Interstellar communication. VI. Searching X-ray spectra for narrowband communication

TL;DR

This study searches archival X-ray spectra for narrowband signals indicative of interstellar communication, finding mostly natural sources and setting a limit on artificial signals at less than 0.01%.

Contribution

It provides the first empirical limit on artificial narrowband X-ray signals in archival data, advancing the search for extraterrestrial communication.

Findings

19 sources with monochromatic signals identified

Most signals are likely natural in origin

Artificial signals constitute less than 0.01% of sources

Abstract

We have previously argued that targeted interstellar communication has a physical optimum at narrowband X-ray wavelengths $\lambda\approx1\,$nm, limited by the surface roughness of focusing devices at the atomic level (arXiv:1711.05761). We search 24,247 archival X-ray spectra (of 6,454 unique objects) for such features and present 19 sources with monochromatic signals. Close examination reveals that these are most likely of natural origin. The ratio of artificial to natural sources must be <0.01%. This first limit can be improved in future X-ray surveys.