Simultaneous Multiband Photometry of the Early Optical Afterglow of GRB 240825A with Mephisto

Gamma-ray bursts (GRBs) are the most luminous transients in the Universe. The interaction of the relativistic jet with the circumburst medium produces an afterglow and generates multiwavelength emission. In this work, we present simultaneous multiband photometry of GRB 240825A with the Multi-channel Photometric Survey Telescope (Mephisto) and analyze its temporal and spectral properties. The measurement began 128 s after the GRB trigger and continued until the fourth day, when the afterglow essentially diminished and the measured brightness was close to that of the host galaxy. Based on the multiband light curves in the uvgriz bands, we find that the optical flux density satisfies F nu,obs proportional to t-1.34 nu-2.48 with a spectral index of 2.48, much larger than those of most other GRBs. To reconcile the measured much softer spectral energy distribution (SED) with that predicted by the standard afterglow model, an extra host-galaxy extinction of EB-V similar to (0.37-0.57) mag is required. We interpreted this excess as arising from a dense circumburst medium. We further find that the SED of the optical afterglow hardened as the afterglow decayed and the color excess EB-V decreased similar to 0.26 mag from 100 to 3000 s after the GRB trigger. Finally, we analyze the properties of the host galaxy of GRB 240825A based on data from the Sloan Digital Sky Survey, the Pan-STARRS survey, and the HSC-SSP survey. For a host redshift of z = 0.659, the stellar mass and star formation rate of the host galaxy are estimated to be log(M*/M circle dot)=10.0-0.3+0.3 and log(SFR/M circle dot yr-1)=0.6-3.3+0.8 , respectively, pointing to a gas-rich, star-forming, medium-size galaxy.