Upper limits to the number of Oort Cloud Objects based on serendipitous occultation events search in X-rays

TL;DR

This study analyzed 1.6 million seconds of X-ray data from RXTE to search for occultation events caused by Oort Cloud Objects, setting upper limits on their population without relying on specific comet injection models.

Contribution

First observational upper limits on Oort Cloud Object populations derived from serendipitous occultation searches in X-ray data, independent of comet injection models.

Findings

No occultation events detected in the data.

Upper limits are about 1000 times higher than theoretical estimates for the inner Oort Cloud.

Upper limits are about one million times higher for the outer Oort Cloud.

Abstract

Using all the RXTE archival data of Sco X-1 and GX 5-1, which amount to about 1.6 mega seconds in total, we searched for possible occultation events caused by Oort Cloud Objects. The detection efficiency of our searching approach was studied with simulation. Our search is sensitive to object size of about 300 m in the inner Oort Cloud, taking 4000 AU as a representative distance, and of 900 m in the outer Oort Cloud, taking 36000 AU as the representative distance. No occultation events were found in the 1.6 Ms data. We derived upper limits to the number of Oort Cloud Objects, which are about three orders of magnitude higher than the highest theoretical estimates in the literature for the inner Oort Cloud, and about six orders higher for the outer Oort Cloud. Although these upper limits are not constraining enough, they are the first obtained observationally, without making any model assumptions about comet injection. They also provide guidance to such serendipitous occultation event search in the future.