X-rays Trace the Volatile Content of Interstellar Objects

TL;DR

This paper proposes using X-ray spectroscopy to analyze the volatile composition of interstellar objects, offering a new method to characterize them and gain insights into planet formation beyond our Solar System.

Contribution

It develops a model predicting X-ray flux from interstellar objects based on their chemical makeup, highlighting the potential of X-ray observations for compositional analysis.

Findings

X-ray emission from H2 and N2 can be detected via charge exchange.

XMM-Newton could have observed 1I/'Oumuamua's X-ray emission.

Follow-up X-ray observations are recommended for newly discovered ISOs.

Abstract

The non-detection of a coma surrounding 1I/`Oumuamua, the first discovered interstellar object (ISO), has prompted a variety of hypotheses to explain its nongravitational acceleration. Given that forthcoming surveys are poised to identify analogues of this enigmatic object, it is prudent to devise alternative approaches to characterization. In this study, we posit X-ray spectroscopy as a surprisingly effective probe of volatile ISO compositions. Heavily ionized metals in the solar wind interact with outgassed neutrals and emit high-energy photons in a process known as charge exchange, and charge exchange induced X-rays from comets and planetary bodies have been observed extensively in our Solar System. We develop a model to predict the X-ray flux of an ISO based on its chemical inventory and ephemeris. We find that while standard cometary constituents, such as H$_2$O, CO$_2$, CO, and dust are best probed via optical or infrared observations, we predict strong X-ray emission generated by charge exchange with extended comae of H$_2$ and N$_2$ -- species which lack strong infrared fluorescence transitions. We find that XMM-Newton would have been sensitive to charge exchange emission from 1I/`Oumuamua during the object's close approach to Earth, and that constraints on composition may have been feasible. We argue for follow-up X-ray observations of newly discovered ISOs with close-in perihelia. Compositional constraints on the general ISO population could reconcile the apparently self-conflicting nature of 1I/`Oumuamua, and provide insight into the earliest stages of planet formation in extrasolar systems.