Time Lags in Gamma-Ray Bursts Reveal Spectral Evolution and Emission Mechanisms
Researchers have shown that time lags between high- and low-energy photons in gamma-ray bursts (GRBs) serve as reliable tracers of spectral evolution and can distinguish between different physical emission components. Analyzing two exceptionally bright bursts — GRB 160625B and GRB 190114C — using Fermi satellite data, the team found that positive lags reflect the expected hard-to-soft softening of prompt emission, while negative lags signal the emergence of a separate, independent high-energy spectral component. The findings offer a practical diagnostic tool for probing the physical mechanisms driving GRB prompt emission and early afterglows.
A study accepted for publication in Astronomy & Astrophysics demonstrates that time lags in gamma-ray bursts — the delay between the arrival of hard (high-energy) and soft (low-energy) photons — can be used to identify and separate distinct emission components across different energy bands. Using joint time-resolved spectral analysis of Fermi GBM and LAT Low Energy (LLE) data spanning 10 keV to 100 MeV, the authors examined GRB 160625B and GRB 190114C in detail. GRB 160625B showed consistently positive lags consistent with a single spectral component undergoing hard-to-soft evolution, and analysis of its high-energy exponential cutoff yielded bulk Lorentz factor estimates of approximately 120–250, suggesting emission originating above the photosphere. GRB 190114C, by contrast, exhibited negative lags in the 30–100 MeV band, coinciding with a delayed high-energy power-law component that became dominant after roughly 2.5 seconds into the burst. Multi-wavelength comparisons suggest some compatibility with an early afterglow origin for this component, though the authors note that internal dissipation mechanisms cannot be ruled out. The study establishes time lags as an effective observational proxy for spectral evolution, with positive lags tracing prompt emission softening and negative lags flagging the onset of a new, spectrally distinct high-energy component.
What's missing
The study is based on only two GRBs, both exceptionally bright, which limits the generalizability of the conclusions to the broader GRB population. The physical origin of the negative-lag high-energy component in GRB 190114C remains unresolved — the authors acknowledge that both external shocks (early afterglow) and internal dissipation are viable explanations.
What different sources said
- arXiv astro-phCenter
Time lags as proxy of spectral evolution in gamma-ray bursts
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