Mini-supernovae from white dwarf–neutron star mergers: Viewing-angle-dependent spectra and light curves

Context. 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 56Ni mixed in. Numerical simulations reveal that this unbound ejecta should be strongly polar-dominated, which may cause the subsequent radioactive-powered thermal transient to be viewing-angle-dependent–an issue that has so far received limited investigation. Aims. We investigated how the intrinsically nonspherical geometry of WD–NS wind ejecta affects the viewing-angle dependence of the thermal transients. Methods. Using a two-dimensional axisymmetric ejecta configuration and incorporating heating from the radioactive decay of 56Ni, we employed a semi-analytical discretization scheme to simulate the observed viewing-angle-dependent photospheric evolution, as well as the resulting spectra and light curves. Results. The observed photosphere evolves over time and shows a strong dependence 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 M⊙ of ejecta and 0.01 M⊙ of synthesized 56Ni, 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 ∼1040–1042 erg s−1. The typical peak timescales are expected to be 3–10 d. Conclusions. We present the first exploration of 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. Our study highlights the critical role of geometry in the interpretation of WD–NS mini-supernovae and motivates further exploration of their diversity in observation.