A Straight and Narrow Ionized Filament

We report the discovery of an extremely narrow, extremely linear, ionized filament. The filament is ∼2.°5 long and has an Hα surface brightness of ∼0.5 R. It is at high galactic latitude, stretching from (l, b) = (140.°8°, 39.°0) to (138.°0, 37.°7). The filament is approximately Y-shaped. The vertical segment of the Y is ∼1.°2 long and ∼20″ wide, and the widest separation of the two diagonal segments is ∼5′. We discuss four possible origins for this feature: (1) an extremely low density, nearby jet; (2) an unusually linear filament associated with some large-scale nearby nebula, perhaps even the Local Bubble; (3) an ionized trail left by mechanical input from a star or compact object moving through the interstellar medium (ISM); or (4) an ionized trail left by photoionization ("Fossil Strömgren Trail") from a star or compact object. We favor this last hypothesis, and derive some of the basic properties for an ionized trail. Regardless of whether this latter hypothesis applies to this specific filament, the basic properties of such a trail, its length, width, and brightness, are interesting, predictable, and should be observable behind some white dwarfs. If the filament is a photoionized trail, then the source should be closer than a few hundred parsecs, with a measurable proper motion and a luminosity of hydrogen ionizing photons of ≲1044 ergs s-1. We have searched for such sources in line with the filament and find one candidate, the X-ray source RX J094247.2+700238. If the M dwarf binary star Gl 360 is the optical counterpart of the X-ray source, as has been thought, then its proper motion eliminates it as a candidate, and we have no other potential candidate to leave a trail. We note the similarity of this structure to long, narrow features ("canals") observed as depolarizing regions against the Galactic synchrotron background and find that this emission filament may also be detectable as a region of Faraday depolarization. We suggest future tests for ascertaining the origin of this filament and discuss how this structure might be useful to constrain the thermal and velocity structure of the nearby ISM.