A super-Chandrasekhar mass type Ia supernova progenitor at 49 pc set to detonate in 23 Gyr
Double white dwarf binaries are a leading explanation to the origin of typeIa supernovae, but no system exceeding the Chandrasekhar mass limit (1.4M$_\odot$) has been found that will explode anywhere close to a Hubble time.Here, we present the super-Chandrasekhar mass double white dwarfWDJ181058.67+311940.94 whose merger time ($22.6\pm1.0$ Gyr) is of the sameorder as a Hubble time. The mass of the binary is large, combining to$1.555\pm0.044$ M$_\odot$, while being located only 49 pc away. We predict thatthe binary will explode dynamically via a double detonation destroying bothstars just before they merge, appearing as a subluminous type Ia supernova witha peak apparent magnitude of about $m_V=-16$ (200,000 times brighter thanJupiter). The observationally-derived birthrate of super-Chandrasekhar massdouble white dwarfs is now at least $6.0\times10^{-4}$ yr$^{-1}$ and theobserved rate of type Ia supernovae in the Milky Way from such systems isapproximately $4.4\times10^{-5}$ yr$^{-1}$, while the predicted type Iasupernova rate in the Milky Way from all progenitor channels is about sixtytimes larger. Hence, WDJ181058.67+311940.94 mitigates the observed deficit ofmassive double white dwarfs witnessed in volume-complete populations, butfurther evidence is required to determine the majority progenitors of type Iasupernovae.