Mini-supernovae from white dwarf-neutron star mergers: Viewing-angle-dependent spectra and lightcurves

Unstable mass transfer may occur during white dwarf-neutron star (WD-NS) mergers, in which the WD can be tidally disrupted and form an accretion disk around the NS. Such an accretion disk can produce unbound wind ejecta, with synthesized $^{56}\mathrm{Ni}$ mixed in. Numerical simulations reveal that this unbound ejecta should be strongly polar-dominated, which may cause the following radioactive-powered thermal transient to be viewing-angle-dependent. This issue has so far received limited investigation. We investigate how the intrinsically non-spherical geometry of WD-NS wind ejecta affects the viewing-angle dependence of the thermal transients. Using a two-dimensional axisymmetric ejecta configuration and incorporating heating from the radioactive decay of $^{56}\mathrm{Ni}$, we employ a semi-analytical discretization scheme to simulate the observed viewing-angle-dependent photospheric evolution, as well as the resulting spectra and lightcurves. The observed photosphere evolves over time and depends strongly on the viewing angle: off-axis observers can see deeper, hotter inner layers of the ejecta and larger projected photospheric areas compared to on-axis observers. For a fiducial WD-NS merger producing 0.3 solar mass of ejecta and 0.01 solar mass of synthesized $^{56}\mathrm{Ni}$, the resulting peak optical absolute magnitudes of the transient span from ~ -12 mag along the polar direction to ~ -16 mag along the equatorial direction, corresponding to luminosities of $10^{40}$-$10^{42}$ erg s$^{-1}$. The typical peak timescales are expected to be 3-10 d. We for the first time explore the viewing-angle effect on WD-NS merger transients. Since their ejecta composition and energy sources resemble those of supernovae, yet WD-NS merger transients are dimmer and evolve more rapidly, we propose using “mini-supernovae” to describe the thermal emission following WD-NS mergers.

💡 Research Summary

This paper investigates the thermal transients that follow the merger of a white dwarf (WD) with a neutron star (NS), proposing the term “mini‑supernovae” (mSNe) for these events. The authors focus on the fact that, unlike the roughly spherical ejecta from typical supernovae, the wind outflows generated by the accretion disk in a WD‑NS merger are highly anisotropic, being strongly concentrated toward the polar directions. Using results from Fernández et al. (2019), they construct a two‑dimensional, axisymmetric ejecta model in spherical coordinates, describing the latitudinal mass distribution with a log‑linear function of μ = cos θ. The total ejecta mass (Mₑⱼ ≈ 0.3 M⊙) and the synthesized ⁵⁶Ni mass (M_Ni ≈ 0.01 M⊙) are set as fractions of the original WD mass, reflecting typical values from previous hydrodynamic studies.

Radioactive decay of ⁵⁶Ni → ⁵⁶Co → ⁵⁶Fe supplies the heating source. The authors adopt a semi‑analytical discretization scheme: the ejecta is divided into angular zones, each treated as a separate slab with its own density, velocity, and temperature evolution. For each zone they compute the location of the photosphere (optical depth τ ≈ 2/3) as a function of time, and derive the corresponding photospheric temperature. By projecting the photospheric surface onto the observer’s line of sight, they obtain the viewing‑angle‑dependent projected area and effective temperature, which together determine the bolometric luminosity via L = σ A_proj T_eff⁴.

The key result is a strong dependence of observable properties on viewing angle. An observer looking down the polar axis sees a relatively thin column of material, leading to a cooler, smaller photosphere and a peak absolute magnitude of about –12 mag (≈10⁴⁰ erg s⁻¹). In contrast, an equatorial observer peers through a denser column, revealing deeper, hotter layers and a larger projected photosphere, producing a peak magnitude of roughly –16 mag (≈10⁴² erg s⁻¹). The peak timescales range from 3 to 10 days, set primarily by the ejecta mass and the diffusion time. Spectrally, the transients start blue/UV‑dominated and evolve toward optical and near‑infrared as the photosphere cools; the equatorial view retains a bluer spectrum for a longer period because of its higher temperature.

Parameter studies show that increasing the ejecta mass shifts the peak to later times and modestly raises the luminosity, while raising the ⁵⁶Ni fraction dramatically brightens the event. The authors also discuss differences expected for CO versus ONe white dwarfs, which affect both the total ejecta mass and the nickel yield.

In the discussion, the paper emphasizes that these “mini‑supernovae” fill a gap between classical supernovae (which are brighter and evolve on weeks‑to‑months timescales) and kilonovae (which are fainter and powered by r‑process heating). The pronounced anisotropy implies that a single event can appear dramatically different depending on orientation, a factor that must be accounted for when interpreting transient surveys. The authors suggest that multi‑band light‑curve modeling, combined with polarization measurements, could constrain the viewing angle and thus confirm a WD‑NS origin. They also advocate for future three‑dimensional radiative‑transfer simulations and for systematic searches in wide‑field surveys such as ZTF and LSST to identify these fast, sub‑luminous transients.

Overall, the study provides the first comprehensive, angle‑dependent modeling of WD‑NS merger thermal emission, establishes the “mini‑supernova” nomenclature, and outlines observational strategies to detect and characterize this previously under‑explored class of astrophysical explosions.