A pair of 19th-century Maine lighthouses that went up for auction in May have attracted competing bids, federal officials said. The top bid for the Halfway Rock Light Station off Harpswell is $31,500, and the top bid for Boon Island Light Station off York is $12,000. Bidding on Boon Island closes Aug. 12 but could be extended; there is no closing date for Halfway Rock Light. Halfway Rock was built in 1871 and features a 76-foot-tall tower on two acres of ledge about 10 miles out. Boon Island was built in 1855 and includes a 133-foot tower on a tiny island about 6 miles offshore.