I don't know what it is, but it's actually visible at everything below 100GHz, for whatever that is worth.
I may be wrong here, but they could be the observations blocked by the Sun, with that arc being the Sun's track on a sky.
I'll add up to my comment: the other wavelengths have the arc shifted, which would mean that the observation is performed from a different location or (more likely) at a different time of the year.