High-energy and nuclear astrophysics are truly in a golden period. I conduct theoretical research in the exciting area of compact stars, neutron stars and white dwarfs. Neutron stars are the most dense objects in nature, and have long fascinated astronomers and physicists alike. With X-ray telescopes such as Chandra and XMM to study these objects, nuclear experiments such as heavy-ion collisions to study the nuclear force, and the promise of gravitational wave detectors such as LIGO, our knowledge of these enigmatic objects and the nature of dense matter is rapidly improving. Many neutron stars accrete gas from a binary, solar-like companion. As this gas accumulates on the surface of the neutron star, unstable nuclear reactions produce bursts of X-rays.
New observations of these nuclear processes provide clues about the properties of superdense matter, but these observations also challenge our understanding of how the fuel is accreted and burned. Particularly exciting are the recently discovered “superbursts,” energetic explosions that are believed to be powered by unstable fusion of carbon-12. The “crust” of the neutron star, where the density is less than nuclear, must be sufficiently hot in order for the superburst to ignite. By studying these superbursts we can learn about the physics of the core, which cools by emitting neutrinos.
Recently, I have collaborated on the first calculation of electron captures in the crust of an accreting neutron star using realistic nuclear physics input, and have investigated the “sinking” of heavy nuclei in the outer layers. These calculations quantified the heating of the neutron star from these reactions, and the results have been used in simulations of the “freezing” of ions into a lattice in the neutron star crust. A surprise from these simulations is that the “ashes” of these bursts chemically separate as they are compressed to high densities. In an exciting new development, a superburst was detected from an intermittently, or transient, accreting neutron star. Because the crust cools when the neutron star is not accreting, it was previously thought that superbursts could not occur in this system, because the temperature in the crust would be too cool for carbon-12 fusion to occur.
A Strong Shallow Heat Source In the Accreting Neutron Star MAXI J0556-332,” A. Deibel, A. Cumming, E.F. Brown, and D. Page, Astrophys. Jour. Lett., in press (2015)
Strong neutrino cooling by cycles of electron capture and β- decay in neutron star crusts, H. Schatz et al., Nature, 505, 62 (2014)
The Equation of State from Observed Masses and Radii of Neutron Stars, A. W. Steiner, J. M. Lattimer, and E. F. Brown, Astrophys. J. 722, 33 (2010)
Mapping Crustal Heating with the Cooling Lightcurves of Quasi-Persistent Transients, E. F. Brown and A. Cumming, Astrophys. J. 698, 1020 (2009).
Possible Resonances in the 12C+12C Fusion Rate and Superburst Ignition, R.L. Cooper, A.W. Steiner, and E.F. Brown, Astrophys. Jour., 702, 660 (2009)