Deep Crustal Heating of Neutron Stars in an Accreting Binary

Undergraduate #81
Discipline: Physics
Subcategory: Astronomy and Astrophysics

Zachary Langford - Everett Community College

In the case of a neutron star in a binary system, the neutron star will accrete matter from its binary companion due to its immense gravity and close proximity. When this occurs, the accreted matter will form an accretion disk which will cause a torque on the magnetic field, decreasing the rotational period, and eventually fall onto the star and push the crust into the core of the star. The crust of the neutron star is in nuclear statistical equilibrium, but the new accreted matter will not be. This will cause the star to start producing heat due to the nuclear interactions in the newly accreted crust. The heat added from nuclear interactions depends on what interactions are happening which depends on the density the accreted matter reaches. Current calculations of this ‘deep crustal heating’ assume a fully accreted crust, meaning the initial crust of the star has been pushed down into the core and is now comprised completely of accreted matter. This does not fit with our observational data of certain accreting binary systems. The energy we observe is lower than we would expect. Since we know that accretion only happens about 1-10% of the time it is reasonable to say that the crust may not be fully accreted, but a combination of accreted matter and original crust. Using neutron star equations of state, that systematically span known uncertainties, we look at the magnetic field evolution, period, and depth of the accreted crust as a function of time the star has been accreting matter.

References: Haensel, P., Zdunik, J. L., A&A, 227 (1990). Wang, J., Zhang, C. M., Zhao, Y. H., Kojima, Y., Yin, H. X., Song, L. M., A&A, 526 (2011). Wijnads, R., Degenaar, N., Page, D., MNRAS, 432 (2013).

Funder Acknowledgement(s): This work was done as a part of the REU program in Physics and Astronomy at Texas A&M - Commerce funded by the National Science Foundation under grant no. PHY-1359409.

Faculty Advisor: William Newton, william.newton@tamuc.edu

Role: I created and used computer models of the period evolution and the deep crustal heating, as well as generated a number of equations of state using various Skyrme models and programs written by William Newton.