Thanks Joel,
I don't think that is gradient, because there shouldn't really be any. It was imaged from a dark site with an 88mm image circle. I used flats, so it would be surprising to have any gradients left. And I hadn't noticed any gradients in the original subs. The left side of the image is clear where it is in the shadow of a dust lane of the Eagle nebula, which leads me to believe it is either ionized O2 or perhaps reflected dust? And it also seems to be the same colour as the dust? O2? at the bottom edge of the Omega nebula, leading me to believe that it is of the same stuff. You can also see traces of it in this image, around the Lagoon nebula.
http://www.astrobin.com/full/253968/B/
Dean