IFR C208 WITH N90 AT 4000 FT EXPERIENCED NMAC WITH UNKNOWN VFR TFC BETWEEN LAYERS; CTLR ISSUED TFC IMMEDIATELY PRIOR TO EVENT.

Narrative

WHILE ENRTE FROM OXC TO ACK ON AN IFR FLT PLAN UNDER CTL OF NEW YORK APCH; WE RECEIVED A TCAS TFC ALERT. CAPT AND I LOOKED TO OUR 9 O'CLOCK POS FOR TFC. WE WERE CLBING AT APPROX 600 FPM ABOVE A LOWER CLOUD DECK AND BELOW A HIGHER DECK. SHORTLY AFTER OUR TCAS ALERT; NEW YORK CALLED AND ADVISED OF VFR TFC AT 9 O'CLOCK POS AND 2 MI. OUR TCAS SHOWED THE TFC TO BE 300 FT BELOW AND AT 9 O'CLOCK WITHIN 2 MI. WE SAW A LOW WING TWIN ENG ACFT HEADING TO THE SW AT APPROX 200-300 FT BELOW US. THE ACFT PASSED DIRECTLY BELOW US. THE ACFT CONTINUED UNDER US AND WAS OUT OF SIGHT IN A MATTER OF SECONDS. I TOLD NEW YORK THAT THAT WAS A CLOSE SITUATION AND THE CTLR REPLIED THAT THE TARGET JUST APPEARED AND HAD PROBABLY JUST TURNED ITS XPONDER ON. IT IS POSSIBLE THAT THE TWIN HAD CLBED THROUGH THE UNDERCAST IN IMC WITHOUT AN ATC CLRNC AND HAD NOT TURNED ON THEIR XPONDER UNTIL IN VMC. THIS COULD HAVE BEEN AVOIDED HAD THE PLT(S) OF THE TWIN OPERATED WITH THEIR MODE C XPONDER OPERATING AND FOLLOWED THE APPROPRIATE RULES FOR FLT IN IMC. AT THE TIME OF THE INCIDENT THE ACFT WAS NOT MAINTAINING 1000 FT ABOVE THE CLOUDS.