Light Transport; Low Wing; 2 Turbojet Eng
Flight Crew Air Transport Pilot (ATP)
Workload; Situational Awareness; Distraction
Conflict Airborne Conflict; Deviation - Altitude Excursion From Assigned Altitude
Air Traffic Control Provided Assistance; Flight Crew Took Evasive Action
Aircraft
We were on the JIIMS2 arrival into PHL descending through 17500 FT with LNAV/VNAV engaged with a clearance to cross 'RAADS' intersection at 15000 FT.  Our decent rate was standard approx 2000 FT/min 3 deg.  Suddenly traffic directly in front of us showed on TCAS. The controller pointed the traffic out to us and said the traffic just appeared to do a 360.  I started to shallow the descent since the traffic was below us when the controller assigned us a heading of 360 for traffic.  Less than a split second later the TCAS gave us a Resolution Advisory (RA) with a steep descent which I complied with immediately and simultaneously complied with the new heading assignment given by the controller. Once we heard the 'Clear of Conflict' we advised the controller and he cleared us direct to JIIMS intersection.  We never went below our crossing altitude given of 15000 FT during the Traffic Advisory (TA)/RA descent for this traffic which we never saw but could have been a Drone since maneuvering at an altitude of approx 17;500 FT does not make any sense and the controller was not in contact with this traffic.
Pilot describes a situation where they are in a descent and get a RA at the same time the Controller issues traffic.  They descend as the RA suggests and turn to a heading issued by the Controller.  Traffic is not seen but suggested to be a drone by the pilot.