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

TL;DR

This study searches for pre-discovery images of interstellar comet 2I/Borisov, finds it as early as December 2018, and analyzes its activity onset, nucleus size, and behavior, revealing similarities to new solar system comets.

Contribution

First detection of interstellar comet 2I/Borisov in pre-discovery images, providing insights into its activity onset and physical properties.

Findings

Detected comet Borisov in ZTF images from May 2019

Identified the comet in images from December 2018 at 7.8 au

Suggested activity onset around 8.6 au, possibly driven by volatiles other than water

Abstract

Comet 2I/Borisov, the first unambiguous interstellar comet ever found, was discovered in August 2019 at $\sim3$ 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 $\sim0.5$---$10 \mathrm{km^2}$ 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.