From the source: [the President] shall have power to grant reprieves and pardons for offenses against the United States, except in cases of impeachment.
http://www.law.cornell.edu/constitution/articleii#section2
It's fair to argue that there is no precedent, but that just means it hasn't happened before, not that it cannot happen.
There are substantial differences between legislative immunity and a pardon; the latter carries an imputation of guilt and acceptance of a confession of it [...].
The facts, which involve the effect of a pardon of the President of the United States tendered to one who has not been convicted of a crime nor admitted the commission thereof, and also the necessity of acceptance of a pardon in order to make it effective are stated in the opinion."
- Burduck v. United States, 236 US 79 (1915), Supreme Court of the United States, internal citations and quotes omitted.
https://supreme.justia.com/cases/federal/us/236/79/case.html
This is not without precedent.
There is no precedent for pardoning a dead person who was not convicted of a crime because it is a legal impossibility as described above.
Aaron died before his case was finally adjudicated (i.e, until all appeals were exhausted), so like the Enron guy, in the eyes of the law, he is not and never can be guilty of the crimes he was charged with.
Again, for emphasis: you cannot pardon someone without a conviction.