B757 INBOUND TO PHL TAKES EVASIVE ACTION FROM UNKNOWN TFC.

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WE WERE BEING VECTORED FOR RWY 9L AT PHL. WE WERE AT 4000 FT; HDG 210 DEGS ASSIGNED. AT THIS POINT; I SAW WHAT APPEARED TO BE VERY CLOSE TFC OUT THE R WINDOW. IT APPEARED TO BE VERY CLOSE AND CLOSING ON US RAPIDLY AT THE SAME ALT. APCH CTL WAS TALKING TO OTHER TFC; SO THERE WASN'T TIME TO QUERY HIM CONCERNING CONFLICT. I SHOUTED OUT 'TFC' SEVERAL TIMES TO THE CAPT; THEN TOOK EVASIVE ACTION. I DISENGAGED THE AUTOPLT AND TURNED APPROX 90 DEGS TO THE L AND LOST 300 FT OF ALT. THE CAPT ADVISED ATC THAT WE WERE AVOIDING TFC ON OUR R SIDE. THE CTLR RPTED NO TFC NEAR US. THE CTLR THEN GAVE CLRNC TO DSND TO 3000 FT AND FLY HDG 240 DEGS; WHICH WE COMPLIED WITH. THE REMAINDER OF THE APCH AND LNDG WERE UNEVENTFUL. SUPPLEMENTAL INFO FROM ACN 580337: THE CTLR RPTED HE SAW NO TFC ON OUR R SIDE NEAR OUR ACFT. THERE WAS NO RA OR TA FROM THE TCASII WHICH WAS OPERATING NORMALLY. THE CTLR INSTRUCTED US TO CONTINUE DSCNT TO 3000 FT FROM 3700 FT AND FLY A HDG OF 240 DEGS TO CONTINUE THE DOWNWIND. THE ENTIRE DEV EPISODE LASTED ABOUT 15 SECONDS.