No Neutron Star Companion to the Lowest Mass SDSS White Dwarf
Anderson, Scott F.; Heinke, Craig; Kilic, Mukremin; Agüeros, Marcel A.; Kleinman, Scot J.; Camilo, Fernando; Freire, Paulo; Liebert, James W.; Silvestri, Nicole M.
United States, Canada, Germany
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). 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. 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.