Flight Crew Air Transport Pilot (ATP)
ATC Issue All Types; Aircraft Equipment Problem Less Severe
General None Reported / Taken
Aircraft; Environment - Non Weather Related
We were level at FL290 with a clearance to cross BUOYY at FL240.  We had not started down when we observed a TCAS target approaching the aircraft 1700 feet below us and climbing towards us.  We stayed level as it climbed to with 900 feet of our altitude and came within approximately 1.5 miles to our right hand side.  We decided to question Boston center about the traffic as we had been cleared to descend to FL240.  I asked if they were talking to the traffic climbing toward us from below and they said only that they were talking to a B777 descending towards us from above.  Both myself; and the First Officer are sure the TCAS was displaying a climbing target.  Also the First Officer is sure he saw two separate targets; one climbing towards us and one descending towards us.  Boston Center had no observed target where we claimed one to be.  When we landed we were given a phone number and asked to call the Center.  I spoke with two different managers; including three separate calls with the QC manager for Boston Center.  He seemed very concerned as they had no known target on their radar and he also indicated 'something similar had happened a week or so ago in the north of their sector' I called Maintenance Control on his behalf; and asked if the aircraft saved TCAS information.  Maintenance spoke with 'Safety' and I was told that no; the aircraft does not save TCAS information.
EMB170 Captain describes a TCAS target climbing towards his aircraft after having received a descent clearance.  When ATC is questioned they reply that there is no target at that location and altitude.