LGT ON IFR ARR ROUTE WAS GIVEN TRAFFIC ON UNK-SMT B WHO LATER TOOK EVASIVE ACTION MISS THE LGT.

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I WAS FLYING THE ACFT AT 11000' MSL LEVEL FLT AIRSPD 250 KTS. THE CAPT WAS WORKING THE RADIOS. ATC ADVISED; 'TFC 10-11 O'CLOCK; 6 MI; 10000' SWBOUND; UNVERIFIED.' OUR CREW THOUGHT IT STRANGE THAT THE TFC WAS NOT AT A VFR ALT. WE ALL LOOKED FOR THE TFC. ATC: 'TFC NOW (BROKEN XMISSION) MI 10500' TURNING WBOUND.' S/O OR JUMPSEAT RIDER: 'THERE; 11 O'CLOCK; JUST ABOVE THE HORIZON.' THE CAPT STATED; 'WATCH HIM; HE'S CLBING.' THE ACFT APPEARED TO BE A LIGHT TWIN ENG ACFT. ATC: '10700.' THE ACFT WAS WINGS LEVEL OFF THE LEFT SIDE OF OUR ACFT; AT WHAT LOOKED LIKE THE SAME ALT. I WAS ABOUT TO TAKE EVASIVE ACTION WHEN I SAW THE LIGHT TWIN MAKE A VERY STEEP RIGHT TURN. THE CAPT STATED; 'HE'S GOING TO MISS US!' THE ACFT PASSED OFF THE LEFT SIDE OF OUR ACFT. THE CAPT STATED TO BOS CENTER; 'DAMN; THAT WAS TOO CLOSE.' ATC RESPONDED THAT 'HE HOPED THE TARGET HAD US VISUALLY.' I FEEL BOS CENTER FELT; ON THE FIRST CALLOUT; THAT THERE WOULD NOT BE A CONFLICT WITH THE TFC. ATC REALIZED THE PROB WHEN THE TARGET TURNED WBOUND AND BEGAN TO CLB. I DON'T THINK ATC HAD TIME TO VECTOR US AWAY FROM THE TFC AND THAT HE DID THE PROPER THING BY KEEPING US INFORMED OF THE TARGET'S ALT AND HDG. SUPPLEMENTAL INFO FROM ACN 85767: ATC: '10700';' TARGET NOT IDENTED BY HEAD ON PROFILE AS A LIGHT TWIN; WINGS LEVEL AT LEFT SIDE OF MY WINDSHIELD AT OUR ALT. I REACHED FOR THE YOKE TO BREAK RIGHT; (F/O STATED LATER HE WAS ABOUT TO ALSO) BUT SAW THE TWIN MAKE A VERY STEEP RIGHT BANK; AND I STATED; 'HE'S GOING TO MISS US!' TWIN PASSED MY LEFT SLIDING WINDOW SHOWING US HIS UNDERSIDE. AS I FOLLOWED THE SMT Y LOOKING AFT; HE APPEARED TO LEVEL HIS WINGS AND APPEARED TO PASS BEHIND US. I STATED TO ATC; 'DAMN; THAT WAS TOO CLOSE;' AND ATC ANSWERED TO THE EFFECT THAT HE HAD HOPED THE TARGET HAD US VISUALLY. I BELIEVE THAT ATC FELT; THE SAME AS I DID; THAT ON THE FIRST CALLOUT THERE WOULD BE NO CONFLICT; (IE; THE SMT Y HDG SW WOULD PUT HIM FAR TO OUR RIGHT WHEN OUR PATHS CONVERGED). HE REALIZED THE PROB WHEN HE SAW THE TARGET TURN W AND CLB; BUT DID NOT HAVE TIME TO VECTOR US. I FEEL THAT HIS ACTION IN KEEPING US INFORMED WAS ABOUT ALL HE COULD DO.