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

TL;DR

The paper investigates how the non-spherical wind ejecta from unstable WD–NS mass transfer affects the viewing-angle dependence of Ni-powered thermal transients. It builds a semi-analytical, 2D axisymmetric ejecta model with a Ni-rich wind, calculates the time-dependent photosphere and temperature, and predicts angle-dependent spectra and lightcurves. The results show peak optical magnitudes between $-12$ and $-16$ mag and peak times of $3$–$10$ d, with luminosities of $\sim 10^{40}$–$10^{42}$ erg s$^{-1}$; the transients are dubbed "mini-supernovae" due to their SN-like energy source yet lower brightness and faster evolution. The work emphasizes geometry as a key factor in WD–NS transients and motivates future, more detailed radiative-transfer treatments and opacities to better interpret observations in the era of multi-messenger astronomy.

Abstract

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.

Figures (7)

• Figure 1: Sectional drawings of the temperature evolution within the ejecta at $t = 0.5\,\mathrm{d}$, $1\,\mathrm{d}$, $2\,\mathrm{d}$, $4\,\mathrm{d}$, $8\,\mathrm{d}$, and $16\,\mathrm{d}$ following the WD--NS merger of our fiducial model. Here, $\varv_{z}$ and $\varv_e$ denote the ejecta velocities along the polar axis ($\theta = 0$) and the equatorial plane ($\theta = \pi/2$), respectively. The colour scale indicates the logarithmic temperature. Given the 2D axisymmetry of the system in spherical coordinates, the temperature structure can be extended to the entire ejecta.
• Figure 2: Sectional views of the photosphere evolution in velocity space of our fiducial model, as seen by observers at different viewing angles. The line of sight is fixed to be vertically downward in the planes of the figure, and only the cross-section through the ejecta center is shown. Here, $\varv_{\parallel}$ and $\varv_{\perp}$ represent the velocity components parallel and perpendicular to the line of sight, respectively. Colored lines indicate the photospheric contours at various epochs following the WD--NS merger.
• Figure 3: Emergent spectra of WD--NS mSNe for $\theta_{\text{view}} = 0^{\circ}$, $15^{\circ}$, $30^{\circ}$, $45^{\circ}$, $60^{\circ}$, and $90^{\circ}$. Colored lines show the total observed emergent spectra at different epochs after the WD--NS merger of our fiducial model, assuming a luminosity distance of $D_{\mathrm{L}} = 10\,\mathrm{pc}$. The derived photospheric blackbody temperatures, $T^{\rm BB}_{\rm phot}$, are shown in the labels.
• Figure 4: Viewing-angle-dependent $ugriz$ lightcurves of WD--NS mSNe for the fiducial model. Six different viewing angles $\theta_{\rm view}$ are considered: $0^\circ$ (top left), $15^{\circ}$ (top right), $30^{\circ}$ (middle left), $45^{\circ}$ (middle right), $60^{\circ}$ (bottom left), and $90^{\circ}$ (bottom right).
• Figure 5: Optical luminosity lightcurve of WD--NS mSNe for our fiducial model. Different colors indicate different viewing angles at $\theta_{\text{view}} = 0^{\circ}$, $15^{\circ}$, $30^{\circ}$, $45^{\circ}$, $60^{\circ}$, and $90^{\circ}$. Solid and dashed lines correspond to the $g$- and $r$-band luminosities, respectively.