GRBs & Magnetars

Gamma-ray Birth Cries Suggest Massive Magnetic Engines

Swift's Burst Alert Telescope (BAT) detected the GRB in the constellation Pictor on 29 July 2006.

XRT picked up GRB 060729 (named for the date of its first observation) 124 seconds after the BAT detected it. Normally, XRT monitors an afterglow for a week or two until it fades to near invisibility. But GRB 060729's afterglow started off so bright and faded so slowly that XRT could regularly monitor it for months, and the instrument still was able to detect it in late November. The burst's relatively close proximity to Earth, about 5 billion light-years, also was a factor in XRT's ability to monitor the afterglow for such an extended period.

The slow fading of the X-ray afterglow has several important ramifications for our understanding of GRBs. "It requires a larger energy injection than we normally see in bursts, and may require continuous energy input from the central engine," says astronomer Dirk Grupe of Penn State University, in University Park, Pennsylvania, who is lead author of the international team that reports these results in a paper scheduled to appear in the June 20, 2007 issue of the Astrophysical Journal.

One possibility is that the central engine, perhaps an accreting black hole, ejected multiple shells of material at near light speed. Forward shells may have decelerated when they slammed into interstellar gas, allowing back shells to catch up and slam into them with tremendous force. The resulting shock waves could have powered the afterglow and made it shine brightly in X-rays.

But another possibility is that the GRB's central engine was a magnetar -- a neutron star with an ultra-powerful magnetic field. The magnetar's magnetic field acts like a brake, forcing the star's rotation rate to spin down rapidly. The energy of this spin-down can be converted into magnetic energy that is continuously injected into the initial blast wave that triggered the GRB. Calculations show that this energy could power the observed X-ray afterglow and keep it shining for months.
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"Quantum events are taking place all around us. They are very, very small. Some of these small quantum events caught up in the process of rapid expansion of space became galaxies along the way. During inflation, quantum fluctuations can produce not only galaxies, but also new parts of the universe. An infinite number of worlds could exist with different types of physical laws operating among them." - Andrei Linde
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