Gamma-Ray Burst in a Binary System

Ze-Cheng Zou, Bin-Bin Zhang, Yong-Feng Huang, Xiao-Hong Zhao

Published 2021-06-24Version 1

Typical models of gamma-ray bursts (GRBs) assume that they happen in isolated central engine systems, with little attention being paid to the fact that most stars (including those evolving as GRB progenitors) are in binary or multi-body systems. This paper will study in detail the observational effects when a GRB occurs in a binary system. We show that the blockage by the companion star becomes nonnegligible when it is located within a typical GRB jet opening angle (e.g., 10 degrees) and at an orbit of 67 au, assuming the GRB emission site is at $10^{15}$ cm radius. In such a case, an on-axis observer will see a GRB with a similar temporal behavior but 25% dimmer. On the other hand, an off-axis observer outside the jet opening angle (hence missed the original GRB) can see a delayed "reflected" GRB, which is much fainter in brightness, much wider in the temporal profile and slightly softer in energy. Our study can naturally explain the origin of some low-luminosity GRBs. Moreover, we also point out that the companion star may be shocked if it is located inside the GRB emission site, which can give rise to an X-ray transient or a GRB followed by a delayed X-ray bump on top of X-ray afterglows.