Maxim Ludwig has a commendable backing back in the Mystics. More valuably, he’s got a strong partnership going with Angel Olsen. The Los Angeles singer-songwriter has been touring and performing regularly with Olsen in recent months, including on a track for the recent Lou Reed tribute album. For his new single, “Mercury Avenue,” Ludwig is again duetting with Olsen, and the result is downright radiant. It’s like a twenty-first transmogrification of one of Tom Petty’s songs of vulnerable ache, and Olsen’s harmonies give the finished product an extra dose of shimmering beauty.