INDIANAPOLIS – Jimmie Johnson made waves a month ago when he showed up, nonchalantly, as the most newsworthy guest at IndyCar’s season-opening two-day test session at Circuit of the Americas. He was a guest of the Arrow McLaren SP team, and rumors grew rapidly about what the NASCAR legend’s future in the other sector of American auto racing might be.

He was vague, at first, but there’s little doubt of Johnson’s sincerity now. Wednesday, the seven-time Cup series champion tweeted photos from inside the cockpit of an Indy car, and when asked what it meant, the 44-year-old driver – who announced late last year his plans to retire from full-time Cup competition – said it bluntly: he’s in preparation mode for an impending IndyCar debut.

“It means my friends at @ArrowMcLarenSP offered to let me jump in the car and test with them next month at @BarberMotorPark,” he tweeted.

Johnson, the four-time Brickyard 400 champion, didn’t provide any other details about a date or time of said test, but the IndyCar series does visit Barber Motorsports Park, which is just outside Birmingham, Ala., next month for an April 5 race, the second stop on the series’ 17-race schedule.

Johnson all but ruled out a debut in IndyCar for 2020 while at COTA, though. With more than two-dozen NASCAR races left in his final full season, trying to pull double-duty one weekend would be taxing, though it’s certainly been done before. But like part-time IndyCar driver James Hinchcliffe, whose three-race 2020 slate has opened him up for what he calls his “gap year," as well as former F1 champ Fernando Alonso, who for the second consecutive year is taking advantage of his empty calendar to try to tackle the Indy 500, exploration within motorsports is often much easier with free time to get up to speed.

“For me, this is a learning experience, and IndyCar has always been a bucket list category for me, to come see these cars, be around these cars,” Johnson said at COTA. “(Next year) is open for me, and who knows what opportunity might come along in IndyCar, sports cars or back to my off-road roots.

“I’m just taking it all in and see what comes of it.”

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Johnson did add that he’s “closed the door” on a possible run at the Indy 500, as well as IndyCar oval track racing in general, tbut the series’ variety of road and street races – of which there are 12 on the 2020 slate – would give him ample opportunities to find a fit in 2021 – possibly with the team he’s already built a strong relationship with from the start and one that has a track record of hosting fan-favorites in long-time popular driver Hinchcliffe, as well as Alonso.

“I’m definitely learning,” said Johnson, who two weeks ago posted a video from driving on an IndyCar simulator machine. “This is a totally different world that what I’ve been used to, but it’s a world I dreamed of racing in when I was a kid growing up in San Diego.

“My heroes were the guys that raced in the Indy 500. I’d go to the Long Beach Grand prix and watch, hang out, and I wanted to be in a car going by.”

Email IndyStar motor sports reporter Nathan Brown at nlbrown@gannett.com. Follow him on Twitter: @By_NathanBrown.