Absentee ballots for the state's primary election are available starting Monday for registered voters who won't be able to get to the polls on Sept. 6. State Representative David Linsky, a Natick Democrat, is running unopposed on the Democratic ballot, while Republican William Callahan is running as a write-in candidate in the Republican primary, and needs at least 150 votes to face Linsky in the November final election for the Fifth Middlesex District seat, which also includes Sherborn and Millis. Callahan, who retired in June  from a 30-year  career in the Air Force and Massachusetts Army National Guard, and served as a battalion and brigade commander in Iraq, said on his campaign website that he is running as a write-in candidate because Department of Defense rules kept him from meeting the filing deadlines. The last day to register to vote in time for the primary is Aug. 17.