condensed matter - What has been observed as the "Hawking radiation" emitted by a black hole analog?

I've noticed this paper which explains that they have observed the "Hawking radiation" emitted by a black hole analog. In which sense the Bose-Einstein condensate described by the paper can be considered a black hole analog? It is essentialy that the "analog of an event horizon" for sound waves is created or are there other relevant aspects? And what has been observed which mimics Hawking radiation? Is there something in this experimental observation that can only be explained by quantum effects, such as phonons?