Rotation Period of 3I/ATLAS After Perihelion from Jet Position Angle Wobble and Photometric Variability

TL;DR

This study determines the post-perihelion rotation period of the interstellar visitor 3I/ATLAS by combining two independent diagnostics: jet position-angle wobble derived from HST imaging processed with Larson–Sekanina filtering, and a Gr-band photometric time series. The jet-PA analysis yields $P_{ m jet} = 7.20 \pm 0.05$ h, while the photometric analysis provides $P_{ m phot} = 7.136 \pm 0.001$ h; together they support a rotation period of roughly $P \approx 7.1$ h, with the jet precessing about the rotation axis by about $20^{\circ}$ and the axis aligned with the Sun–3I/ATLAS line within $\sim 20^{\circ}$. The two methods are complementary, with the jet morphology reflecting orientation of a persistent active source and the coma brightness modulated by jet orientation and illumination geometry. The results underscore the value of concurrent morphological and photometric constraints to disentangle rotation states and activity-driven modulations in comets and interstellar visitors, while acknowledging aliasing and systematics that require denser future observations. The inferred Sun-aligned rotation state has implications for torque processes acting on the nucleus and for interpreting activity-driven variability in the coma.

Abstract

We determine the post-perihelion rotation period of 3I/ATLAS using two independent diagnostics: the temporal modulation of the position angle (PA) of a persistent jet-like feature, and a time-series photometric light curve in the Gr (R) band. For the jet morphology, we measure the PA at multiple epochs by applying the Larson-Sekanina Rotational Gradient filter to Hubble Space Telescope images between November 20, 2025 and December 27, 2025, and model the phase-folded PA curve with weighted least-squares Fourier series up to two harmonics while scanning trial periods P to identify minima in \c{hi}2(P). For the photometry, we adopt the best-fit period from an independent 30-minute binned analysis (from a 0.25 meter telescope MPC L92) based on a refined \c{hi}2(P) profile for a sinusoidal model with nightly offsets. We find that the jet-PA modulation gives a period Pjet = 7.20 +/-0.05 h (adopting a conservative uncertainty dominated by sparse sampling and systematics), while the photometry yields Pphot = 7.136+/-0.001 h (formal 1σ uncertainty). Although the periods differ slightly, the offset is plausibly attributable to non-Gaussian systematics and aliasing. The combined data supports a post-perihelion rotation period of 7.1 h associated with precession of the jet structure around the rotation axis by 20°. The rotation axis is aligned with the sunward direction to within 20°

Figures (6)

• Figure 1: Jet position angle (PA) measurements versus time. Error bars show the adopted $\sigma_{\rm PA}=3^\circ$.
• Figure 2: Phase-folded jet PA measurements at $P=7.20$ h (phase 0--1). The best-fit $K=2$ model.
• Figure 3: Residuals of the jet-PA fits at $P=7.20$ h for $K=2$. The reduced residual structure for $K=2$ supports including a second harmonic.
• Figure 4: Photometry folded at $P_{\rm phot}=7.136$ h (phase 0--1, 30-minute bins). Points are coloured by observing night. The curve is the best-fit 2 harmonic model from the photometric analysis.
• Figure 5: Residual photometry (phase 0-1, 30-minute bins) best-fit 2 harmonic model from the photometric analysis.