General information
GRB 260429B / EP260429a is reported as a long-duration GRB detected by AstroSat CZTI and associated with the Einstein Probe WXT transient, with EP-FXT follow-up identifying a fading X-ray source and COLIBRÍ later reporting a fading optical counterpart candidate.
Wavelength coverage
- Gamma-ray / hard X-ray: AstroSat CZTI detection in 100–500 keV (Veto) and faintly in 20–200 keV; Konus-Wind detection mentioned (private communication).
- X-ray: EP-FXT follow-up found two 0.5–10 keV sources in the WXT error region; one source faded markedly between two epochs.
- Optical: COLIBRÍ r and z imaging provided early non-detections at the WXT position, and later two-epoch imaging indicating a fading uncatalogued optical source.
No observations are mentioned in UV, near-IR, mid-IR, radio, TeV, neutrinos, or gravitational waves.
Lightcurve and spectrum
- CZTI (Veto, 100–500 keV): light curve peaks at 2026-04-29 14:42:59.27 UTC; T90 = 34 (+6, -9) s; peak count rate 147 (+31, -33) counts/s above background with total 1802 (+485, -554) counts.
- EP-FXT (0.5–10 keV): EPF_J160221.2-093607 flux decreased from 2.1e-12 to 5.8e-13 erg cm^-2 s^-1 between ~16.8 hr and ~55.2 hr; EPF_J160226.1-093337 stayed ~6.6e-13 erg cm^-2 s^-1.
- COLIBRÍ: candidate optical source measured at r = 21.95 ± 0.05 and z = 21.08 ± 0.04 (first epoch), then not detected at ~27.7 days down to r > 23.90 and z > 23.07 (3σ).
Redshift
What’s special vs typical
- A specific optical counterpart candidate is proposed based on fading and spatial coincidence with the likely fading EP-FXT afterglow source, rather than only upper limits.