Pre-discovery Activity of New Interstellar Comet 2I/Borisov Beyond 5 AU

Abstract

Comet 2I/Borisov, the first unambiguous interstellar comet ever found, was discovered in August 2019 at $\sim 3$ au from the Sun on its inbound leg. No pre-discovery detection beyond 3 au has yet been reported, mostly due to the comet's proximity to the Sun as seen from the Earth. Here we present a search for pre-discovery detections of comet Borisov using images taken by the Catalina Sky Survey (CSS), Pan-STARRS and Zwicky Transient Facility (ZTF), with a further comprehensive follow-up campaign being presented in \citet{Bolin2019}. We identified comet Borisov in ZTF images taken in May 2019 and use these data to update its orbit. This allowed us to identify the comet in images acquired as far back as December 2018, when it was 7.8 au from the Sun. The comet was not detected in November 2018 when it was 8.6 au from the Sun, possibly implying an onset of activity around this time. This suggests that the activity of the comet is either driven by a more volatile species other than H${}_2$O, such as CO or CO${}_2$, or by exothermic crystallization of amorphous ice. We derive the radius of the nucleus to be $<7$ km using the non-detection in November 2018, and estimate an area of $\sim 0.5$---$10\mathrm{k}{\mathrm{m}}^2$ has been active between December 2018 and September 2019, though this number is model-dependent and is highly uncertain. The behavior of comet Borisov during its inbound leg is observationally consistent with dynamically new comets observed in our solar system, suggesting some similarities between the two.

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Cited by in corpus (18)

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• Outburst and Splitting of Interstellar Comet 2I/Borisov
• Coma Anisotropy and the Rotation Pole of Interstellar Comet 2I/Borisov
• Constraints on the Occurrence of 'Oumuamua-Like Objects
• Visible and near-infrared observations of interstellar comet 2I/Borisov with the 10.4-m GTC and the 3.6-m TNG telescopes
• Compact pebbles and the evolution of volatiles in the interstellar comet 2I/Borisov
• A ZTF Look at Optical Variability of Young Stellar Objects in the North America and Pelican Nebulae Complex
• Interstellar Comet 2I/Borisov exhibits a structure similar to native Solar System comets
• Interstellar comet 2I/Borisov as seen by MUSE: C${}_2$, NH${}_2$ and red CN detections
• Oort Cloud Comets Discovered Far from the Sun
• Pre-perihelion Monitoring of Interstellar Comet 2I/Borisov
• Near-Discovery SOAR Photometry of the Third Interstellar Object: 3I/ATLAS
• Chasing Nomadic Worlds: A New Class of Deep Space Missions
• Constraining the orientation of the spin axes of extrasolar minor bodies 1I/2017 U1 (`Oumuamua) and 2I/Borisov
• Is Interstellar Object 2I/Borisov a Stardust Comet? Predictions for the Post Perihelion Period
• Sublimation of Water Ice from a Population of Large, Long-Lasting Grains Near the Nucleus of 2I/Borisov?