Photospheric radius expansion during magnetar bursts

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.

12 pages, 3 figures, ApJ in press

References in corpus (10)

• The Fermi Gamma-Ray Burst Monitor
• Physics of Strongly Magnetized Neutron Stars
• Thermonuclear (type-I) X-ray bursts observed by the Rossi X-ray Timing Explorer
• 1E 1547.0-5408: a radio-emitting magnetar with a rotation period of 2 seconds
• A Swift gaze into the 2006 March 29th burst forest of SGR 1900+14
• Discovery of a new Soft Gamma Repeater: SGR J0418+5729
• The 2008 May burst activation of SGR 1627-41
• Biases for neutron-star mass, radius and distance measurements from Eddington-limited X-ray bursts
• SGR 1806-20 about two years after the giant flare: Suzaku, XMM-Newton and INTEGRAL observations
• Suzaku Observation of the New Soft Gamma Repeater SGR 0501+4516 in Outburst

Cited by in corpus (9)

• Magnetars: the physics behind observations
• The Five Year Fermi/GBM Magnetar Burst Catalog
• The wind nebula around magnetar Swift J1834.9-0846
• Fermi/GBM Observations of SGR J0501+4516 Bursts
• Compton scattering S-matrix and cross section in strong magnetic field
• Extended Tails from SGR 1806-20 Bursts
• Models of hydrostatic magnetar atmospheres at high luminosities
• Spectral Modification of Magnetar Flares by Resonant Cyclotron Scattering
• On the X-ray emission mechanisms of the persistent source and very low-fluence bursts of SGR J0501+4516