Published materials (books, magazine and journal articles, newspaper articles) written at the time about a particular event.  While these are sometimes accounts by participants, in most cases they are written by journalists or other observers.  The important thing is to distinguish between material written at the time of an event as a kind of report, and material written much later, as historical analysis.

These works are generally written by journalists, theorists, academics, politicians, and historians.  This is where a New York Times article can be used as a primary source.  If you are writing a paper on the bombing of Cambodia and Laos in the Vietnam War, it is acceptable to use newspaper articles of the time alleging these facts.  The Chomsky book is a book written as an assessment of a current event -- at the time of an event -- and can be used.  Again, with these materials, please consult your professor, as they may have a different opinion as it pertains to your paper topic.