Riverbet against unknown?

Party Poker No-Limit Hold'em, $ Hero (9 handed) Hand History Converter Tool from FlopTurnRiver.com (Format: FlopTurnRiver)

MP3 ($57.70)
CO ($52.50)
Button ($8.97)
SB ($49)
Hero ($50)
UTG ($47)
UTG+1 ($54.10)
MP1 ($64.45)
MP2 ($47.50)

Preflop: Hero is BB with A, K. SB posts a blind of $0.25.
2 folds, MP1 calls $0.50, 4 folds, SB (poster) raises to $0.75, Hero raises to $2.5, MP1 calls $2.50, SB calls $2.

Flop: ($9) T, 2, Q (3 players)
SB checks, Hero checks, MP1 bets $4, SB folds, Hero calls $4.

Turn: ($17) J (2 players)
Hero checks, MP1 bets $7, Hero raises to $21, MP1 calls $14.

River: ($59) J (2 players)


Final Pot: $59