FLC OF GOV G4 ACFT HAS NMAC WITH A BE BARON IN THE TFC PATTERN. TA'S HAD BEEN ISSUED; BUT NOT FOR THIS PARTICULAR ACFT. TCASII ALERTED THE FLC.

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WE WERE IFR IN VMC (VISIBILITY 50 SM) XING THE LAWRENCE VOR IN CONTACT WITH BOSTON APCH CTL. WE WERE CLEAN; HEADING SWBOUND DOING APPROX 200 KTS AND WERE CLRED FOR THE VISUAL APCH TO RWY 5 AND TOLD TO CONTACT HANSCOM TWR. WE STARTED DSNDING TO PATTERN ALT OF 1500 FT AND WENT FLAPS 20 DEGS; SPD 160 KIAS AND PUT THE GEAR DOWN; THIS WAS ACCOMPLISHED BEFORE GETTING TO THE ARPT BOUNDARY. HANSCOM TWR ISSUED MULTIPLE TA'S AT 2 AND 3 O'CLOCK POS. I HAD ONE TFC IN SIGHT AND STILL LOOKING FOR THE OTHER WHEN WE HAD A TCASII TA AT 9 O'CLOCK POS ABOUT 400 FT BELOW. THIS WAS FOLLOWED ALMOST IMMEDIATELY BY A TFC RESOLUTION ALERT (RA) WITH THE NORMAL AURAL TONES. I IMMEDIATELY ASKED THE PLT IN THE L SEAT IF HE HAD THE TFC IN SIGHT AND HE RESPONDED YES. WE CONTINUED TO GET THE AURAL WARNINGS AND I AGAIN ASKED THE OTHER PLT IF HE STILL HAD THE TFC. HE SAID YES. I THEN OBSERVED A BEECH BARON B58 COME OUT FROM UNDER THE AIRPLANE IN A L TURN WITH 2 PEOPLE INSIDE. I ESTIMATED TOTAL SEPARATION TO BE 200 FT DIAGONALLY. THE OTHER ACFT WAS GOING AWAY FROM US. WITH THE EVENT OVER WE COMPLETED OUR APCH AND LNDG UNEVENTFULLY. I FOUND MYSELF SOMEWHAT DISTRACTED BY THE TA'S ISSUED BY THE TWR AND DO NOT RECALL ANY CONVERSATION ABOUT THE PRESENCE OF THE BARON TFC. I ALSO SAY THE TCASII DID ITS JOB AND ALERTED US OF THE TFC IN TIME TO AVOID IT IF WE HAD NOT HAD THE TFC IN SIGHT. THE OTHER PLT ALSO ACTED PROPERLY IN DISCONNECTING THE AUTOPLT AND WAS READY FOR EVASIVE ACTION HAD IT BEEN NECESSARY.