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Abstract: On August 24th 2008 the new magnetar SGR 0501+4516 discovered by SWIFTemitted a bright burst with a pronounced double-peak structure in hard X-rays,reminiscent of the double-peak temporal structure seen in some brightthermonuclear bursts on accreting neutron stars. In the latter case this is dueto Photospheric Radius Expansion PRE: when the flux reaches the Eddingtonlimit, the photosphere expands and cools so that emission becomes softer anddrops temporarily out of the X-ray band, re-appearing as the photospheresettles back down. We consider the factors necessary to generate double-peakedPRE events, and show that such a mechanism could plausibly operate in magnetarbursts, despite the vastly different emission process. Identification of themagnetic Eddington limit in a magnetar would constrain magnetic field anddistance and could, in principle, enable a measurement of gravitationalredshift. It would also locate the emitting region at the neutron star surface,constraining the burst trigger mechanism. Conclusive confirmation of PRE eventswill require more detailed radiative models for bursts. However for SGR0501+4516 the predicted critical flux using the magnetic field strengthinferred from timing and the distance suggested by its probable location in thePerseus arm of our Galaxy is consistent with that observed in the August 24thburst.



Author: Anna L. Watts, Chryssa Kouveliotou, Alexander J. van der Horst, Ersin Gogus, Yuki Kaneko, Michiel van der Klis, Ralph A.M.J. Wije

Source: https://arxiv.org/



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