Photospheric radius expansion during magnetar bursts

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.

💡 Research Summary

The paper investigates a bright hard‑X‑ray burst from the newly discovered magnetar SGR 0501+4516 on 24 August 2008, which displayed a pronounced double‑peak temporal structure. Such a morphology is reminiscent of photospheric radius expansion (PRE) events observed in thermonuclear (type I) X‑ray bursts from accreting neutron stars, where the flux reaches the Eddington limit, the photosphere expands and cools, the emission temporarily shifts out of the X‑ray band, and a second peak appears when the photosphere contracts. The authors ask whether a similar mechanism can operate in magnetar bursts despite the very different energy release processes (magnetic reconnection rather than nuclear burning) and the presence of ultra‑strong magnetic fields (10¹⁴–10¹⁵ G).

First, the observational data from Swift/BAT and Konus‑Wind are presented. The burst lasts ≈0.5 s and shows two distinct peaks separated by ≈0.2 s. Spectral analysis reveals that the first peak is hard (30–150 keV), followed by a rapid softening that pushes most of the flux below the detector’s sensitivity, and then a second hard peak of comparable intensity. This pattern matches the classic PRE signature.

The theoretical section derives the conditions required for PRE in a magnetized environment. The standard Eddington luminosity (L_{\rm Edd}=4\pi GMc/\kappa) must be modified to include magnetic suppression of the electron scattering opacity and the additional magnetic pressure term. The resulting “magnetic Eddington limit” can be written as

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