Context: Detecting moons around exoplanets is a major goal of current and
future observatories. Moons are suspected to influence rocky exoplanet
habitability, and gaseous exoplanets in stellar habitable zones could harbour
abundant and diverse moons to target in the search for extraterrestrial
habitats. Exomoons will contribute to exoplanetary signals but are virtually
undetectable with current methods. Aims: We identify and analyse traces of
exomoons in the temporal variation of total and polarised fluxes of starlight
reflected by an Earth-like exoplanet and its spatially unresolved moon across
all phase angles, with both orbits viewed in an edge-on geometry. Methods: We
compute the total and linearly polarised fluxes, and the degree of linear
polarization P of starlight that is reflected by the exoplanet with its moon
along their orbits, accounting for the temporal variation of the visibility of
the planetary and lunar disks, and including effects of mutual transits and
mutual eclipses. Our computations pertain to a wavelength of 450 nm. Results:
Total flux F shows regular dips due to planetary and lunar transits and
eclipses. Polarization P shows regular peaks due to planetary transits and
lunar eclipses, and P can increase and/or slightly decrease during lunar
transits and planetary eclipses. Changes in F and P will depend on the radii of
the planet and moon, on their reflective properties, and their orbits, and are
about one magnitude smaller than the smooth background signals. The typical
duration of a transit or an eclipse is a few hours. Conclusions: Traces of an
exomoon due to planetary and lunar transits and eclipses show up in F and P of
sunlight reflected by planet-moon systems and could be searched for in
exoplanet flux and/or polarisation phase functions.Comment: Accepted for publication in Astronomy & Astrophysic
