Researchers from the Yunnan Observatories of the Chinese Academy of Sciences have conducted a new study on the temporal evolution of the afterglow from gamma-ray burst GRB 240825A. The study offers new evidence to better understand the physical environment surrounding gamma-ray bursts and provides insights into the mechanisms that govern their afterglow emission.

The findings were recently published in The Astrophysical Journal.

Long-duration gamma-ray bursts (LGRBs) are widely believed to form from the core collapse of massive stars, usually occurring in dense star-forming regions. NASA's Swift satellite detected GRB 240825A on August 25, 2024, and observed an unusually bright optical counterpart.

Early measurements yielded an X-ray afterglow spectral index of 0.79 and a significantly softer optical afterglow spectral index of 2.48, compared with a typical value near 1. Under standard models, a gamma-ray burst is classified as "optically dark" when its observed optical afterglow flux falls below the level predicted from its X-ray spectral index.

The team carried out follow-up observations of the GRB 240825A afterglow using both space- and ground-based facilities, including the Lijiang 2.4-meter telescope, and performed a systematic analysis of its optical darkness by combining multiwavelength afterglow data. They quantified the optical darkness using the optical-to-X-ray spectral index (β_ox).

Results revealed a clear temporal evolution: optical darkness initially decreased, reaching a minimum approximately 1,000 seconds after the burst — satisfying the criterion for an optically dark gamma-ray burst — before increasing again. By roughly 11 hours post-burst, the afterglow no longer met the definition of an optically dark event, marking a distinct transition to an optically bright phase.

Further analysis of the optical and X-ray spectral energy distribution (SED) of the GRB 240825A afterglow indicated that extinction in its surrounding medium declined gradually over time. This suggests the burst likely originated in a dense circumburst environment.

The findings provide new observational clues for understanding the origins of optically dark gamma-ray bursts and the evolution of their surrounding environments.

This work was supported by the National Key R&D Program of China, the National Natural Science Foundation of China, and the Yunnan Revitalization Talent Support Program.