GRB 251129A

General information

GRB 251129A was detected by SVOM/ECLAIRs (with supporting X-ray detections/localizations from SVOM/MXT, Einstein Probe and Swift), and an optical counterpart was rapidly identified near RA 224.3286 deg, Dec 79.2930 deg. The event drew extensive follow-up, including evidence for ultra-long prompt/high-energy activity and unusual late-time afterglow behavior.

Wavelength coverage

• Gamma-ray / hard X-ray: SVOM/ECLAIRs reported emission detected over a long interval in 4–120 keV (up to ∼3200 s).
• X-ray: SVOM/MXT, Einstein Probe (WXT/FXT), and Swift/XRT reported an uncatalogued X-ray counterpart with spectral fits and temporal decay behavior.
• UV/optical: Swift/UVOT and many ground-based facilities reported detections and photometry (including multi-band u,g,r,i,z and early color evolution).
• Radio: e-MERLIN reported a C-band non-detection early, followed by a significant VLA C-band detection at ∼16 days. No observations were mentioned in the NIR, mm/sub-mm, or GeV–TeV bands.

Lightcurve and spectrum

• Duration: SVOM/ECLAIRs reports detection from ~T0-41 s to ~T0+3200 s (4–120 keV), suggesting an ultra-long GRB / extended emission component.
• High-energy spectrum/flux: ECLAIRs time-averaged spectra in 4–120 keV are fit by power laws with photon indices ∼1.7, 2.1, and 2.0 in successive intervals, with flux declining from ∼1.5e-8 to ∼1.5e-9 erg/cm^2/s.
• X-ray temporal behavior: Swift/XRT reports a steep power-law decay with α ≈ 6.23; EP/FXT reports absorbed power-law fits with photon index ∼1.98 at T0+2643 s, softening in later ToOs, and a strong flux decline over hours.
• Optical evolution: early optical fading with a reported slope ∼0.7 (SVOM/VT) and color becoming bluer between ∼0.5 h and ∼4–50 h; late-time imaging (∼9–27 d) shows a rebrightening in u,g,r,i,z, reaching r=22.24±0.04 at 27.2 d, exceeding the inferred host level.
• Optical spectroscopy: a late-time GTC spectrum shows a continuum and host-galaxy emission lines used for redshift determination.
• Radio evolution: e-MERLIN reports a 3σ upper limit of 0.066 mJy at ∼4–5 d, while VLA detects ∼75±8 µJy at 5 GHz and ∼62±9 µJy at 7 GHz at ∼16 d.

Redshift

• z = 0.505 from GTC spectroscopy via detection of [OII], [OIII], Hα, Hβ, and Hγ in an underlying galaxy; the same report interprets the late optical rebrightening as an emerging associated supernova at this redshift.
• An earlier GTC spectroscopic report claimed z = 1.460 based on metal absorption features (CrII, FeII/FeII*, MgII, NiII, SiII, ZnII), creating a reported discrepancy between absorption- and emission-based redshift determinations.

What’s special vs typical

• Ultra-long (∼3200 s) hard X-ray activity and unusual multi-wavelength temporal/spectral evolution were highlighted as atypical for standard long-GRB behavior.
• Clear late-time optical rebrightening consistent with a supernova emergence, with contemporaneous host-galaxy emission-line redshift measurement (z=0.505).
• Late-time radio detection after an earlier non-detection suggests a rising/emerging radio component on week timescales.
• Conflicting spectroscopic redshift claims (z=0.505 vs z=1.460) make the physical interpretation and environment of the burst unusually uncertain based on the published circulars.