  On August 24th 2008 the new magnetar SGR 0501+4516 (discovered by SWIFT)
emitted a bright burst with a pronounced double-peak structure in hard X-rays,
reminiscent of the double-peak temporal structure seen in some bright
thermonuclear bursts on accreting neutron stars. In the latter case this is due
to Photospheric Radius Expansion (PRE): when the flux reaches the Eddington
limit, the photosphere expands and cools so that emission becomes softer and
drops temporarily out of the X-ray band, re-appearing as the photosphere
settles back down. We consider the factors necessary to generate double-peaked
PRE events, and show that such a mechanism could plausibly operate in magnetar
bursts, despite the vastly different emission process. Identification of the
magnetic Eddington limit in a magnetar would constrain magnetic field and
distance and could, in principle, enable a measurement of gravitational
redshift. It would also locate the emitting region at the neutron star surface,
constraining the burst trigger mechanism. Conclusive confirmation of PRE events
will require more detailed radiative models for bursts. However for SGR
0501+4516 the predicted critical flux (using the magnetic field strength
inferred from timing and the distance suggested by its probable location in the
Perseus arm of our Galaxy) is consistent with that observed in the August 24th
burst.
