Bursts and Flares from Highly Magnetic Pulsars

Abstract

In January 1979, for the first time, recurrent short (<100ms) and intense γ-ray bursts were detected from a source in the direction of the Galactic center, later named SGR1806-20. Two months later, in March 1979, rapid (~8s) coherent X-ray pulsations were discovered during an extremely intense flare (FX ~ 10⁴⁴ ergs s^-1 cm^-2) from SGR0506-66, the prototype of the magnetar class. Since then, many advances have been achieved in the study of this class of high energy emitting isolated neutron stars, thought to shine in the X-rays due to the decay of the strongest magnetic fields present in the Universe (> 10¹⁵ Gauss). The results obtained through the study of the transient phenomena (mainly burst and flares) displayed by some of them and covering more than 10 orders of magnitudes in terms of time scale variability (from fraction of milli-seconds up to years) and of flux (from 10^-9 up to 1 ergs s^-1 cm^-2), will be presented and discussed with particular emphasis on the most recent results obtained. Among others are: the discovery of rapid QPOs in the giant flares of SGR1806-20 and SGR1900+14, the study of intermediate flares from SGR1900+14, 1E1547.0-5408 and SGR0501+4516. The high frequency oscillations discovered in the tails of giant flares from two magnetars are thought to be the first direct detections of seismic vibrations from neutron stars. The possibility of starquakes associated with the giant flares triggering global vibrations opens up the prospect of using seismology to study the interior structure and composition of neutron stars, and to infer information on the crust thickness. This is a major breakthrough in the study of the nature of matter under conditions of extreme pressure. The time resolved spectroscopic studies of the flare emission from magnetarswas instead used to infer information related to the neutron star and magnetosphere structure, the emitting processes, and the confined fireball properties. In particular, within the magnetar model, the majority of these findings can be accounted for in terms of thermalised emission from the E-mode and O-mode photospheres.