The (Double) White Dwarf Binary SDSS 1257+5428
  SDSS 1257+5428 is a white dwarf in a close orbit with a companion that has
been suggested to be a neutron star. If so, it hosts the closest known neutron
star, and its existence implies a great abundance of similar systems and a rate
of white-dwarf neutron-star mergers similar to that of the type Ia supernova
rate. Here, we present high signal-to-noise spectra of SDSS 1257+5428, which
confirm an independent finding that the system is in fact composed of two white
dwarfs, one relatively cool and with low mass, and the other hotter and more
massive. With this, the demographics and merger rate are no longer puzzling
(various factors combine to lower the latter by more than two orders of
magnitude). We show that the spectra are fit well with a combination of two
hydrogen model atmospheres, as long as the lines of the higher-gravity
component are broadened significantly relative to what is expected from just
pressure broadening. Interpreting this additional broadening as due to
rotation, the inferred spin period is short, about 1 minute. Similarly rapid
rotation is only seen in accreting white dwarfs that are magnetic; empirically,
it appears that in non-magnetized white dwarfs, accreted angular momentum is
lost by nova explosions before it can be transferred to the white dwarf. This
suggests that the massive white dwarf in SDSS 1257+5428 is magnetic as well,
with B~10^5 G. Alternatively, the broadening seen in the spectral lines could
be due to a stronger magnetic field, of ~10^6 G. The two models could be
distinguished by further observations.
