Optical transients from non-explosive double white-dwarf mergers: the case of a central neutron star remnant

Abstract

Discoveries of ultra-massive magnetic white dwarfs (WDs) and peculiar pulsars have been proposed to originate in double white dwarf (DWD) mergers. There are three possible post-merger central remnants of non-explosive mergers: 1) a stable sub-Chandrasekhar WD; 2) a rapidly rotating super-Chandrasekhar WD; 3) a neutron star (NS). In this work, we explore the thermal transient arising from non-explosive DWD mergers that leave an NS remnant from the prompt collapse of the merged core. The transient is powered by the cooling of the expanding dynamical ejecta, with energy injection from magnetic dipole radiation, which depends on the dipole factor $D = B_d^2/P_0^4$, with $B_d$ and $P_0$ being the surface magnetic field strength and initial rotation period of the newborn NS. We simulate lightcurves in the Legacy Survey of Space and Time (LSST) bands and estimate the horizon and detection rates for these transients across a range of model parameters. We find LSST detection horizons upper limits ranging $30$--$1020$ Mpc and corresponding detection rates $10^2$--$10^6$ yr$^{-1}$ for $\log D = 24$--$40$. Accounting for the survey cadence, we find that only configurations with $\log D = 36$--$40$ are detectable within $240$--$990$ Mpc, with detection rates $10^4$--$10^6$ yr$^{-1}$. Combined searches across surveys can compensate for the low cadence and improve the detection rates of fast and less energetic sources. Multi-wavelength campaigns can aid in detecting the spindown radiation at higher energies observable after the optical transient. Observations of these transients will provide direct evidence of the non-explosive DWD mergers, characterise the remnants and progenitor parameters, and the fraction of explosive and non-explosive mergers.

Figures (6)

• Figure 1: Time evolution of heating by the source ($H_{\rm inj} = H_{\rm sd}$, light-blue solid line), the effective injection into the ejecta ($H_{\rm eff}$, dark-blue dot-dashed line), and the thermal radiation emitted ($L_{\rm rad}$, red solid line). The bolometric luminosity ($L_{\rm bol}$) is the sum of the radiated luminosity from the cooling of the ejecta ($L_{\rm rad}$, black dotted line) and the leaked luminosity ($L_{\rm leak}$, green solid line).
• Figure 2: Isotropic luminosity radiated by the ejecta powered by NS remnant with dipole factors $\log D=24$--40, corresponding to configurations with magnetic fields $B_d=10^{10}$--$10^{14}$ G and initial rotation periods $P_0 = 1$--$100$ ms. The change in the slope of the curves at late times indicates the transition to the adiabatic cooling regime. The dotted and dashed blue curves are configurations with the same dipole factor, $\log D = 36$, but with different magnetic field and rotation period, {$B_d = 10^{14}$ G, $P_0 = 10$ ms} and {$B_d = 10^{12}$ G, $P_0 = 1$ ms}.
• Figure 3: Temporal evolution of the thermal spectra of the ejecta heated by the NS remnant of a DWD merger. The optical range is marked by the shaded region between $10^{14}$--$10^{15}$ Hz. The dipole factor characterises the strength of the injected dipole radiation. In the upper plot, lightcurves corresponding to $\log D = 24, 28$ and $32$ (red, yellow and green, respectively) overlap. Even at $t \sim 1$ day, $\log D = 24$ and $28$ are overlapping, indicating that low dipole factors lead to similar early-time behaviour. The dotted and dashed blue lightcurves correspond to configurations with {$B_d = 10^{14}$ G, $P_0 = 10$ ms} and {$B_d = 10^{12}$ G, $P_0 = 1$ ms}, which show how the behaviour of the transient does not depend on the dipole factor alone (see text for details).
• Figure 4: A grid of DWD merger transients powered by NS remnant with varying magnetic fields and rotations. The lightcurves, in absolute magnitudes, are simulated in the LSST ugrizy bands. The configurations with $\log D = 24$ and $28$ produce similar lightcurves and are therefore presented together. The configurations with $\log D = 36$ show a slight variation in the decay time. Here, we show the fastest case.
• Figure 5: Comparison of the lightcurve of the optical transient powered by fallback accretion onto a central WD left by a DWD merger (solid: this work, dashed: Ref. 2023ApJ...958..134S).