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Periodic Wobble of the Post-Perihelion Jet Structure Around 3I/ATLAS

Toni Scarmato, Abraham Loeb

Abstract

We analyse data on the post-perihelion morphology, including jet position angles (PAs) and coma dominated photometry of the interstellar object 3I/ATLAS. From Hubble Space Telescope (HST) images processed with a Larson Sekanina rotational gradient filter, we measure the PAs of three main persistent jet like features between November 30 and December 27, 2025 and fit a weighted Fourier model in a period scan. The dominant jet PA wobble yields Pjet = 7.20 +/- 0.05 h. An independent Gr (R band) time-series photometry data set, using two different apertures from MPC station L92, analyzed with nightly offsets and 30 minute binning, gives Pphot = 7.136 +/- 0.001 h (formal 1sigma), with a semiamplitude A about 0.311 mag and scatter sigmajit about 0.089 mag. The close agreement beetwen the periods supports a characteristic post-perihelion period of about 7.1 h. We interpret this period as an attitude precession/nutation (non principal axis rotation) traced by jet orientation and coma flux redistribution. The jet structure precesses about the rotation axis with a characteristic angular excursion of order about 20 degs, and the rotation axis is aligned with the sunward direction to within about 20 degs.

Periodic Wobble of the Post-Perihelion Jet Structure Around 3I/ATLAS

Abstract

We analyse data on the post-perihelion morphology, including jet position angles (PAs) and coma dominated photometry of the interstellar object 3I/ATLAS. From Hubble Space Telescope (HST) images processed with a Larson Sekanina rotational gradient filter, we measure the PAs of three main persistent jet like features between November 30 and December 27, 2025 and fit a weighted Fourier model in a period scan. The dominant jet PA wobble yields Pjet = 7.20 +/- 0.05 h. An independent Gr (R band) time-series photometry data set, using two different apertures from MPC station L92, analyzed with nightly offsets and 30 minute binning, gives Pphot = 7.136 +/- 0.001 h (formal 1sigma), with a semiamplitude A about 0.311 mag and scatter sigmajit about 0.089 mag. The close agreement beetwen the periods supports a characteristic post-perihelion period of about 7.1 h. We interpret this period as an attitude precession/nutation (non principal axis rotation) traced by jet orientation and coma flux redistribution. The jet structure precesses about the rotation axis with a characteristic angular excursion of order about 20 degs, and the rotation axis is aligned with the sunward direction to within about 20 degs.
Paper Structure (21 sections, 3 equations, 6 figures)

This paper contains 21 sections, 3 equations, 6 figures.

Table of Contents

  1. Introduction
  2. Observations and Data
  3. Morphology and jet PA measurements
  4. Photometry and Astrometrica apertures
  5. Methods
  6. Harmonic representation of PA oscillations
  7. Photometric folding and stationary tests
  8. Results
  9. Dominant jet structure: PA wobble period from HST morphology
  10. Coma photometry: small-aperture solution ($r_{\rm eff}\approx6.2"$)
  11. Coma photometry: large-aperture solution ($r_{\rm eff}\approx42.2"$)
  12. Minor jets: additional PA periodicity from Jet 2 and Jet 3
  13. Jet 2.
  14. Jet 3.
  15. Summary of periods
  16. ...and 6 more sections

Figures (6)

  • Figure 1: Schematic geometry (not to scale), with North up and East to the left. The projected spin axis is indicated in the sunward direction (PA$=110^\circ$) and anti-sunward direction (PA$=290^\circ$). Mean jet PAs and measured PA oscillation half-amplitudes are: Jet 1, $55^\circ\pm12.8^\circ$; Jet 2 (anti-sunward), $290^\circ\pm20^\circ$; Jet 3, $170^\circ\pm12.6^\circ$. The scheme show the anti-sunward jet in PA=290° wobbling with period of 7.2 h, that is dominant in the photometric variation also.
  • Figure 2: Large-aperture ($N=30$, $r_{\rm eff}\approx42.2"$) photometry: four-night subset. Top: $\Delta\chi^2$ period scan over 6.8--7.4 h for $k=1$ and $k=2$ Fourier models (per-night offsets allowed). Bottom: phase-folded light curve at $P=7.142$ h after removing per-night offsets; the $k=2$ model refines the waveform shape while preserving inter-night phase coherence.
  • Figure 3: Same as Figure \ref{['fig:phot4n']}, but for the full five-night data set including December 27, 2025. Top: period scan within 6.8--7.4 h. Middle: fold at the four-night reference period $P=7.142$ h highlighting degraded stationarity introduced by the fifth night. Bottom: fold at the best $k=1$ period from the five-night scan, $P=7.158$ h.
  • Figure 4: Inner-coma jets of 3I/ATLAS on 2025-12-27 UT. Two principal jet structures remain separated by $\sim120^\circ$, but the emission broadens into a fan. Such morphological changes modulate the coma brightness within a fixed aperture and can alter the amplitude/shape (and sometimes the apparent phase) of the photometric signal.
  • Figure 5: Jet 2 diagnostic plots from the PA time series analysis: (top) PA versus time by observing segment with the best-fit model drawn within each night; (middle) period scan (2--6 h zoom); (bottom) detrended phase fold at $P_2=2.9$ h. Error bars correspond to the reported $\pm3^\circ$ values.
  • ...and 1 more figures