Faint rapid red transients from neutron star–CO white dwarf mergers

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 postprocess 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 56 Ni (few×10 −3 M ) result 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 rapidlyevolving 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.

Spatially Resolved X-Ray Study of Supernova Remnant G306.3–0.9 with Unusually High Calcium Abundance

“…At 8 kpc, where the ISM mixing requirement of NS-WD merger models can be satisfied, the SN explosion energy would be only ∼ 4.5 × 10 49 erg, which is too low for a typical SN explosion or the 1D NS-WD merger model of Margalit & Metzger (2016). Although the more sophisticated 2D simulations of Zenati et al (2019Zenati et al ( , 2020 obtained similarly low explosion energy of 10 48 -10 49 erg for NS-CO WD mergers, they showed that the He WD cases cannot lead to any thermonuclear explosion or mass ejection. On the contrary, the explosion energy at 20 kpc is consistent with Ca-rich transient SN 2005E and He shell detonation models (Perets et al 2010;Waldman et al 2011).…”

“…One of its significant characteristics is the Ca-rich ejecta and the mass of the Ca element may reach ∼ 10 −1 M e . A natural explanation for Ca-rich ejecta is the burning of He-rich matter and there are two leading nucleosynthesis models so far: (i) the accreted WD He shell detonation (Waldman et al 2011); and (ii) the merger of a He-rich WD and a NS (Margalit & Metzger 2016;Zenati et al 2020). Both models predict very large Ar/Si and Ca/Si ratios in the ejecta.…”

Spatially Resolved X-ray Study of Supernova Remnant G306.3$-$0.9 with Unusually High Calcium Abundance

“…One of its significant characteristics is the Ca-rich ejecta and the mass of the Ca element may reach ∼ 10 −1 M ⊙ . A natural explanation for Ca-rich ejecta is the burning of He-rich matter and there are two leading nucleosynthesis models so far: i) the accreted WD He shell detonation (Waldman et al 2011); ii) the merger of a He-rich WD and a NS (Margalit & Metzger 2016;Zenati et al 2020). Both models predict very large Ar/Si and Ca/Si ratios in the ejecta.…”

“…At 8 kpc, where the ISM mixing requirement of NS-WD merger models can be satisfied, the SN explosion energy would be only ∼ 4.5 × 10 49 erg, which is too low for a typical SN explosion or the 1D NS-WD merger model ofMargalit & Metzger (2016). Although the more sophisticated 2D simulations ofZenati et al (2019Zenati et al ( , 2020 obtained similarly low explosion energy of 10 48 -10 49 erg for NS-CO WD mergers, they showed that the He WD cases…”

Detectability of neutron star — White dwarf coalescences by eROSITA and ART-XC

“…Thus, bright transients might appear at different wavelengths, including optical range. 2 In [2] the authors studied the outcome of NS-WD mergers in some details from the point of view of electromagnetic counterparts (see also a recent e-print [8]). In particular, they suggested that due to jet formation a high-energy transient can appear.…”