When using the phrase "Author Unknown," there are a number of synonyms which can be equally effective. Some of these include "anonymous author," "unattributed source," "unknown writer," "mystery author," and "unnamed source." Each of these phrases convey the idea that the creator of a particular work is not known, and they can be used in a variety of contexts, such as in academic papers, newspaper articles, or in reference to works of art or literature. Ultimately, the use of these synonyms can provide a more elegant and nuanced way of acknowledging the source of a piece of work, while also respecting the anonymity of its creator.