Multi-Wavelength Post-Perihelion Polarimetry of Interstellar Comet 3I/ATLAS
Abstract
We present new post-perihelion polarimetric observations of the third discovered interstellar object, 3I/ATLAS (C/2025 N1; hereafter 3I), obtained using FORS2 at the Very Large Telescope, ALFOSC at the Nordic Optical Telescope, and FoReRo2 at the 2 m Ritchey-Chrétien-Coudé telescope. The observations span phase angles 1-30∘, providing the most extensive polarimetric phase angle coverage obtained for an interstellar object to date. The post-perihelion measurements reveal that the unusual polarimetric properties of 3I persist across perihelion and match recent independent post-perihelion observations, indicating no significant evolution in the polarimetric properties. The increased phase angle coverage allowed us to further constrain the minimum polarisation to −2.9% at phase angle 5.5∘. Multi-band BVRI observations reveal a wavelength dependence of polarisation, with the negative polarisation branch becoming deeper and shifted toward smaller phase angles at longer wavelengths. The polarimetric colour is predominantly red and increases with phase angle, consistent with behaviour observed in other comets. Imaging and polarimetric maps show a substantially more extended coma after perihelion, while the polarisation distribution itself remains smooth and spatially homogeneous. Numerical modelling suggests that the unusually deep polarisation phase curve originates from moderately porous dust aggregates consisting of weakly absorbing sub-micron monomers, resulting in much brighter dust than that in typical solar system comets.
Related
- Post-perihelion Photometric and Spectroscopic Monitoring of the Interstellar Comet 3I/ATLAS
- Very Large Telescope observations of interstellar comet 3I/ATLAS III: High-resolution monitoring of CN and forbidden oxygen emission across the perihelion passage with ESPRESSO
- Post-perihelion dust activity and a candidate icy-grain halo in long-period comet C/2023 A3 (Tsuchinshan--ATLAS)
- Characterization of the anisotropic dust activity of interstellar comet 3I/ATLAS
- High-resolution optical spectroscopy reveals the distinctive volatile composition of the interstellar comet 3I/ATLAS
BibTeX
@misc{gray2026multiwavelength,
title = {{Multi-Wavelength Post-Perihelion Polarimetry of Interstellar Comet 3I/ATLAS}},
author = {Zuri Gray and Galin Borisov and Ludmilla Kolokolova and Johannes Markkanen and Yuna G. Kwon and Stefano Bagnulo and Alberto Cellino and Rosemary C. Dorsey and Grigori Fedorets and Mikael Granvik and Eric MacLennan and Antti Penttilä and Karri Muinonen and Olga Muñoz and Philippe Bendjoya and Maxime Devogèle and Simone Ieva and Francisco J. García-Izquierdo},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.04932},
eprint = {2609.04932},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.04932},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Multi-Wavelength Post-Perihelion Polarimetry of Interstellar Comet 3I/ATLAS
AU - Zuri Gray
AU - Galin Borisov
AU - Ludmilla Kolokolova
AU - Johannes Markkanen
AU - Yuna G. Kwon
AU - Stefano Bagnulo
AU - Alberto Cellino
AU - Rosemary C. Dorsey
AU - Grigori Fedorets
AU - Mikael Granvik
AU - Eric MacLennan
AU - Antti Penttilä
AU - Karri Muinonen
AU - Olga Muñoz
AU - Philippe Bendjoya
AU - Maxime Devogèle
AU - Simone Ieva
AU - Francisco J. García-Izquierdo
PY - 2026
DA - 2026/09/04
PB - arXiv
DO - 10.48550/arxiv.2609.04932
UR - https://arxiv.org/abs/2609.04932
AB - We present new post-perihelion polarimetric observations of the third discovered interstellar object, 3I/ATLAS (C/2025 N1; hereafter 3I), obtained using FORS2 at the Very Large Telescope, ALFOSC at the Nordic Optical Telescope, and FoReRo2 at the 2 m Ritchey-Chrétien-Coudé telescope. The observations span phase angles 1-30^∘, providing the most extensive polarimetric phase angle coverage obtained for an interstellar object to date. The post-perihelion measurements reveal that the unusual polarimetric properties of 3I persist across perihelion and match recent independent post-perihelion observations, indicating no significant evolution in the polarimetric properties. The increased phase angle coverage allowed us to further constrain the minimum polarisation to −2.9% at phase angle 5.5^∘. Multi-band BVRI observations reveal a wavelength dependence of polarisation, with the negative polarisation branch becoming deeper and shifted toward smaller phase angles at longer wavelengths. The polarimetric colour is predominantly red and increases with phase angle, consistent with behaviour observed in other comets. Imaging and polarimetric maps show a substantially more extended coma after perihelion, while the polarisation distribution itself remains smooth and spatially homogeneous. Numerical modelling suggests that the unusually deep polarisation phase curve originates from moderately porous dust aggregates consisting of weakly absorbing sub-micron monomers, resulting in much brighter dust than that in typical solar system comets.
ER -