University of California, Lick Observatory
Neutron Stars on Fire
Neutron stars in low mass X-ray binaries accrete hydrogen and helium from a companion star. A few times a day, for ten seconds at a time, they light up as the fuel on their surfaces ignites and burns in a thermonuclear runaway. These "Type I X-ray bursts" outshine the accretion luminosity, giving a brief chance to study the neutron star directly. In recent years, there has been much excitement and progress in this field. Millisecond period brightness oscillations during bursts directly measure the spin frequency of the neutron star, and allow us to probe the magnetic field and ignition and spreading of the burning. Rare, spectacular "superbursts" have been observed, 1000 times more energetic than usual, indicating a new regime of nuclear burning, most likely ignition of carbon in a sea of heavy elements produced by the rp process. I discuss these new findings and describe our general understanding of the physics of these flashes, and what we're learning about these neutron stars.