JWST unveils a dust-obscured supernova associated with a gamma-ray burst

Abstract

Recent observations have challenged the duration-based classification of gamma-ray bursts (GRBs), making the detection of an associated supernova (SN) or kilonova crucial for conclusively identifying their progenitors. We investigate the origin of GRB 240825A at $z=0.659$, whose prompt emission gave conflicting indications of a merger or collapsar origin and for which no SN was detected in deep ground-based observations. We analyze JWST/NIRSpec observations obtained 66.5 days (40.1 days in the rest frame) after the burst. We identify a SN signal using a pixel-by-pixel decomposition of the 2D spectrum and isolate it by jointly modeling the host and transient emission. We constrain the line of sight extinction from the afterglow spectral energy distribution and characterize the host through broadband photometry and nebular emission lines. Despite a dusty line of sight ($A_V=1.37\pm0.08$ mag), with tentative evidence of a 2175 {\AA} bump, and a bright host galaxy ($M_B=-20.5$ AB mag), we identify a point-like source at the GRB position with broad spectral features similar to those of SN 1998bw. The SN contributes $\sim$10-15% of the NIRSpec flux and would contribute only $\sim$1% of the total flux in a ground-based optical image at the same epoch. Its extinction-corrected luminosity is comparable to SN 1998bw, although it remains sensitive to the host subtraction and template assumptions. The host is an extended, dusty, star-forming galaxy with $\log(M_\star/M_\odot)=10.12\pm0.05$, SFR $=5.88\pm1.60\,{\rm M_\odot\,yr^{-1}}$, and a super-solar metallicity of $12+\log({\rm O/H})=8.87\pm0.04$, above the metallicity typically observed for collapsar GRB hosts. The detection of a dust-obscured SN firmly establishes the massive-star origin of GRB 240825A and shows that JWST can uncover SNe in dusty, chemically enriched environments that were previously largely inaccessible.