* Astronomy

Title: No Neutron Star Companion To The Lowest Mass SDSS White Dwarf
Authors: Marcel Agueros (Columbia U), Craig Heinke (U of Alberta), Fernando Camilo (Columbia U), Mukremin Kilic (CfA), Scott Anderson (U of Washington), Paulo Freire (Arecibo Obs), Scot Kleinman (Gemini Obs), James Liebert (Steward Obs), Nicole Silvestri (U of Washington)

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.

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