On 27 December 2004, a giant flare from an object known as a soft gamma-ray repeater, SGR 1806-20, was detected by a whole host of satellites. SGRs are characterized by short, recurrent bursts of soft gamma-rays and are prime candidates for magnetars — isolated neutron stars with extremely intense magnetic fields. In the case of SGR 1806-20, its flare was so bright — the largest such explosion ever seen in the Universe — that it ‘blinded’ all of the detectors onboard the satellites. When their sight returned, however, the tail-end of the hyperflare was still visible. In this week’s Astrophysical Journal, GianLuca Israel and colleagues1 reveal the existence of a variety of oscillations in this tail, enabling them to probe the inside of a neutron star to an unprecedented level.

Based on high-resolution data taken with the Rossi X-Ray Timing Explorer (RXTE), the group analysed in detail the timing of X-ray signals from the flare. They have detected rapid quasi-periodic oscillations (QPOs) within the pulsating tail of the event, which appear 170–220 seconds after the onset of the explosion and die away after about ten minutes. The frequency of these QPOs, 92.5 hertz, is much larger than the rotation of the neutron star and strongly suggests that they originate from resonant oscillation modes of the star. In addition, there is evidence for further oscillations at about 18 and 30 hertz between 200 and 300 seconds after the main event.

So what do these vibrations mean? The frequency of such modes depends on the star radius and on the sound (or Alfvén) speed in the stellar interior. This in turn depends on the magnetization of the material through which the waves propagate. As such, the frequency of the detected QPOs is extremely informative.

In the context of magnetars, the main spike of the flare corresponds to a plasma fireball that travels away from the neutron star at relativistic speeds, sending seismic shocks throughout the star. This fireball can be held back by the star’s magnetic field and, on cooling, can create the long pulsating tails observed. As the seismic energy dissipates, the Alfvén waves may be temporarily enhanced, giving rise to rapid oscillations which are 'imprinted' in the X-rays given off (the 92.5-hertz QPOs). The lower-frequency harmonics can be accounted for by secondary fracturing of the star’s crust.