17 U.S.C. § 204: Execution of transfers of copyright ownership

(a) A transfer of copyright ownership, other than by operation of law, is not valid unless an instrument of conveyance, or a note or memorandum of the transfer, is in writing and signed by the owner of the rights conveyed or such owner’s duly authorized agent. (b) A certificate of acknowledgement is not required for the validity of a transfer, but is prima facie evidence of the execution of the transfer if—(1) in the case of a transfer executed in the United States, the certificate is issued by a person authorized to administer oaths within the United States; or (2) in the case of a transfer executed in a foreign country, the certificate is issued by a diplomatic or consular officer of the United States, or by a person authorized to administer oaths whose authority is proved by a certificate of such an officer.