A new type of transient high-energy source in the direction of the Galactic Centre

Statistical Properties of SGR 1806-20 Bursts

We present statistics of SGR 1806-20 bursts, combining 290 events detected with the Rossi X-Ray Timing Explorer/Proportional Counter Array, 111 events detected with the Burst and Transient Source Experiment, and 134 events detected with the International Cometary Explorer. We find that the fluence distribution of bursts observed with each instrument are well described by power laws with indices 1.43, 1.76, and 1.67, respectively. The distribution of time intervals between successive bursts from SGR 1806-20 is described by a lognormal function with a peak at 103 s. There is no correlation between the burst intensity and either the waiting times until the next burst or the time elapsed since the previous burst. In all these statistical properties, SGR 1806-20 bursts resemble a self-organized critical system, similar to earthquakes and solar flares. Our results thus support the hypothesis that the energy source for soft gamma repeater bursts is crustquakes due to the evolving, strong magnetic field of the neutron star, rather than any accretion or nuclear power.

Is GRO J1744-28 a strange star?

The unusual hard x-ray burster GRO J1744-28 recently discovered by the Compton Gamma-Ray Observatory can be modeled as a strange star with a dipolar magnetic field of </=10(11) gauss. According to this model, when the accreted mass of the star exceeds some critical mass, its crust breaks, resulting in the conversion of the accreted matter into strange matter and a release of energy. Subsequently, a fireball forms and expands relativistically outward. The expanding fireball interacts with the surrounding interstellar medium, causing its kinetic energy to be radiated in shock waves and producing a burst of x-ray radiation. The burst energy, duration, interval, and spectrum derived from such a model are consistent with the observations of GRO J1744-28.