On the Dynamics of Suddenly Heated Accretion Disks around Neutron Stars

Type I X-ray bursts and superbursts on neutron stars release sudden and intense radiation fields into their surroundings. Here, we consider the possible effects of these powerful explosions on the structure of the accretion disk. The goal is to account for the apparent evolution of the innermost regions of the accretion disk around 4U 1820–30 during a superburst. Three different processes are considered in detail: radiatively or thermally driven outflows, inflow due to Poynting-Robertson drag, and a structural change to the disk by X-ray heating. Radiatively driven winds with large column densities can be launched from the inner disk, but only for L/LEdd & 1, which is expected to be obtained only at the onset of the burst. Furthermore, the predicted mass outflow rate is less than the accretion rate in 4U 1820–30. Estimates of the Poynting-Robertson or radiative drag timescale shows that it is a very efficient means of removing angular momentum from the gas. However, the analytical results are likely only applicable at the innermost edge of the disk. X-ray heating gives a change in the disk scale height that is correlated with the blackbody temperature, as seen in the evolution during the 4U 1820–30 superburst. If this change in the scale height can alter the surface density, then the viscous time (with α ∼ 0.03–0.2) is the closest match to the 4U 1820–30 results. We expect, however, that all three processes are likely ongoing when an accretion disk is subject to a sudden heating event. Ultimately, a numerical simulation of a disk around a bursting neutron star is required to determine the exact response of the disk. Magnetic truncation of the accretion flow is also considered and applied to the 4U 1820–30 X-ray reflection results. Subject headings: accretion, accretion disks — radiation mechanisms: thermal — stars: individual (4U 1820– 30) — stars: neutron — X-rays: bursts