GRB 260128A

General information

Einstein Probe detected the fast X-ray transient EP260128a, associated with GRB 260128A, and Fermi-GBM later found a consistent sub-threshold GRB-like signal. Multiple optical/NIR follow-ups reported mostly non-detections, with one early faint z-band candidate reported within the EP localization.

Wavelength coverage

• Gamma-rays: Fermi-GBM targeted search found a sub-threshold transient consistent with the EP position; best-fit Band template (GCN 43556).
• X-rays: EP-WXT/FXT detected and characterized the transient in 0.5–10 keV (GCN 43549, 43554).
• Optical: Liverpool Telescope and NOT reported early imaging (with a tentative/faint candidate in NOT z), and later deeper limits from NOT and COLIBRÍ (GCN 43553, 43555, 43570, 43572).
• Near-IR: TNG/NICS reported J-band limits (GCN 43570). No reports in radio, mm/sub-mm, UV, or TeV were mentioned.

Lightcurve and spectrum

• Duration: WXT refined analysis indicates the event lasted ~100 s (GCN 43554).
• X-ray spectrum/flux (WXT): absorbed power law with photon index 0.05±0.73 (NH fixed to Galactic 5.84×10^20 cm^-2); unabsorbed 0.5–4 keV flux 6.98(-4.40/+11.93)×10^-9 erg s^-1 cm^-2 (GCN 43554).
• X-ray spectrum/flux (FXT): absorbed power law with photon index 1.99±0.06 (NH fixed to Galactic 5.84×10^20 cm^-2); unabsorbed 0.5–10 keV flux (2.76±0.15)×10^-11 erg s^-1 cm^-2; no additional absorption required (GCN 43554).
• Gamma-ray timing/significance: most significant GBM targeted-search signal at T0+145 s on a 16 s timescale with FAR 1.1e-05 Hz; also a signal at T0+70 s with FAR 6.5e-05 Hz; light curve described as multi-peaked (GCN 43556).
• Gamma-ray spectrum: best-fit “normal” Band template with Epeak=230 keV, alpha=-1.0, beta=-2.3 (GCN 43556).
• Optical candidate/limits: NOT reported a z-band source at z=23.07±0.26 (AB) at 1.004 hr with no r-band detection (GCN 43555); subsequent stacked observations reported non-detections with limits including z>23.2 and J>22.0 at ~23 hr (GCN 43570) and i>23.6, z>23.1 at ~29–36 hr (GCN 43572).

Redshift

What’s special vs typical

• A faint z-band-only optical candidate led to a suggestion of a potentially very high-redshift event (z >~5), though this remains unconfirmed (GCN 43555).
• The GRB-like signal was recovered in the more sensitive GBM targeted search (sub-threshold to onboard triggering) with a high EP/GBM spatial association probability (GCN 43556).