Gamma-Ray Burst in a Binary System

Regardless of their different types of progenitors and central engines, gamma-ray bursts (GRBs) were always assumed to be standalone systems after they formed. Little attention has been paid to the possibility that a stellar companion can still accompany a GRB itself. This paper investigates such a GRB-involved binary system and studies the effects of the stellar companion on the observed GRB emission when it is located inside the jet opening angle. Assuming a typical emission radius of $\sim10^{15}\,$cm, we show that the blockage by a companion star with a radius of $R_\mathrm{c}\sim67\,\mathrm{R_\odot}$ becomes non-negligible when it is located within a typical GRB jet opening angle (e.g., $\sim10$ degrees) and beyond the GRB emission site. 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.