pre-noun adjectival
the ... in question; the very ...; the ... itself
Pre-noun adjectival (rentaishi) that specifies the exact person or thing being discussed, often with a nuance of 'the very one' or 'the one at issue'. Common in news and formal contexts.
当の本人は何も知らなかった。
The person in question knew nothing about it.
当の事件について詳しく説明してください。
Please explain the incident in question in detail.