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Phys.org

Temporal evolution of GRB 240825A afterglow provides insight into origins of optically dark gamma-ray bursts

Li Yali, Chinese Academy Of Sciences Thu, 12 Feb 2026 17:00:03 EST · Science, Space
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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.

This article has been reviewed according to Science X's editorial process and policies . Editors have highlighted the following attributes while ensuring the content's credibility:

Evolution of optical darkness (β OX ) and multiwavelength light curves for the GRB 240825A afterglow. Credit: The Astrophysical Journal (2026). DOI: 10.3847/1538-4357/ae1e7d

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.

Publication details Rui-Zhi Li et al, Time Evolution of Optical Darkness in Gamma-Ray Burst Afterglow: The Case of GRB 240825A, The Astrophysical Journal (2026). DOI: 10.3847/1538-4357/ae1e7d Journal information: Astrophysical Journal

— Source: Phys.org (https://phys.org/news/2026-02-temporal-evolution-grb-240825a-afterglow.html)

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