Time Evolution of Optical Darkness in GRB Afterglow: The Case of GRB 240825A

Abstract

Long-duration gamma-ray bursts (GRBs) are believed to occur in star-forming regions. The multiwavelength follow-up observations of the early afterglow of GRB 240825A provided insights into the evolution of the optical-to-X-ray spectral feature of the afterglow. We comprehensively investigate the evolution of X-ray spectral properties through time-resolved spectral analysis and calculate optical darkness ($\beta_\mathrm{OX}$) to reveal the physical properties of the afterglow. The X-ray-to-optical SEDs of afterglow in different time intervals are fitted to derive the extinction curves. The $\beta_\mathrm{OX}$ exhibits a trend of decreasing and then increasing, reaching its minimum value at $\sim1000\mathrm{\,s}$ post-trigger. However, at 11 hours post-trigger, $\beta_\mathrm{OX}$ does not meet the criteria for an optically dark burst. The extinction curves in different time intervals indicate that GRB 240825A occurred in a dust-obscured environment.