Narrative:

Near midair collision with small aircraft. We were arriving into ZZZ from ZZZ1. We were on the zzzzz arrival; south bound; at 6000 ft. Just outside of ZZZZZ1 intersection. ATC gave us a traffic advisory of a target at 1 to 2 o'clock less than a mile; no altitude. I was pm (pilot monitoring) and the first officer (first officer) was PF (pilot flying). We both quickly got the traffic insight at probably less than 1000 ft. Away and climbing at approximately our altitude. They were traveling [east-southeast]; approximately 50-60 degree intercept to our course. The autopilot was immediately disconnected and the PF started a left turn and descent away from the traffic. The traffic continued on its course to within 500 ft. Before obviously seeing us. The small aircraft made an abrupt turn and dive to his left to go behind us. The small aircrafts last minute maneuver relieved us from having to make a more aggressive evasive input to avoid the traffic. We ended up approximately 30 degrees off course and just over 300 ft. Below our assigned altitude. ATC was notified; they assigned us a heading and we corrected back to 6000 ft. ATC apologized and stated the target was intermittent. At no time did we receive a TCAS TA or RA; before during or after the event. The TCAS was working normally as we had other traffic on display. It is our opinion the small aircraft did not have a transponder on. I believe ATC was observing a primary target. At the time we maneuvered away from the small aircraft it was done smoothly without abrupt nor excessive bank or pitch. I do not believe any passengers would have noticed anything out of the ordinary unless they happened to be looking out the window and saw the small aircraft dive away from us. It is however; in my opinion though; had the small aircraft not maneuvered away from us or had ATC notified us of the target any later; then it would have required an immediate and abrupt evasive action to avoid an imminent collision. It's hard to believe in this day and age that an aircraft can be operating in a high density arrival corridor; into busy airport; and not have an operating transponder. If they had a transponder this situation would have easily been avoided well before it was ever a danger.

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Original NASA ASRS Text

Title: Captain reported a NMAC with traffic observed visually.

Narrative: Near midair collision with small aircraft. We were arriving into ZZZ from ZZZ1. We were on the ZZZZZ arrival; south bound; at 6000 ft. just outside of ZZZZZ1 intersection. ATC gave us a traffic advisory of a target at 1 to 2 o'clock less than a mile; no altitude. I was PM (Pilot Monitoring) and the FO (First Officer) was PF (Pilot Flying). We both quickly got the traffic insight at probably less than 1000 ft. away and climbing at approximately our altitude. They were traveling [east-southeast]; approximately 50-60 degree intercept to our course. The autopilot was immediately disconnected and the PF started a left turn and descent away from the traffic. The traffic continued on its course to within 500 ft. before obviously seeing us. The small aircraft made an abrupt turn and dive to his left to go behind us. The small aircrafts last minute maneuver relieved us from having to make a more aggressive evasive input to avoid the traffic. We ended up approximately 30 degrees off course and just over 300 ft. below our assigned altitude. ATC was notified; they assigned us a heading and we corrected back to 6000 ft. ATC apologized and stated the target was intermittent. At no time did we receive a TCAS TA or RA; before during or after the event. The TCAS was working normally as we had other traffic on display. It is our opinion the small aircraft did not have a transponder on. I believe ATC was observing a primary target. At the time we maneuvered away from the small aircraft it was done smoothly without abrupt nor excessive bank or pitch. I do not believe any passengers would have noticed anything out of the ordinary unless they happened to be looking out the window and saw the small aircraft dive away from us. It is however; in my opinion though; had the small aircraft not maneuvered away from us or had ATC notified us of the target any later; then it would have required an immediate and abrupt evasive action to avoid an imminent collision. It's hard to believe in this day and age that an aircraft can be operating in a high density arrival corridor; into busy airport; and not have an operating transponder. If they had a transponder this situation would have easily been avoided well before it was ever a danger.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.