NMAC. FLC OF AN MLG DSNDED FROM ASSIGNED ALT IN RESPONSE TO A TCASII RA DURING A STAR ARR. THE OTHER UNKNOWN ACFT CAME WITHIN 300 FT OF THE RPTR'S ACFT.

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INBOUND TO PHL ON BUNTS ONE ARR. AFTER BUNTS INTXN; DSNDED FROM 8000 FT TO 6000 FT. ACFT PREVIOUS HAD COMPLAINED OF BAD RIDE AT 6000 FT. WE ASKED CTLR FOR LOWER BASED ON OTHER'S COMPLAINTS. CTLR INFORMED US THAT SHE NEEDED US AT 6000 FT. FOR 15 MI WE COMPLIED. IN AND OUT OF CLOUD BASES AT 6000 FT; BROKEN LAYER ABOVE; HDG 170 DEGS FOR DOWNWIND TO BASE LEG SET UP (VECTORED). NEARING PHL ARPT I INITIATED A CALL FOR REQUEST FOR LOWER. RADIO COMS WERE TIGHT AND BUSY. AT END OF XMISSION FOR LOWER; I HEARD A BROKEN COM FOR A HDG TO 090 DEGS -- NO ACFT CALL SIGN. NO ONE ACKNOWLEDGED ATC'S COMMAND. THE DIRECTIVE WAS NOT ISSUED AGAIN. I QUESTIONED APCH TWICE AS TO WHO THE HEADING WAS FOR WITH NO REPLY. CAPT (PF) STAYED ON 170 DEG HDG. CTLR CAME BACK UP; SAYING SHE WAS ON LAND LINE. I QUESTIONED AGAIN (THIRD XMISSION ON SAME QUESTION). SHE DIRECTED US TO TURN HARD 070 DEGS -- VFR TARGET HAD 'POPPED' UP ABOVE US AT 6500 FT. TCASII IDENTED TARGET AS A TA (AMBER); THEN TURNED TO RA (RED) 'DSND; DSND; DSND' JUST AS THE CAPT CLICKED OFF AUTOPLT; BANKED 40 DEGS L AND BEGAN DSCNT AT 4000 FPM. I INFORMED APCH THAT WE WERE DSNDING OUT OF 6000 FT ON AN RA. CAPT STOPPED DSCNT AT 5300 FT AFTER RA CEASED. I INFORMED CTLR THAT WE WERE AT 5300 FT. SHE REPLIED SHE NEEDED US AT 6000 FT. WE COMPLIED; CLBED AND WAS HANDED OFF TO FINAL CTLR FOR A NON EVENTFUL LNDG AT PHL. OUR BEST ESTIMATION IS THAT THE TFC CAUSING THE RA WAS 300 FT ABOVE AT ITS LOWEST ALT; AND AS CLOSE AS 400-500 FT HORIZLY FROM US.