Astrophysics

Observing Starquakes

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Science  15 Nov 2002:
Vol. 298, Issue 5597, pp. 1303
DOI: 10.1126/science.298.5597.1303a

Soft gamma-ray repeaters (SGRs) are a class of rotating neutron stars. Fewer than a dozen SGRs are known, and they are distinctive because, after years of quiet, they suddenly emit extremely luminous and energetic bursts of soft gamma rays. Ibrahim et al. analyzed spectra of SGR 1806–20 collected by the Rossi X-ray Timing Explorer (RXTE) during a series of outbursts in 1996. In the first paper, they discussed the detection of a 5000 eV absorption feature in the spectrum of one precursor, and in the second paper they describe the same absorption feature in other precursors and some of the smaller bursts.

The clear detection of this narrow absorption feature in several spectra and the amount of energy emitted are consistent with proton cyclotron resonances created by a quaking magnetar with a magnetic field strength of 1015 Gauss, one of the strongest magnetic fields ever measured. An outburst is caused by a starquake—literally, the breaking open of the neutron star surface induced by the strong magnetic field. The quaking magnetar releases plasma, and this plasma, caught in the very strong magnetic field, is radiated away from the surface and is also reflected back toward the surface of the star. In doing so, the plasma interacts with hydrogen in the atmosphere or magnetosphere and creates the proton cyclotron resonance observed in the spectra. — LR.

Astrophys. J.574, L51 (2002); Astrophys. J., in press (astro-ph/0210515).

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