SIMBAD references

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.