We report the discovery of a extremely narrow , extremely linear , ionized filament .
The filament is \sim 2.5 ^ { \circ } long and has an H \alpha surface brightness of \sim 0.5 rayleighs .
It is at high galactic latitude , stretching from ( l,b ) = ( 140.8 ^ { \circ } , 39.0 ^ { \circ } ) to ( 138.0 ^ { \circ } , 37.7 ^ { \circ } ) .
The filament is approximately “ Y ” shaped .
The vertical segment of the “ Y ” is \sim 1.2 ^ { \circ } long and \sim 20 ^ { \prime \prime } wide , and the widest separation of the two diagonal segments is \sim 5 ^ { \prime } .
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 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 measureable proper motion and a luminosity of hydrogen ionizing photons of ^ { < } _ { \sim } 10 ^ { 44 } { ergs~ { } s ^ { -1 } } .
We have searched for such sources in line with the filament and find one candidate , the X-ray source RXSJ094247.2+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 interstellar medium .