Two New Calcium-Rich Gap Transients in Group and Cluster Environments

Calcium-rich gap transients are a recently discovered class of transients, characterized by peak luminosities in the “gap” between supernovae and novae, rapid photometric and spectroscopic evolution, and a nebular spectrum dominated by [Ca II] emisison. We present two new events discovered by the Palomar Transient Factory: PTF11kmb and PTF12bho. A striking feature of both transients are their host environments: PTF12bho is an intra-cluster transient in the Coma Cluster, while PTF11kmb is located in a loose galaxy group, and both are offset >20 kpc from any plausible host galaxy. Deep imaging from Subaru and the Hubble Space Telescope constrains the presence of any underlying faint galaxies or globular clusters at the locations of the transients. We discuss the offset distribution and host galaxy demographics of Ca-rich gap transients as a class, showing that both are more extreme than that observed for either Type Ia SNe and even short-duration gamma-ray bursts, and that the majority of events found to date are found in galaxy groups or clusters. Finally, we discuss the implications for the progenitor systems of Ca-rich gap transients.