GRB 251215A

General information

GRB 251215A is a long gamma-ray burst detected by Fermi/GBM and localized to arcminute precision by Swift/BAT-GUANO, with Swift/XRT follow-up identifying a likely fading X-ray afterglow candidate. Optical/UV follow-up reported only upper limits in a direction with significant Galactic extinction.

Wavelength coverage

• Gamma-ray: Detected by Fermi/GBM with prompt-emission duration and spectral/fluence measurements; also detected by Swift/BAT-GUANO, Glowbug, and a CALET trigger associated with the same event.
• X-ray: Swift/XRT detected multiple uncatalogued sources initially; later observations indicate Source 5 is fading at >3σ and is likely the afterglow, with an enhanced XRT position reported.
• Optical/UV: Swift/UVOT reported a white-band upper limit (white > 22.4, uncorrected for extinction); COLIBRÍ reported r > 21.8 and z > 20.5 (uncorrected); MASTER-Net reported limits up to ~20.3 mag in clear band. No observations mentioned in: radio, infrared, millimeter/sub-mm, GeV/TeV.

Lightcurve and spectrum

• T90: ~143 s (50–300 keV; Fermi/GBM).
• Prompt spectrum: Band function with Epeak = 35 ± 7 keV, alpha = −1 ± 0.3, beta = −1.97 ± 0.03 (Fermi/GBM; preliminary).
• Fluence: (2.05 ± 0.06)×10^-5 erg cm^-2 (10–1000 keV; Fermi/GBM).
• 1 s peak photon flux: 5.7 ± 0.2 ph s^-1 cm^-2 (10–1000 keV; Fermi/GBM).
• X-ray fading: Source 5 reported fading with alpha > 0.3; later classified as fading with >3σ significance and likely afterglow (Swift/XRT).

Redshift

What’s special vs typical

• The prompt spectrum is unusually soft for a long GRB (low reported Epeak ~35 keV), suggestive of an X-ray-rich/soft event.
• The line of sight has strong Galactic reddening/extinction (reported E(B−V)=0.87 and Av ~2.8), consistent with a “dark” optical appearance despite X-ray afterglow evidence.
• The burst was detected by multiple space-based instruments including the ISS-based Glowbug technology demonstrator, and Swift/BAT event-mode data were recovered via the GUANO pipeline triggered by external notices.