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Photospheric radius expansion during magnetar bursts

Watts, Anna L. and Kouveliotou, Chryssa and van der Horst, Alexander J. and Göğüş, Ersin and Kaneko, Yuki and van der Klis, Michiel and Wijers, Ralph A. M. J. and Harding, Alice K. and Baring, Matthew G. (2010) Photospheric radius expansion during magnetar bursts. Astrophysical Journal, 719 (1). pp. 190-200. ISSN 0004-637X

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Official URL: http://dx.doi.org/10.1088/0004-637X/719/1/190

Abstract

On 2008 August 24 the new magnetar SGR 0501+4516 (discovered by Swift) emitted a bright burst with a pronounced double-peaked structure in hard X-rays, reminiscent of the double-peaked 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 24 burst.

Item Type:Article
Uncontrolled Keywords:pulsars: individual (SGR 0501+4516); stars: neutron; stars: magnetic field; X-rays: bursts
Subjects:Q Science > QB Astronomy > QB460-466 Astrophysics
Q Science > QB Astronomy
ID Code:14270
Deposited By:Ersin Göğüş
Deposited On:24 Aug 2010 12:00
Last Modified:25 May 2011 14:17

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