ar X iv : a st ro - p h / 98 06 02 4 v 1 2 J un 1 99 8 Is GRO J 1744 - 28 a Strange Star ?

The unusal hard x-ray burster GRO J1744-28 recently discovered by the Compton Gamma-ray Observatory (GRO) can be modeled as a strange star with a dipolar magnetic field ≤ 10 11 Gauss. When the accreted mass of the star exceeds some critical mass, its crust may break, resulting in conversion of the accreted matter into strange matter and release of energy. Subsequently, a fireball may form and expand relativistically outward. The expanding fireball may interact with the surrounding interstellar medium, causing its kinetic energy to be radiated in shock waves, 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. GRO J1744-28 is a new type of x-ray transient source, which was discovered on 2 December 1995 by the Burst And Transient Source Experiment (BATSE) on the Compton Gamma-Ray Observatory (1). The bursts were detected up to energies of ∼ 75 keV with intervals between bursts of about 200 s initially. After 2 days the burst rate dropped to about one per hour (2). However, by 15 January 1996 the burst rate had increased to ∼ 40 per day. The burst durations lasted ∼ 10 s. The burst fluences (25-60 keV) range between