How come a "failed supernova" is, well, "failed"?

Question

I heard about this a while ago but recently came back to it:

https://en.wikipedia.org/wiki/Failed_supernova

which suggests that basically what this is is something where a star is so big and heavy that when it dies, it is "too big to produce a supernova" and the whole thing just collapses straight to a black hole.

However, it also mentions that this type of event does not produce little if any energy burst of its own. But why? Wouldn't the stellar material in the split few seconds from when it starts to implode to it crossing the event horizon of the nascent black hole, heat up tremendously and thus produce a massive radiation flood for a brief moment, resulting in an effect similar to a short gamma burster?

Answer 1

The "free-fall time" in a core-collapse supernova is considerably less than a second. This is the timescale on which the core collapse from something the size of the Earth to something the size of a neutron star/black hole event horizon.

The neutrino emission from a supernova is where about 99% of the collapse energy ends up. Most of the neutrinos are emitted in a burst over the course of about 10 seconds after the collapse is halted.

Gamma rays come about much later, because the envelope of the star is opaque to electromagnetic radiation. Short gamma ray bursts are actually thought to arise in merging neutron stars where there is no overlying envelope.

The bottom line is, if the collapse isn't halted and the core collapses into a black hole, then there is no time or opportunity for radiation and neutrinos to be produced and emerge before the core disappears into the event horizon (or from the point of view of a distant observer, everything gets redshifted towards infinity on a timescale much shorter than any period when neutrinos and radiation can be produced in significant quantities).