10nl - AA raised on the river, what's my play?

Poker Stars $0.05/$0.10 No Limit Hold'em - 8 players -

UTG+1: $5.90
MP1: $2.00
MP2: $5.65
CO: $6.05
BTN: $15.40
SB: $10.30
BB: $1.70
Hero (UTG): $16.25

Pre Flop: ($0.15) Hero is UTG with A A
Hero raises to $0.40, 4 folds, BTN calls $0.40, SB calls $0.35, 1 fold

Flop: ($1.30) 9 3 K
SB checks, Hero bets $0.90, BTN calls $0.90, SB folds

Turn: ($3.10) 4
Hero bets $1.50, BTN calls $1.50

River: ($6.10) 4
Hero bets $2.50, BTN raises to $6.50, Hero ???

No reads, what's my play?