Faint Rapid Red Transients from Neutron Star - CO White Dwarf Mergers

Mergers of neutron stars (NS) and white dwarfs (WD) may give rise to observable explosive transientevents. We use 3D hydrodynamical (SPH) simulations, as well as 2D hydrodynamical-thermonuclearsimulations (using the FLASH AMR code) to model the disruption of CO-WDs by NSs, whichproduce faint transient events. We post-process the simulations using a large nuclear network andmake use of the SuperNu radiation-transfer code to predict the observational signatures and detailedproperties of these transients. We calculate the light-curves (LC) and spectra for five models of NS -CO-WD mergers. The small yields of56Ni (few×10−3M) result in faint, rapidly-evolving reddenedtransients (RRTs) with B (R) - peak magnitudes of at most∼−12 (−13) to∼−13 (−15), muchshorter and fainter than both regular and faint/peculiar type-Ia SNe. These transients are likely to beaccompanied by several months-long, 1–2 mag dimmer red/IR afterglows. We show that the spectraof RRTs share some similarities with rapidly-evolving transients such as SN2010x, although RRTsare significantly fainter, especially in the I/R bands, and show far stronger Si lines. We estimate thatthe upcoming Large Synoptic Survey Telescope could detect RRTs at a rate of up to∼10−70 yr−1,through observations in the R/I bands. The qualitative agreement between the SPH and FLASHapproaches supports the earlier hydrodynamical studies of these systems.