Compact Objects Merging with Stars as an Origin of Ultra-Long Gamma-Ray Bursts and Luminous Fast Blue Optical Transients

Abstract: Ultra-long gamma-ray bursts (ULGRBs) and luminous fast blue optical transients (LFBOTs) are two rare classes of engine-driven transients whose physical connection remains unknown. It has been suggested that both may arise from the mergers of a massive helium core with a compact object. We investigate this common origin by reanalyzing the optical counterpart of the highly unusual GRB 111209A/SN 2011kl associated with an ULGRB in the context of a recently developed, analytical LFBOT model. We find that SN 2011kl is broadly consistent with an LFBOT origin, exhibiting a rapid, luminous and blue early emission. However, compared to the LFBOT population, SN~2011kl features a longer "plateau" of emission ~2 weeks post-merger, suggesting an extended pre-merger mass-loss history, as well as stronger UV suppression. We additionally compare the host galaxy environments of five ULGRBs to those of LFBOTs and classical LGRBs. We find that ULGRBs, similar to LFBOTs and long GRBs, tend to occur in lower mass (<10^10 solar masses) galaxies with higher amounts of active star formation than observed for field galaxy populations at similar redshifts. Together, these results support a shared progenitor for at least a subset of ULGRBs and LFBOTs.