Selected Publications: Low-temperature triple-alpha rate in a full three-body model," N.B. Nguyen, F.M. Nunes, I.J. Thompson, and E.F. Brown, Phys. Rev. Lett., in press (2012)
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)
First Superburst from a Classical Low-mass X-ray Binary Transient. L. Keek et al., Astron. Astrophys., in press (2008)
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.
Cutaway of an accreting neutron star, with a recent calculation (Keek et al. 2008) of the temperature across the crust shown in the inset plot. The calculation is for an intermittently accreting neutron star that produced an energetic explosion just 50 days after it started accreting, before the crust could heat sufficiently. The plot shows the temperature when accretion began (solid line) and at the time of the superburst (dotted line). In order for the superburst to have ignited, the dotted line should pass through the red rectangle. The fact that it doesnt suggests that another source of heating must be present in the crust, or that the fusion cross section is much larger than currently estimated.