Title: ROME IV. The Arecibo Search for Substellar Magnetospheric Radio Emissions in Purported Exoplanet-Hosting Systems at 5 GHz

Abstract: Plasma flow-obstacle interactions, such as those between an exoplanet's magnetosphere and the host star's stellar wind, may lead to detectable radio emissions. Despite many attempts to detect magnetospheric (auroral) radio emissions from exoplanets, a reproducible, unambiguous detection remains elusive. This fourth paper of the ROME (Radio Observations of Magnetized Exoplanets) series presents the results of a targeted radio survey of nine nearby systems that host exoplanet, brown dwarf, or low-mass-stellar companions conducted with the Arecibo radio telescope at $\sim$5 GHz. This search for magnetospheric radio emissions has the greatest sensitivity ($\sim$1 mJy during $<$1 s integration times) and collected full Stokes parameters over the largest simultaneous bandpass of any survey to date. It is also the first survey to search for radio emission from brown dwarfs of spectral class Y, which may illuminate open questions regarding their magnetism, interior and atmospheric structure, and formation histories. No magnetospheric radio emissions from substellar companions were detected. These results are examined within the context of recent theoretical work on plasma flow-obstacle interactions, and radio emissions observed from the solar system planets and ultracool dwarfs.