Pre-perihelion detection of a wobbling high-latitude jet in the interstellar comet 3I/ATLAS

Abstract

We present observations of the detection of a faint high-latitude jet in the inner coma of comet 3I/ATLAS that coincides with the broad plume detected in visible images along PA $280\pm 10^{\circ }$. A detailed analysis shows that the jet was clearly detected on seven nights (2025, August~3, 5, 18, 19, 21, 24, and~29). The jet maintains an almost, though not perfectly, constant position angle (PA) throughout these epochs. High-precision PA measurements at a projected distance of 6000~km from the cometary optocenter reveal a periodic modulation centered at ~ 280 degrees, consistent with a high-latitude jet undergoing precessional motion around the sky-projected spin axis of the nucleus. This is the first periodic jet-angle modulation detected in an interstellar comet. The derived periodicity of $7.74\pm 0.35$ h may imply a nucleus rotation period of $P_{\mathrm{rot}}=15.48\pm 0.70$ h if the jet originates from a single active source near one of the poles. This value is slightly shorter than the period of $P_{\mathrm{rot}}=16.79\pm 0.23$ h derived from the photometric time series. From the measured PA range, the sky-projected orientation of the spin axis is derived as $\mathrm{PA}=280.7\pm 0.2^{\circ }$

6 pages, 4 figures and 1 table