ar X iv : a st ro - p h / 98 10 44 9 v 1 2 8 O ct 1 99 8 Fireball / Blastwave Model and Soft γ - ray Repeaters

Soft γ-ray repeaters are at determined distances and their positions are known accurately. If observed, afterglows from their soft γ-ray bursts will provide important clues to the study of the so called " classical γ-ray bursts ". On applying the popular fireball/blastwave model of classical γ-ray bursts to soft γ-ray repeaters, it is found that their X-ray and optical afterglows are detectable. Monitoring of the three repeaters is solicited. Since their discovery nearly thirty years ago, γ-ray bursts (GRBs) have made one of the biggest mysteries in astrophysics, primarily because they have remained invisible at wavelengths other than γ-rays so that the distances are unknown. 1 The recent detection of X-ray, optical and even radio afterglows from some GRBs located by the Italian-Dutch BeppoSAX satellite has opened up a new era. 2,3 The possible host galaxy of GRB 970228 and the determined redshift 0.835 < z < 2.1 for GRB 970508 strongly indicate a cosmological origin. Fireball model becomes the most popular and successful model for GRBs. After the main GRB, the blastwave generated between the GRB ejecta and the interstellar medium (ISM) provides a natural explanation for the power-law decay of the observed low energy afterglows. 2−4 We call such a GRB scenario as a fireball/blastwave model. However, so few GRBs have been located rapidly and accurately enough for us to search for their afterglows, that the cosmological origin of GRBs and the correctness of the fireball/blastwave model still need more tests. GRBs occurring at a definite distance and in a fixed direction would be ideal for checking the model. Here we suggest that soft γ-ray repeaters (SGRs), whose nature is much clearer, might be good candidates. As a subtle class of GRBs, SGRs are characterized mainly by their soft spectra and unpredictable recurrences. 5 There are only three known SGRs: 0526−66, 1806−20 and 1900+14, all of them have been tentatively associated with supernova remnants (SNRs), indicating a neutron star origin. Recently, Hurley et al. (1997) seem to have observed a new (4th) soft γ-ray