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

Toni Scarmato, Loeb, Abraham|arXiv (Cornell University)|Jan 15, 2026

Astrophysical Phenomena and Observations0 citations

TL;DR

The paper derives the post-perihelion rotation period of 3I/ATLAS using jet position angle wobble and Gr-band photometry, yielding near-consistent periods around 7.1 hours from two independent methods.

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°

Motivation & Objective

• Motivate measuring the rotation state of the interstellar interloper 3I/ATLAS after perihelion.
• Determine the rotation period using two independent diagnostics: jet position angle modulation and time-series photometry.
• Assess consistency and systematic uncertainties between morphology-based and photometric period estimates.

Proposed method

• Model the phase-folded jet PA time series with a truncated Fourier series up to two harmonics at fixed trial periods to find minima in chi-squared.
• Scan periods to identify the best jet-period that minimizes chi-squared for K=2 and compare with a K=1 model.
• Analyze time-series Gr-band photometry with a 2-harmonic sinusoid and nightly offsets using weighted least squares to determine a separate photometric period.
• Use an independent 30-minute binned photometric analysis from a ground-based telescope to provide an additional period constraint.
• Adopt conservative uncertainties for jet PA due to sparse sampling and systematics.
• Phase-fold and compare the jet-based period with the photometric period to assess consistency.

Experimental results

Research questions

• RQ1 What is the post-perihelion rotation period of 3I/ATLAS as inferred from jet morphology and photometric variability?
• RQ2 How consistent are the jet-based and photometric period estimates, and what are the likely sources of discrepancy?
• RQ3 What does the comparison imply about the rotation state and jet precession of 3I/ATLAS?

Key findings

• Jet-PA modulation yields P_jet = 7.20 ± 0.05 hours.
• Photometry yields P_phot = 7.136 ± 0.001 hours.
• The two periods agree at the few-percent level, suggesting a nucleus rotation timescale of ~7.1 hours.
• Rotation axis is aligned with the sunward direction to within ~20 degrees.
• The data imply precession of the jet around the rotation axis by about 20°, influencing observed jet morphology and coma brightness modulation.