I discuss the possibility of Photospheric Radius Expansion (PRE) during magnetar bursts. Identification of PRE would enable a determination of the magnetic Eddington limit (which depends on field strength and neutron star mass and radius), and shed light on the burst mechanism. This work is inspired by a Fermi GBM observation of a burst of the magnetar SGR 0501+4516 on 2008 August 24, which showed a double peaked light curve reminiscent of PRE in thermonuclear X-ray bursts. To investigate whether PRE could occur in magnetar bursts, we have modeled hydrostatic atmospheres in a strong radial magnetic field, determining both their maximum extent and photospheric temperatures. We find that spatially-extended atmospheres cannot exist in such a field configuration: typical maximum extent for magnetar-strength fields is 10m (as compared to 200 km in the non-magnetic case).
Achieving balance of gravitational and radiative forces over a large range of radii, which is critical to the existence of extended atmospheres, is rendered impossible in strong fields due to the dependence of opacities on temperature and field strength. We conclude that high luminosity bursts in magnetars does not lead to expansion and cooling of the photosphere, as in the non-magnetic case. We also find the maximum luminosity that can propagate through a hydrostatic magnetar atmosphere to be lower than previous estimates. We find that the photospheres associated with the two different polarization modes are always very close together, both spatially and in temperature. This has implications for the interpretation of spectral fits of Fermi GBM data, which are often well described by two black bodies. Our results show that these two black bodies cannot be ascribed to extended photospheres.