利用Mephisto对GRB 240825A早期光学余辉的同步多波段测光研究
Simultaneous Multiband Photometry of the Early Optical Afterglow of GRB 240825A with Mephisto
AI总结：
本研究利用Mephisto望远镜对GRB 240825A早期光学余辉开展同步多波段测光，发现其光谱指数偏软，需引入致密暴周介质导致的额外消光解释，还分析了余辉演化及宿主星系性质。
摘要：
伽马射线暴（GRB）是宇宙中光度最高的暂现天体。相对论性喷流与暴周介质相互作用会产生余辉并激发多波段辐射。本研究利用多通道测光巡天望远镜（Multi-channel Photometric Survey Telescope，Mephisto）获取了GRB 240825A的同步多波段测光数据，并分析了其时间演化与光谱特性。观测测量在GRB触发后128秒启动，持续至第4天，此时余辉已基本衰减，测得的亮度接近宿主星系的亮度。基于uvgriz波段的多波段光变曲线，我们发现光流量密度满足$F_{ν,{\rm obs}}\npropto t^{-1.34}ν^{-2.48}$，其光谱指数为2.48，远大于绝大多数其他GRB的光谱指数。为了协调测得的偏软的光谱能量分布（SED）与标准余辉模型的预测结果，需要引入大小为$E_{B-V}\n(0.37-0.57)$星等的额外宿主星系消光。我们将这一超额消光解释为致密暴周介质导致的结果。我们进一步发现，随着余辉的衰减，其光学余辉的SED逐渐变硬，且从GRB触发后100秒到3000秒，色余$E_{B-V}$下降了约0.26星等。最后，我们基于SDSS、PanSTARRS和HSC-SSP巡天的数据分析了GRB 240825A宿主星系的性质。在宿主星系红移$z=0.659$的前提下，估算得到宿主星系的恒星质量为$\nlog(M_*/M_\nodot)=10.0^{+0.3}_{-0.3}$，恒星形成率为$\nlog({\rm SFR}/M_{\ndot}{\rm yr}^{-1})= 0.6^{+0.8}_{-3.3}$，表明这是一个富气体、正在形成恒星的中等质量星系。
英文摘要：
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 seconds 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 $F_{ν,{\rm obs}}\propto t^{-1.34}ν^{-2.48}$ 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 $E_{B-V}\sim(0.37-0.57)$ 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 $E_{B-V}$ decreased $\sim0.26$ mag from 100 seconds to 3000 seconds after the GRB trigger. Finally, we analyze the properties of the host galaxy of GRB~240825A based on data from the SDSS, PanSTARRS and HSC-SSP surveys. For a host redshift of $z=0.659$, the stellar mass and star formation rate of the host galaxy are estimated to be $\log(M_*/M_\odot)=10.0^{+0.3}_{-0.3}$ and $\log({\rm SFR}/M_{\odot}{\rm yr}^{-1})= 0.6^{+0.8}_{-3.3}$, respectively, pointing to a gas-rich, star-forming, medium-size galaxy.
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