High-Resolution Spectroscopy of the O/Ne-Rich Ultracompact Binary 4U 1626-67

Chandra/HETGS spectroscopy of 4U 162667 detects Doppler pairs of broad Keplerian Ne/O lines, and Ne/O absorption edges much stronger than ISM values.Based on the inferred local Ne/O ratio, the mass donor is likely the lowmasscore of a crystallized CO or ONeMg white dwarf, and may be the prototypefor an emerging class of ultracompact binaries with exotic donors. We proposean 80 ks XMM observation with 3 principal goals: (1) to establish the donor composition by measuring the edges of Ne,O,Mg,and C; (2) to study the accretionflow structure through Doppler tomography using the Hlike lines; (3) to studydisk warping by correlating line strengths with the known UV variability of thesource. The 20 ks XMM GTO observation will not meet any of these goals. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]