Ultracompact X-Ray Binaries with Neon-rich Degenerate Donors

Abstract

There are three low-mass X-ray binaries (4U 0614ϩ091, 2S 0918Ϫ549, and 4U 1543Ϫ624) for which broad-line emission near 0.7 keV was previously reported. A recent high-resolution observation of 4U 0614ϩ091 with the Chandra Low-Energy Transmission Grating Spectrometer (LETGS) found evidence of an unusually high Ne/O abundance ratio along the line of sight but failed to detect the previously reported 0.7 keV feature. We have made a search of archival ASCA spectra and identified a fourth source with the 0.7 keV feature, the 20 minute ultracompact binary 4U 1850Ϫ087. In all four of these sources, the 0.7 keV residual is eliminated from the ASCA spectra by allowing excess photoelectric absorption due to a nonsolar relative abundance of neon, just as in the LETGS spectrum of 4U 0614ϩ091. The optical properties of these systems suggest that all four are ultracompact (minutes) binaries. We propose that there is excess neon local to each of these sources, P ! 80 orb as also found in the ultracompact binary pulsar 4U 1626Ϫ67. We suggest that the mass donor in these systems is a low-mass, neon-rich degenerate dwarf and that the binaries are all ultracompact. Subject headings: binaries: close — stars: individual (2S 0918Ϫ549, 4U 0614ϩ091, 4U 1543Ϫ624, 4U 1850Ϫ087) — X-rays: binaries