There's a couple of options; the most obvious one is radiothermal heating from radioactive elements in the core, or even just remnant heat from the core slowly

There's a couple of options; the most obvious one is radiothermal heating from radioactive elements in the core, or even just remnant heat from the core slowly working its way up. And if there are pairs of these, then their orbits might be keeping their cores moving and warm due to tides.