GRB 260401A

General information

GRB 260401A triggered multiple high-energy instruments on 2026-04-01 around 10:53:26 UTC, with reports describing it as a likely long GRB. Localizations from Fermi/GBM and BALROG are consistent with a bright, temporally coincident MAXI/GSC X-ray transient.

Wavelength coverage

• Gamma-ray / hard X-ray (keV–MeV): Fermi/GBM (localization, lightcurve, spectrum/fluence), CALET/CGBM (durations), GECAM-B (T90, spectrum/fluence), SVOM/GRM (+ ECLAIRs via shield; spectra/fluence/flux), NuSTAR CsI shields (prompt detection).
• X-ray (4–10 keV): MAXI/GSC detection with reported bright scan-averaged flux and scan-to-scan upper limits.
• Optical: MASTER-Net follow-up reporting only upper limits (no OT claimed). No observations were mentioned in these circulars for: radio, infrared, ultraviolet, TeV.

Lightcurve and spectrum

• Timing/duration:

  • CALET/CGBM: single pulse; T90 = 4.4 ± 0.6 s (40–1000 keV). (44177)
  • Fermi/GBM: bright episode with multiple peaks; T90 ~ 27 s (50–300 keV). (44179)
  • GECAM-B: multiple pulses; T90 = 28.8 (+0.1/-0.2) s (70–6000 keV). (44187)
  • SVOM/GRM: multiple pulses; T90 = 24 ± 2 s (15–5000 keV). (44188)
  • NuSTAR CsI shields: single peak lasting ~10 s; peak ~3000 cps with baseline ~1000 cps; no evidence above 100 keV in CZT detectors. (44178)
  • MAXI/GSC: not significant in previous (09:20 UT) and next (12:26 UT) transits; upper limit 20 mCrab for each. (44176)

• Spectral/fluence/flux:

  • Fermi/GBM time-averaged (T0-0.8 to T0+32.1 s): cutoff power law, index = -1.78 ± 0.01, Epeak = 239 ± 1 keV; fluence (10–1000 keV) = (1.80 ± 0.02)e-05 erg/cm^2; 1-s peak photon flux (10–1000 keV) = 21.1 ± 0.4 ph/s/cm^2. (44179)
  • GECAM-B time-averaged (T0 to T0+15 s): power law, index = -1.51 (+0.08/-0.07); fluence (10–1000 keV) = (2.20 (+0.11/-0.10))e-05 erg/cm^2. (44187)
  • SVOM/GRM time-averaged (T0-10 to T0+10 s): cutoff power law, index = -1.64 ± 0.06, Epeak = 321 (+141/-77) keV; fluence (10–1000 keV) = (1.44 (+0.06/-0.07))e-05 erg/cm^2. (44188)
  • SVOM/GRM 1-s peak (T0-0.1 to T0+0.9 s): cutoff power law, index = -1.26 ± 0.07, Epeak = 391 (+84/-59) keV; energy flux (10–1000 keV) = (3.28 ± 0.15)e-06 erg/cm^2/s. (44188)
  • MAXI/GSC: scan-averaged 4–10 keV flux 5700 ± 200 mCrab. (44176)

• Optical limits:

  • MASTER-Net reports clear-band limits including a coadded limit of 19.0 mag at ~6.36e4 s after T0. (44180)

Redshift

What’s special vs typical

• Very strong prompt multi-instrument coverage spanning gamma-ray to X-ray (Fermi/GBM, SVOM/GRM+ECLAIRs-shield, GECAM-B, CALET/CGBM, NuSTAR shields, MAXI/GSC). (44174, 44176-44179, 44187, 44188)
• Bright prompt X-ray transient reported by MAXI/GSC (5700 mCrab in 4–10 keV during scan). (44176)
• Notable spread in reported T90/duration across instruments/analyses (from ~4 s to ~29 s). (44177, 44179, 44187, 44188)