No Neutron Star Companion To The Lowest Mass SDSS White Dwarf

TL;DR

This study used radio and X-ray observations to search for a neutron star companion to the lowest mass white dwarf, but found no evidence, suggesting the companion is likely another white dwarf.

Contribution

First combined radio and X-ray observational analysis to rule out a neutron star companion to this low-mass white dwarf.

Findings

No pulsar signals detected in radio data.

No X-ray emission detected from the companion.

Companion likely another white dwarf, not a neutron star.

Abstract

SDSS J091709.55+463821.8 (hereafter J0917+4638) is the lowest surface gravity white dwarf (WD) currently known, with log g = 5.55 +/- 0.05 (M ~ 0.17 M_sun; Kilic et al. 2007a,b). Such low-mass white dwarfs (LMWDs) are believed to originate in binaries that evolve into WD/WD or WD/neutron star (NS) systems. An optical search for J0917+4638's companion showed that it must be a compact object with a mass >= 0.28 M_sun (Kilic 2007b). Here we report on Green Bank Telescope 820 MHz and XMM-Newton X-ray observations of J0917+4638 intended to uncover a potential NS companion to the LMWD. No convincing pulsar signal is detected in our radio data. Our X-ray observation also failed to detect X-ray emission from J0917+4638's companion, while we would have detected any of the millisecond radio pulsars in 47 Tuc. We conclude that the companion is almost certainly another WD.