Upper Limits on O VI Emission From Voyager Observations

Abstract

We have examined 426 Voyager fields distributed across the sky for O VI λλ1032, 1038 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)=(117.3d,50.6d). 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.