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.