NUMERICAL-STUDIES OF PHOTON BUBBLE INSTABILITY IN A MAGNETIZED, RADIATION-DOMINATED ATMOSPHERE

An initially static, plane-parallel, radiation pressure supported expo nential atmosphere in a strong magnetic field is found to be unstable to the growth of buoyant low-density regions or ''photon bubbles'' in linear stability analysis. Here we present a series of numerical studi es of the photon bubbles in such an atmosphere carried out using a qua si-two-dimensional radiation hydrodynamical code. When a single-mode p erturbation is applied to the atmosphere, we find that the growth of t hese bubbles is in good agreement with the linear theory. When the evo lution becomes nonlinear, the growth of the photon bubbles is found to be an efficient mechanism of energy transport. The presence of the lo w-density regions helps to increase the photon diffusion speed by a fa ctor of several, while the buoyancy of the bubbles serves to transport energy via advection. Multimode studies, consisting of a perturbation with the linear combination of two single modes and a random perturba tion, suggest that there is a tendency toward merger of the photon bub bles in their transport properties. The small wavenumber modes eventua lly dominate in radiation pressure, while the large wavenumber modes, although having higher growth rates, also saturate more quickly, and t heir contribution to the energy transport can be truncated. A grid stu dy and the multimode calculations indicate that the energy transport t hrough the atmosphere is well represented as long as the photon bubble mode with optical depth of similar to 10 wavelength is well resolved. Possible applications of the photon bubble instability includes the s ettling layer in the accretion column of a neutron star undergoing sup er-Eddington accretion. The enhanced energy transport may manifest its elf in the emergent spectrum from the accreting pulsars and in the sho rt-time variability in the light curves.