Soft Gamma-ray Repeaters in Clusters of Massive Stars

Abstract

Infrared observations of the environment of the two Soft Gamma-ray Repeaters (SGRs) with the best known locations on the sky show that they are associated to clusters of massive stars. Observations with ISO revealed that SGR 1806-20 is in a cluster of giant massive stars, still enshrouded in a dense cloud of gas and dust. SGR 1900+14 is at the edge of a similar cluster that was recently found hidden in the glare of a pair of M5 supergiant stars. Since none of the stars of these clusters has shown in the last years significant flux variations in the infrared, these two SGRs do not form bound binary systems with massive stars. SGR 1806-20 is at only ~ 0.4 pc, and SGR 1900+14 at ~ 0.8 pc from the centers of their parental star clusters. If these SGRs were born with typical neutron star runaway velocities of ~ 300 km/s, they are not older than a few 10$^{3}$ years. We propose that SGR 1806-20 and SGR 1900+14 are ideal laboratories to study the evolution of supernovae explosions inside interstellar bubbles produced by the strong winds that prevail in clusters of massive stars.