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What is the fire arc of a Monolith's particle whip? Can it fire in a 45 degree arc on its front facing, or 360 degrees all the way around?

It is not listed as being hull-mounted, and there is no defining barrel or turret. If the crystal is the emitter, than anything the crystal can see. If the spark plug over the crystal is the emitter, than hull-mounted based on that.

From the previous edition of Codex Necrons:

Most dramatic of its armaments is the particle whip, channeled through a glowing focus crystal atop the vehicle.

So at least fluffwise, it probably safest to draw line of sight from the crystal (which is severly block by the Monolith itself on three of its four sides).

I've always played it as a turret (360 degree arc), but this is definitely not expressly stated. I assumed that the actually beam was coming from the area between the crystal and the arc above it, so if an opponent insisted that the Monolith was blocking LoS for its own weapon, it wouldn't be too much trouble to pivot the model around.

As a non-Necron player, I've always played it as such (the few time my opponent was able to fire it before a lance shot blew up the monolith):

Range is measured from the crystal on top. LOS is drawn from the hull of the monolith.

Obviously not RAW, but RAW don't function in this case.