Post-perihelion Photometric and Spectroscopic Monitoring of the Interstellar Comet 3I/ATLAS
Abstract
The third interstellar object, comet 3I/ATLAS, was observed using various Indian telescopes during its post-perihelion evolution. We carried out imaging and spectroscopic observations of the comet at multiple epochs using the 0.5-m, 1.2-m, and 2.5-m telescopes of the Physical Research Laboratory and the 2-m Himalayan Chandra Telescope (HCT) operated by the Indian Institute of Astrophysics. Additionally, time-series photometry was obtained on 7 epochs using the 0.132-m refractor of Cepheid Observatory. The spectroscopic observations of 3I/ATLAS reveal signatures of molecular emission from CN, C2, and C3, as commonly observed in the emission spectra of solar system comets. The production rates and rate ratios indicate that the composition of the 3I/ATLAS transitioned from carbon-depleted (pre-perihelion) to carbon-typical (post-perihelion). The change in carbon composition could be due to exposure of fresh material from subsurface layers near perihelion. We also performed photometric observations to determine broadband colours and the spin period. The measured colours are B−V= 1.03 ± 0.13 mag, V−R= 0.59 ± 0.11 mag, R−I= 0.08 ± 0.04 mag, g−r= 0.52 ± 0.09 mag, and r−i= 0.13 ± 0.08 mag. The colours are redder than the Long-Period Comets of the solar system and the Sun in B−V and V−R, but near solar in R−I. The measured colours remain nearly constant throughout the observing period and are consistent with the reported pre-perihelion values. The colours and the reflectivity gradient match the photometric colours of the `ultrared-material rich' objects in the solar system. The nuclear radius derived from non-gravitational force lies between 0.45 and 0.65 km.
Versions
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BibTeX
@misc{ahuja2026postperihelion,
title = {{Post-perihelion Photometric and Spectroscopic Monitoring of the Interstellar Comet 3I/ATLAS}},
author = {Goldy Ahuja and Shashikiran Ganesh and K. Aravind and Emmanuel Jehin and Bhavya Ailawadhi and Vikrant K. Agnihotri and B. Arvind and Pallavi Saraf and Alka Singh and Deekshya R. Sarkar and Sunil Chandra and Prithish Halder and Devendra Sahu and T. Sivarani and Anil Bhardwaj},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.22596},
eprint = {2609.22596},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.22596},
}
RIS
TY - UNPB
TI - Post-perihelion Photometric and Spectroscopic Monitoring of the Interstellar Comet 3I/ATLAS
AU - Goldy Ahuja
AU - Shashikiran Ganesh
AU - K. Aravind
AU - Emmanuel Jehin
AU - Bhavya Ailawadhi
AU - Vikrant K. Agnihotri
AU - B. Arvind
AU - Pallavi Saraf
AU - Alka Singh
AU - Deekshya R. Sarkar
AU - Sunil Chandra
AU - Prithish Halder
AU - Devendra Sahu
AU - T. Sivarani
AU - Anil Bhardwaj
PY - 2026
DA - 2026/09/18
PB - arXiv
DO - 10.48550/arxiv.2609.22596
UR - https://arxiv.org/abs/2609.22596
AB - The third interstellar object, comet 3I/ATLAS, was observed using various Indian telescopes during its post-perihelion evolution. We carried out imaging and spectroscopic observations of the comet at multiple epochs using the 0.5-m, 1.2-m, and 2.5-m telescopes of the Physical Research Laboratory and the 2-m Himalayan Chandra Telescope (HCT) operated by the Indian Institute of Astrophysics. Additionally, time-series photometry was obtained on 7 epochs using the 0.132-m refractor of Cepheid Observatory. The spectroscopic observations of 3I/ATLAS reveal signatures of molecular emission from CN, C_2, and C_3, as commonly observed in the emission spectra of solar system comets. The production rates and rate ratios indicate that the composition of the 3I/ATLAS transitioned from carbon-depleted (pre-perihelion) to carbon-typical (post-perihelion). The change in carbon composition could be due to exposure of fresh material from subsurface layers near perihelion. We also performed photometric observations to determine broadband colours and the spin period. The measured colours are B−V= 1.03 ± 0.13 mag, V−R= 0.59 ± 0.11 mag, R−I= 0.08 ± 0.04 mag, g−r= 0.52 ± 0.09 mag, and r−i= 0.13 ± 0.08 mag. The colours are redder than the Long-Period Comets of the solar system and the Sun in B−V and V−R, but near solar in R−I. The measured colours remain nearly constant throughout the observing period and are consistent with the reported pre-perihelion values. The colours and the reflectivity gradient match the photometric colours of the `ultrared-material rich' objects in the solar system. The nuclear radius derived from non-gravitational force lies between 0.45 and 0.65 km.
ER -