Hydrogen Burning on Magnetar Surfaces

We compute the rate of diffusive nuclear burning for hydrogen on the surface of a "magnetar" (soft gamma-ray repeater [SGR] or anomalous X-ray pulsar [AXP]). We find that hydrogen at the photosphere will be burned on an extremely rapid timescale of hours to years, depending on the composition of the underlying material. Improving on our previous studies, we explore the effect of a maximally thick "inert" helium layer, previously thought to slow down the burning rate. Since hydrogen diffuses faster in helium than through heavier elements, we find this helium buffer actually increases the burning rate for magnetars. We compute simple analytic scalings of the burning rate with temperature and magnetic field for a range of core temperatures. We conclude that magnetar photospheres are very unlikely to contain hydrogen. This motivates theoretical work on heavy element atmospheres that are needed to measure the effective temperature from the observed thermal emission and constrains models of AXPs that rely on magnetar cooling through thick light element envelopes.