The iPTF detection of the most recent outburst of the recurrent nova system RX J0045.4+4154 in the Andromeda Galaxy has enabled the unprecedented study of a massive ( M > 1.3 M _ { \odot } ) accreting white dwarf ( WD ) .
We detected this nova as part of the near daily iPTF monitoring of M31 to a depth of R \approx 21 mag and triggered optical photometry , spectroscopy and soft X-ray monitoring of the outburst .
Peaking at an absolute magnitude of M _ { R } = -6.6 mag , and with a decay time of 1 mag per day , it is a faint and very fast nova .
It shows optical emission lines of He/N and expansion velocities of 1900 to 2600 km s ^ { -1 } 1–4 days after the optical peak .
The Swift monitoring of the X-ray evolution revealed a supersoft source ( SSS ) with kT _ { eff } \approx 90 - 110 { eV } that appeared within 5 days after the optical peak , and lasted only 12 days .
Most remarkably , this is not the first event from this system , rather it is a recurrent nova with a time between outbursts of approximately 1 year , the shortest known .
Recurrent X-ray emission from this binary was detected by ROSAT in 1992 and 1993 , and the source was well characterized as a M > 1.3 M _ { \odot } WD SSS .
Based on the observed recurrence time between different outbursts , the duration and effective temperature of the SS phase , MESA models of accreting WDs allow us to constrain the accretion rate to \dot { M } > 1.7 \times 10 ^ { -7 } { M _ { \odot } { yr } } ^ { -1 } and WD mass > 1.30 M _ { \odot } .
If the WD keeps 30 \% of the accreted material , it will take less than a Myr to reach core densities high enough for carbon ignition ( if made of C/O ) or electron capture ( if made of O/Ne ) to end the binary evolution .