title: An ultramassive, fast-spinning white dwarf in a peculiar binary system.
abstract: White dwarfs typically have masses in a narrow range centered at about 0.6 solar mass (M(o)). Only a few ultramassive white dwarfs (mass > 1.2 M(o)) are known. Those in binary systems are of particular interest, because a small amount of accreted mass could drive them above the Chandrasekhar limit, beyond which they become gravitationally unstable. Using data from the x-ray multimirror mission (XMM)-Newton satellite, we show that the x-ray pulsator RX J0648.0-4418 is a white dwarf with mass > 1.2 M(o), based on dynamical measurements only. This ultramassive white dwarf in a post-common envelope binary with a hot subdwarf can reach the Chandrasekhar limit, and possibly explode as a type Ia supernova, when its helium-rich companion will transfer mass at an increased rate through Roche lobe overflow.
authors.affiliation: Istituto Nazionale di Astrofisica-Istituto di Astrofisica Spaziale e Fisica cosmica di Milano, via E. Bassini 15, 20133 Milano, Italy. sandro@iasf-milano.inaf.it
authors.forename: S
authors.initials: S
authors.lastname: Mereghetti
authors.forename: A
authors.initials: A
authors.lastname: Tiengo
authors.forename: P
authors.initials: P
authors.lastname: Esposito
authors.forename: N
authors.initials: N
authors.lastname: La Palombara
authors.forename: G L
authors.initials: GL
authors.lastname: Israel
authors.forename: L
authors.initials: L
authors.lastname: Stella