Most scientists found the idea too much to swallow. In 1992, Robert Duncan and Christopher Thompson put forward a model of a new type of pulsar, a pulsar with extraordinary properties. The theory attempted to explain a number of strange, so far unexplained sources of gamma rays called Soft Gamma Ray Repeaters (SGRs). These distant objects unpredictably emitted strong bursts of gamma rays, quietening down over a matter of seconds, and then remaining dormant for years only to flare up again unexpectedly. What was curious about these objects was the same thing that attracted Jocelyn Bell’s attention more than two decades before. Just as Bell’s ‘scruff’ turned out to be a periodic signal, so the powerful if brief emission of gamma rays from SGRs was not a steady hiss, but a series of spikes. This implied rotation of a small, energetic, and dense object, and the best candidate available was a neutron star. What Duncan and Thompson proposed was that SGRs were a new breed of pulsar, perhaps even outnumbering the normal ones. This new type of pulsar had an important distinguishing characteristic: its magnetic field was hundreds, perhaps thousands of times more powerful than those known to be powering ordinary pulsars. Like so many new horizons of astronomy, the discovery of SGRs that led to the discovery of these magnetic monsters came about completely by accident.
KeywordsNeutron Star Large Magellanic Cloud Intense Magnetic Field Soft Gamma Repeater Giant Flare
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- 1.Subsequent observations allowed astronomers to not only detect gamma ray bursts, but to plot them on the sky. It is now known that these sources of gamma rays are distributed evenly across the sky, confirming their extragalactic origins.Google Scholar
- 2.SGR 1900+14 was initially identified with a young supernova remnant, this time within the Galaxy. During the 1990s it was thought that, like SGR 0526-66 in the LMC, SGR 1900+ 14 had been propelled out of the center of its remnant at the time of the explosion. However, Rob Duncan notes that in 2008 ‘there are now good reasons to believe that the young supernova remnant (SNR) within which SGR 1900+14’s sky position resides (displaced from the center) is just a chance overlap of objects at different distances from Earth and that the SGR was not born in the supernova which made that SNR. Careful observations have shown that SGR 1900+14’s angular position also lies within a compact cluster of young stars in the distant galactic disk. Although intrinsically very luminous, the massive young stars of this cluster are not easy to see in optical light; they are bright only in the infrared (IR) because of many kiloparsecs of intervening galactic dust. Such young, compact star clusters cover only a very tiny fraction of the sky in the galactic plane, a circumstance which makes a chance overlap at different distances from Earth unlikely. But the circumstance which really strongly bolsters this interpretation (i.e., that the SGR is actually in the cluster, and not associated with SNR) is that SGR 1806-20’s angular position also overlaps with a very rare, very compact cluster of bright young, massive stars far across the Galaxy. The a posteori chance of the two most active galactic SGRs having such positions overlapping with these tiny clusters, yet with no physical cluster associations, is something like a million-to-one. On the other hand, there is no young, compact cluster associated with SGR 0526-66. For this SGR, a physical association with the SNR N49 in the LMC still seems plausible.’Google Scholar
- 3.Burst and Transient Source Experiment.Google Scholar
- 4.Specifically, events unfolded thus: In their paper published in 1992 Duncan and Thompson made the connection of the theoretical magnetars with SGRs. Then in a paper published in 1993 (Thompson & Duncan 1993 Ap J 408, 194; sections 14.4 and 15.2) they suggested that the unusual X-ray star 1E 2259+586 was a magnetar. Finally, at a conference in March 1995 (with proceedings published in 1996) Duncan and Thompson suggested that the emerging class of X-ray stars resembling (and including) 1E 2259+586 which they called ‘AXPs’ were magnetars. This was eventually verified when the AXPs were found to emit SGR-like bursts. Now there are nine known AXPs, and a few more good candidates.Google Scholar
- 5.A common hand-held magnet has a field strength of perhaps a hundred gauss, while the strongest magnetic fields created in the laboratory are around 100,000 gauss.Google Scholar