Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Flight Crew Last 90 Days 135; Flight Crew Total 6040; Flight Crew Type 460
Communication Breakdown; Situational Awareness
Conflict NMAC; Deviation - Track / Heading All Types
Flight Crew Took Evasive Action
Human Factors; Airspace Structure
Descending on the SETER4 arrival into ONT; we were given a vector direct PETIS from about BANDS. Shortly after making the turn to direct PETIS we were informed by SoCal that there was a target on the screen with no altitude or type and was not talking to them and he gave us a more westerly heading to avoid the traffic. About a minute or so on the westerly heading he had informed us that the primary target popped up on radar at which time I started looking outside and looking at our TCAS to see if there were any information. Nothing was showing on the TCAS and I saw the airplane in a turn from west to north wing up and level off northward. Both the [Second Officer] and I called out the traffic to the captain and he said that he had the traffic. My first instinct was to fly behind the airplane and keep him in my sight and to keep us separated. The captain then turned to the right bringing us to a parallel or less flight path at the same altitude. It appeared to me that the Cessna was conducting flight training maneuvers. The captain then leveled off the airplane and the Cessna was at our twelve o'clock and closing fast. The [Second Officer] and I started telling the captain to take action and he was not making any correction to avoid the traffic. Both of us had to tell the captain to descend immediately and I was reading myself to take control of the aircraft and the captain did start a slow and steady descent and the I watched the Cessna passing us going northward clearing us approximately 50 feet above us and about 250-300 feet off the right wing. I informed ATC that we had cleared the conflicting aircraft and gave the type; unable to make out the tail number we continued the approach to landing at ONT.
Air Carrier First Officer reported an NMAC during arrival to ONT; and the flying Captain failed to make a timely turn to avoid VFR traffic.