Flux is a science fiction novel by Stephen Baxter. The story of a post-human species, engineered to live in the mantle of a neutron star.

The main protaganist, Dura is ten microns tall, made of nucleonic matter, sees sound, smells photons and swims against magnetic fields. But her incredible world is torn apart as she learns of the true role of her engineered race in Humanity's millennia-long war against the godlike Xeelee.

Although this book stands on its own, it is really part of Baxter's Xeelee Sequence, the neutron star seems to be a weapon and built by humans. It's not explicitly stated, but probably the weapon is to disrupt the Xeelee's greatest engineering work a vast Ring woven from cosmis strings.