Upper Limits on O VI Emission from Voyager Observations

Abstract

We have examined 426 Voyager fields distributed across the sky for O VI (1032/1038 Å) emission from the Galactic diffuse interstellar medium. No such emission was detected in any of our observed fields. Our most constraining limit was a 90% confidence upper limit of 2600 photons cm-2 sr-1 s-1 on the doublet emission in the direction (l, b) = (117.3, 50.6). Combining this with an absorption line measurement in nearly the same direction allows us to place an upper limit of 0.01 cm-3 on the electron density of the hot gas in this direction. We have placed 90% confidence upper limits of less than or equal to 10,000 photons cm-2 sr-1 s-1 on the O VI emission in 16 of our 426 observations.