Soft γ-ray repeaters (SGRs) emit multiple, brief (~0.1-s), intense outburst of low-energy γ-rays. They are extremely rare -three are known our Galaxy and one in the Large Magellanic Cloud. Two SGRs are associated with young supernova remnants (SNRs), and therefore most probably with neutron stars, but it remains a puzzle why SGRs are so different from 'normal' radio pulsars. Here we report the discovery of pulsations in the persistent X-ray flux of SGR1806 - 20, with a period of 7.47 s and a spindown rate of 2.6 x 10-3 s yr-1. We argue that the spindown is due to magnetic dipole emission and find that the pulsar age and (dipolar) magnetic field strength are ~1,500 years and 8 x 1014 gauss, respectively. Our observations demonstrate the existence of 'magnetars', neutron stars with magnetic fields about 100 times stronger than those of radio pulsars, and support earlier suggestions that SGR bursts are caused by neutron-star 'crust-quakes' produced by magnetic stresses. The 'magnetar' birth rate is about one per millennium - a substantial fraction of that of radio pulsars. Thus our results may explain why some SNRs have no radio pulsars.

Item Type:	Refereed Article
Research Division:	Physical Sciences
Research Group:	Astronomical sciences
Research Field:	Galactic astronomy
Objective Division:	Expanding Knowledge
Objective Group:	Expanding knowledge
Objective Field:	Expanding knowledge in the physical sciences
UTAS Author:	Dieters, S (Dr Stefan Dieters)
ID Code:	35594
Year Published:	1998
Web of Science® Times Cited:	878
Deposited By:	Mathematics and Physics
Deposited On:	2005-09-14
Last Modified:	2011-11-22
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