A321
Flight Crew Air Transport Pilot (ATP)
ATC Issue All Types; Conflict Airborne Conflict
Air Traffic Control Issued Advisory / Alert; Air Traffic Control Issued New Clearance; Flight Crew Took Evasive Action
Airspace Structure; Procedure
Arriving EWR on the PHLBO Arrival; NY Approach Control alerted us to traffic that was not displayed on TCAS. In the time it takes you to read this sentence; Approach gave us a heading; we saw the traffic right in front of us just as it displayed on TCAS; and we got a TCAS RA to 'climb.' First Officer accomplished the RA perfectly as we climbed above the traffic. The intruder appeared to be a Cirrus in a climb between the points DYLIN and MERSR on the PHLBO Arrival. We were descending through about 7;000 FT when the incident occurred. When the intruder did appear on the TCAS it was about 2 miles ahead and 300 FT below; in a climb. I estimate we were 500 FT above when we crossed. I'm sure we were close enough for him to hear us; as we filled his windshield just above him.  The profile was required and an uneventful landing followed. As we all know; not all traffic gets displayed on TCAS all the time. First Officer and I were scanning outside as we should while the aircraft was following the RNAV Arrival. We did not spot the intruder because it was climbing in the same direction we were flying; so the smallest cross section was displayed to us. We were looking at its tail. NY Approach must have known about him; but did not alert us until we were close. They probably should have vectored us off the arrival sooner than they did. Keep alert; and scan outside; especially below 10;000 FT.
A321 Captain reports a TCAS RA descending through 7;000 FT on the PHLBO arrival to EWR. An RA climb is initiated to avoid a Cirrus climbing along the arrival route.