A $6.37\mathrm{H}\mathrm{z}$ quasi-periodic oscillating photospheric emission in GRB~240825A

Using data from Swift-BAT and Fermi-GBM, we report the first detection of a high-confidence quasi-periodic oscillation (QPO) in the thermal emission of gamma-ray burst GRB~240825A. The spectral analysis of the burst reveals two radiation components, including a thermal emission dominant in $100\text{–}300\mathrm{k}\mathrm{e}\mathrm{V}$ and a non-thermal emission spanning a wide energy range. During the interval $2.07\text{–}3.25\mathrm{s}$ post-trigger, a strong QPO signal at $6.37\mathrm{H}\mathrm{z}$ ($\gtrsim 5\sigma$ confidence) is identified in the $100\text{–}300\mathrm{k}\mathrm{e}\mathrm{V}$ thermal-dominated band. The variability analysis of the non-thermal component ($15\text{–}30\mathrm{k}\mathrm{e}\mathrm{V}$) uncovered a $0.67\mathrm{H}\mathrm{z}$ QPO, consistent with light-curve modeling using periodic fast-rise exponential decay pulses. The strong QPO in the photospheric emission directly indicates a quasi-periodic oscillating jet. Together with the non-thermal emission variability, we show that this QPO can be explained in terms of a helical structure in the jet, where the viewing angle to the dominant emission region in the jet undergoes periodic changes.