Mini-supernovae from white dwarf-neutron star mergers: Viewing-angle-dependent spectra and lightcurves
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arXiv
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| Natura: | Preprint |
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2025
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| author | Kang, Yacheng Zhu, Jin-Ping Shao, Lijing Zhong, Jiahang Zhang, Jinghao Zhang, Bing |
| author_facet | Kang, Yacheng Zhu, Jin-Ping Shao, Lijing Zhong, Jiahang Zhang, Jinghao Zhang, Bing |
| contents | 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. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2512_04378 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | Mini-supernovae from white dwarf-neutron star mergers: Viewing-angle-dependent spectra and lightcurves Kang, Yacheng Zhu, Jin-Ping Shao, Lijing Zhong, Jiahang Zhang, Jinghao Zhang, Bing High Energy Astrophysical Phenomena Cosmology and Nongalactic Astrophysics Solar and Stellar Astrophysics 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. |
| title | Mini-supernovae from white dwarf-neutron star mergers: Viewing-angle-dependent spectra and lightcurves |
| topic | High Energy Astrophysical Phenomena Cosmology and Nongalactic Astrophysics Solar and Stellar Astrophysics |
| url | https://arxiv.org/abs/2512.04378 |