slack off vs die away

slack off and die away both are verbs.

Word	Noun	Adjective	Verb	Adverb
slack off	No	No	Yes	No
die away	No	No	Yes	No

As verbs, slack off and die away are synonyms defined as:

Other synonyms of slack off include abate, let up, slack.

slack off (verb)	die away (verb)
become less in amount or intensity	become less in amount or intensity