The (Double) White Dwarf Binary SDSS 1257+5428

TL;DR

High-quality spectra of SDSS 1257+5428 reveal it is a binary system of two white dwarfs, with one rapidly rotating and likely magnetic, resolving previous uncertainties about its nature and merger rate implications.

Contribution

This study provides detailed spectral analysis confirming the system consists of two white dwarfs, clarifying its demographics and merger rate, and suggests the white dwarf is magnetic with rapid rotation.

Findings

System is two white dwarfs, not a white dwarf and neutron star.

The hotter, more massive white dwarf shows rapid rotation (~1 min period).

The white dwarf is likely magnetic, with B~10^5 G or 10^6 G.

Abstract

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.