NL50: AKs top pair facing river push.

[guess who]

Hi,

villian is 40/7/3.

ltimate Bet No-Limit Hold'em, $.50 BB (6 handed) by Hand Converter CG

SB ($51.80)
BB ($35.50)
UTG ($100.50)
MP ($49.20)
Hero ($49.40)
Button ($23.85)

Preflop: Hero is CO with A[img]/images/graemlins/diamond.gif[/img], K[img]/images/graemlins/diamond.gif[/img].
UTG calls $0.50, <font color="#666666">1 fold</font>, <font color="#CC3333">Hero raises to $2.25</font>, <font color="#666666">3 folds</font>, UTG calls $1.75.

Flop: ($5.25) 2[img]/images/graemlins/heart.gif[/img], 4[img]/images/graemlins/club.gif[/img], 2[img]/images/graemlins/diamond.gif[/img] <font color="#0000FF">(2 players)</font>
UTG checks, <font color="#CC3333">Hero bets $4</font>, UTG calls $4.

Turn: ($13.25) A[img]/images/graemlins/heart.gif[/img] <font color="#0000FF">(2 players)</font>
UTG checks, <font color="#CC3333">Hero bets $7.5</font>, UTG calls $7.50.

River: ($28.25) 3[img]/images/graemlins/club.gif[/img] <font color="#0000FF">(2 players)</font>
UTG checks, <font color="#CC3333">Hero bets $10</font>, <font color="#CC3333">UTG raises to $58.25</font>, Hero ...?!?

River bet was supposed to be a standard value bet. I didn't think he would ever raise me.
My top pair doesn't seem to be good?! Can't be a busted flush draw, so I only beat A with weaker kicker or a bluff.

Thanks for your thoughts,
guess