Emphatic stress is the placement of greater stress on a particular word or syllable in an utterance. This is done for emphasis, clarification or correction. In this case, the response is a correction of a misapprehension made by the person who asked the question. The emphasised word is the word that is corrected in the question. The questioner has made an error in assuming that the man has read the newspaper and the responder is trying to clarify that the man has not read, but bought the newspaper. This is why the word "bought" is emphasised. 

Did the man read the newspaper?

No. The man BOUGHT the newspaper.