No idea, it's not spec'd.
It may just be the raw photodiode current, since the photosensitive area is, I forget what it is -- is it a thick collector region beneath a thin emitter and base, so the photodiode is effectively the C-B junction, which in turn causes B-E current and amplification? Or is it the other way around, thick emitter or base near the top?
In any case, you'll have to characterize it yourself.
Tim