RPTR'S ACFT; AT 9000 FT; ON VECTORS FROM BOS APCH; RECEIVED A COUNTER INTUITIVE TCASII RA RESULTING IN AN APPARENT LOSS OF SEPARATION. CREW DSNDED ONLY AFTER RECEIVING A SECOND RA.

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LEVEL 9000 FT UNDER BOS APCH CTL AND ON RADAR VECTORS; VFR TFC WAS ISSUED APPROX 10 O'CLOCK; 8500 FT VFR; WSWBOUND; 5 NM. I WAS THE PNF. I ADVISED APCH; 'NO CONTACT; PLEASE KEEP ADVISED!' THE TFC WAS VISUAL ON TCASII. IT REGISTERED -200 FT BELOW AND CLOSING. ANOTHER TA WAS ISSUED BY APCH. STILL THERE WAS NO VISUAL CONTACT. THE TCASII GAVE AN INTRUDER WARNINGS 'TA.' TFC STILL READ -200 FT BELOW. I WARNED THE PF TO BE PREPARED FOR A POSSIBLE TCASII CLB; SINCE THE ACFT WAS REGISTERING -200 FT BELOW. THE TCASII RA SOUNDED WITH A SURPRISING 'DSND' COMMAND. THE COMMAND WAS NOT EXTREMELY LOUD; SO I HESITATED TO INITIATE THE DSCNT. I WAS IN DISBELIEF OF THE COMMAND. A SECOND 'DSND' RA SOUNDED. I INSTRUCTED THE FO TO DISCONNECT THE AUTOPLT AND FOLLOW THE COMMAND. AS WE BEGAN THE DSCNT; I ADVISED BOS APCH; 'ACFT X DSNDING IN RESPONSE TO TCASII RA.' APCH WAS SURPRISED THAT WE WERE DSNDING AND AGAIN ADVISED TFC WAS BELOW AT 8500 FT. IN THE DSCNT; I VISUALLY PICKED UP TFC. WE WERE DSNDING THROUGH HIS ALT AND HE WAS STILL CLOSING. I PUSHED THE CTL YOKE DOWN TO ASSIST THE FO IN EXPEDITING THE DSCNT. PASSING THROUGH APPROX 8300-8400 FT; THE TCASII RPTED 'CLR OF CONFLICT.' WE LEVELED AT 8000 FT. BOS APCH DID NOT INFORM US OF A LOSS OF SEPARATION DUE TO OUR ACTIONS; HOWEVER; I BELIEVE THERE MUST HAVE BEEN DUE TO THE RELATIVE CLOSE PROX OF THE ACFT WHEN I SAW HIM VISUALLY. COMPANY HAS BEEN ADVISED AND THEY ARE CHKING THE ACFT FOR ANY TCASII MALFUNCTIONS. I BELIEVE I MAY HAVE PUT MY ACFT IN JEOPARDY BY FOLLOWING THIS TCASII RA COMMAND. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: NO SIGNIFICANT NEW INFO. RPTR CONFIRMED THAT SHE BELIEVED THE VFR TFC CALLED OUT BY BOS APCH CTL WAS THE SAME TFC ACQUIRED ON THE TCASII SCREEN AND LATER; AS THEY WERE EXECUTING THE RA MANEUVER; ACQUIRED VISUALLY. RPTR INDICATED THAT; AT TIME OF INCIDENT; TCASII SCREEN WAS IN THE 10 MI RANGE AND IN THE 'DOWN' MODE. RPTR STATED THAT THERE WAS ONLY 1 TARGET ON THE TCASII SCREEN. RPTR STATED SHE HAD FILLED OUT A FLT SAFETY RPT AND A CAPT'S RPT. SHE WAS CALLED BY SOMEONE FROM HER COMPANY'S FLT SAFETY RPTING SYS AND TOLD THE INCIDENT WAS BEING INVESTIGATED. RPTR HAS HEARD NOTHING SINCE. RPTR ALSO STATED THAT THERE WERE NO TCASII RELATED WRITE-UPS IN THE ACFT MAINT LOGBOOK.