Faint rapid red transients from Neutron star -- CO white-dwarf mergers

TL;DR

This paper models faint, rapidly-evolving red transients resulting from neutron star and CO white dwarf mergers, predicting their observational signatures and potential detection rates with upcoming surveys.

Contribution

It provides detailed hydrodynamical simulations and radiation transfer modeling of NS-CO WD mergers, revealing their faint transient signatures and spectral features, which were not previously characterized.

Findings

Faint transients with peak magnitudes around -12 to -15 in R-band.

Spectra show strong Si lines and similarities to rapidly-evolving transients.

Estimated detection rate of 10-70 events per year with LSST.

Abstract

Mergers of neutron stars (NS) and white dwarfs (WD) may give rise to observable explosive transient events. We use 3D hydrodynamical (SPH) simulations, as well as 2D hydrodynamical-thermonuclear simulations (using the FLASH AMR code) to model the disruption of CO-WDs by NSs, which produce faint transient events. We post-process the simulations using a large nuclear network and make use of the SuperNu radiation-transfer code to predict the observational signatures and detailed properties of these transients. We calculate the light-curves (LC) and spectra for five models of NS - CO-WD mergers. The small yields of Ni56 (few x 0.001Msun) results in faint, rapidly-evolving reddened transients (RRTs) with B (R) - peak magnitudes of ~ -12 (-13) to ~ -13 (-15), much shorter and fainter than both regular and faint/peculiar type-Ia SNe. We show that the spectra of RRTs share some similarities with rapidly - evolving transients such as SN2010x, though RRTs are significantly fainter, especially in the I/R bands, and show far stronger Si lines. We estimate that the upcoming Large Synoptic Survey Telescope could detect RRTs at a rate of ~ 10 - 70 yr^-1, through observations in the R/I bands.