AQo, all high cards on board

[soah]

Party Poker No-Limit Hold'em, $ BB (6 max, 6 handed) converter

Hero ($173.25)
SB ($98.7)
BB ($107.2)
UTG ($106.2)
MP ($222)
CO ($137)

Preflop: Hero is Button with Q[img]/images/graemlins/spade.gif[/img], A[img]/images/graemlins/diamond.gif[/img]. SB posts a blind of $1.
<font color="#666666">1 fold</font>, <font color="#CC3333">MP raises to $6</font>, CO calls $6, Hero calls $6, <font color="#666666">2 folds</font>.

Flop: ($21) K[img]/images/graemlins/spade.gif[/img], A[img]/images/graemlins/heart.gif[/img], J[img]/images/graemlins/club.gif[/img] <font color="#0000FF">(3 players)</font>
MP checks, CO checks, <font color="#CC3333">Hero bets $15</font>, MP folds, CO calls $15.

Turn: ($51) 7[img]/images/graemlins/club.gif[/img] <font color="#0000FF">(2 players)</font>
CO checks, Hero checks.

River: ($51) A[img]/images/graemlins/club.gif[/img] <font color="#0000FF">(2 players)</font>
<font color="#CC3333">CO bets $55</font>,

Preflop raiser could have nearly anything. Main opponent was completely unknown. Is there any other way to play this? Do I call here? That big of a bet isn't a bluff very often in this game, in my experience. What could he be betting that I beat? AT?