“When someone sins in any of these ways:

If he has seen, heard, or known about something he has witnessed, and did not respond to a public call to testify, he is responsible for his sin.

And tell the Israelites: Take a male goat for a sin offering; a calf and a lamb, male yearlings without blemish, for a burnt offering;

So Aaron approached the altar and slaughtered the calf as a sin offering for himself.