A320
Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 150; Flight Crew Total 7800; Flight Crew Type 1639
Situational Awareness; Communication Breakdown
Conflict NMAC
Flight Crew Requested ATC Assistance / Clarification
Human Factors
Descending VIA FRNCH arrival STAR; ATC leveled us at FL190 then was stepping us down a few thousand feet at a time.  In the vicinity of TOMSN intersection at or near our assigned altitude of 15;000 feet we encountered a light aircraft between 300-500 feet above us traveling in the opposite direction.  I was making a radio call to Approach when I saw the aircraft and said 'AIRCRAFT' before releasing the microphone.  Time from visually acquiring the aircraft to determining it wasn't going to impact us to having it pass was approximately 3-5 seconds.  I then reported 'near midair collision' to ATC.  ATC informed us of traffic at 13;000 feet which I saw below us.  ATC then reported a VFR target squawking 1200 shortly after.  The aircraft may have turned on their transponder after seeing an Airbus nearly hit them.A few miles earlier I had noticed an unfilled white diamond symbol on my TCAS; however it was not in our flight path.  We received no TCAS RA; Traffic Alert or any other guidance or warning.
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A320 flight crew reported an NMAC with a single engine light aircraft in the vicinity of DEN.