Session 84 - Neutron Stars and Their Friends.
Oral session, Thursday, June 13
Historical Society,

[84.02] X-Ray Bursts from Accreting Magnetic Neutron Stars

D. Q. Lamb, M. C. Miller (University of Chicago), R. E. Taam (Northwestern University)

We investigate the mass accretion rate per unit area \dot\sigma and magnetic field strength B for which nuclear burning in the envelope of an accreting neutron star is unstable. When B=0, high \dot\sigma leads to very high temperatures T in the neutron star envelope, due to compressional heating. This stabilizes the nuclear burning. When B\not=0, the electron scattering cross section becomes \sigma_e \gamma \approx (ømega/ømega_c)^2 \sigma_T \approx 10^-2 \sigma_T for all photons traveling along the magnetic field and for photons in the extraordinary mode traveling at large angles to the field with energies \hbar ømega \ll \hbar ømega_C. Thus a very strong magnetic field can dramatically reduce the electron scattering cross section, which is the dominant radiative opacity in the envelope, for radiation escaping outward from the accreted matter. For B < 3 \times 10^12 G, the peak of the blackbody photon number spectrum for T \approx 1 \times 10^8 K (a temperature typical of the neutron star envelope) lies at an energy \hbar ømega > \hbar ømega_C, and the surface magnetic field has little effect on the radiative opacity. Under these conditions, compressional heating again produces very high temperatures in the neutron star envelope, which stabilizes the nuclear burning. Consequently, we do not expect most accretion-powered pulsars to produce Type I X-ray bursts. In contrast, for B \gg 3 \times 10^12 G, the peak of the blackbody photon number spectrum for T \approx 1 \times 10^8 K lies at an energy \hbar ømega \ll \hbar ømega_C for which the electron scattering opacity is dramatically reduced. The enhanced radiative energy transport prevents the neutron star envelope from reaching the very high T otherwise expected for high \dot\sigma. Analytic calculations indicate that under these conditions hydrogen and helium burning can be highly unstable, and consequently that strongly magnetic accreting neutron stars can produce Type I X-ray bursts. MCM acknowledges the support of a Compton Fellowship.