Photospheric radius expansion during magnetar bursts

TL;DR

This paper investigates the possibility of photospheric radius expansion (PRE) during magnetar bursts, similar to thermonuclear bursts, which could help measure magnetic fields, distances, and gravitational redshift in magnetars.

Contribution

It proposes a mechanism for double-peaked PRE events in magnetar bursts and discusses how identifying PRE can constrain key neutron star parameters.

Findings

Predicted critical flux matches observed burst flux in SGR 0501+4516.

Mechanism for double-peaked PRE in magnetar bursts is plausible.

Identification of magnetic Eddington limit can constrain magnetic field and distance.

Abstract

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.