National Aeronautics and Space Administration

Credit: Robert Mallozzi (UAH, MSFC)

An international research team led by Chryssa Kouveliotou has used RXTE to detect 7.5 second pulsations in SGR 1806-20 which, when combined with earlier ASCA observations of the source imply a magnetic field strength about 100 times stronger than the typical neutron star.

SGR 1806-20, a member of the rare class of object known as Soft Gamma Repeaters, was extremely active between October 1996 and November 1997, as evidenced by more than 40 strong bursts detected by BATSE on board CGRO. The BATSE detection triggered a series of five RXTE observations of the source between November 5 -18, 1996.

Power Spectrum Analysis showed the detection of a significant period at 7.47665 seconds; the data folded at this period display a typical X-ray pulsar profile. An significant spindown rate was required to fit the pulse arrival times of the 13 day observation period. With knowledge of the period and it's derivative, the researchers searched the archival ASCA data for periods in this range and uncovered a significant pulsation in a 1993 data set. This elongated time baseline allowed the spindown rate to be more accurately measured, at a value of 8.3 +/- 0.3 x 1e-11 s/s.

A spindown rate of this magnitude implies a superstrong magnetic field, in excess of 1e14 Gauss! An isolated neutron star possessing such a strong magnetic field can be expected to undergo "starquakes", releasing enough energy to power the soft Gamma Ray emissions observed in this class of object. In addition, if up to 10% of neutron stars are formed with such field strengths, and consequently are difficult to observe in radio or X-ray, this might explain the number of supernova remnants without a detectable neutron star at their centers.

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