AB at 4.1 hr after trigger.
• Liverpool Telescope measured g>21.44, r=21.96±0.23, i=21.59±0.13, and z=21.06±0.16 at about 22.5-23.5 hr after trigger.
• FTW/3KK measured r=23.93±0.22 AB at 1.85 days after trigger and detected the source in r, i, and J.
• TNG/NICS measured K=19.9±0.2 Vega at a mid-time of 1.96 days after trigger.
• EP-WXT spectral fitting over 0.5-4 keV found photon index 1.83(-0.56,+0.63), intrinsic NH=2.25(-0.97,+1.21)e22 cm^-2, and average unabsorbed flux 3.17(-0.74,+1.39)e-9 erg s^-1 cm^-2; the estimated peak flux was 1.0e-8 erg s^-1 cm^-2.
• EP-FXT 0.5-10 keV spectrum had photon index 1.96±0.03, intrinsic NH=0.94±0.04e22 cm^-2, and average unabsorbed flux 7.33±0.10e-9 erg s^-1 cm^-2.

Redshift

GTC/OSIRIS+ measured z=0.703 from metal absorption features interpreted as Fe II, Mn II, Mg II, Mg I, and multiple Fe II fine-structure transitions. COLIBRÍ reported that the g-band detection and lack of a break in the grizy SED imply z<3, consistent with the GTC/OSIRIS+ redshift.

What’s special vs typical

• The event had very rapid optical response, with a counterpart found 8 minutes after the EP-WXT trigger and early brightening of about 0.9 mag in 6 minutes.
• The follow-up is unusually rich in the supplied facts, spanning X-ray, optical, NIR, and optical spectroscopy with detections out to nearly two days.
• The combination of bright early optical emission, measured redshift, and detailed X-ray spectral/light-curve characterization makes it a high-value afterglow event, though no confirmed multi-messenger association is reported.