From my understanding, that's just the result of over processing the picture. It was probably a single pixel that was black, and when they "smoothed" out the picture, it probably made that shape. My guess would be that there isn't an actual notch there, but a darker region than what surrounds it. But we probably won't know for sure until we get a Pluto orbiter than can get close to these small satellites (so not for many hundreds of years).
Do you think hydra actually has a notch in the bottom or is that just a shadow from something?
From my understanding, that's just the result of over processing the picture. It was probably a single pixel that was black, and when they "smoothed" out the picture, it probably made that shape. My guess would be that there isn't an actual notch there, but a darker region than what surrounds it. But we probably won't know for sure until we get a Pluto orbiter than can get close to these small satellites (so not for many hundreds of years).