Abstract
We report the discovery of an optical counterpart to a quiescent neutron star in the globular cluster ω Centauri (NGC 5139). The star was found as part of our wide-field imaging study of ω Cen using the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope. Its magnitude and color (R625 = 25.2, B435 - R625 = 1.5) place it more than 1.5 mag to the blue side of the main sequence. Through an Hα filter it is ~1.3 mag brighter than cluster stars of comparable R625 magnitude. The blue color and Hα excess suggest the presence of an accretion disk, implying that the neutron star is accreting from a binary companion and is thus a quiescent low-mass X-ray binary. If the companion is a main-sequence star, then the faint absolute magnitude (M625 11.6) constrains it to be of very low mass (M 0.14 M☉). The faintness of the disk (M435 ~ 13) suggests a very low rate of accretion onto the neutron star. We also detect 13 probable white dwarfs and three possible BY Draconis stars in the 20'' × 20'' region analyzed here, suggesting that a large number of white dwarfs and active binaries will be observable in the full ACS study.
Highlights
! Centauri is a prime target for studies of stellar collisions
The characteristics of the optical counterpart that we have identified suggest the presence of a disk and binary companion, as in a quiescent low-mass X-ray binary
While more than two dozen quiescent low-mass X-ray binary (qLMXB) have been identified in 10 different globular clusters from their X-ray spectra (Heinke et al 2003b), this is only the second optical counterpart found for one of these objects during quiescence. qLMXBs are believed to play an important role in the production of millisecond pulsars; studying them in globular clusters holds the promise of new constraints on neutron star equations of state (Brown et al 1998; Heinke et al 2003a)
Summary
! Centauri is a prime target for studies of stellar collisions. It is nearby (D ’ 5 kpc), massive, and despite a relatively moderate central density has one of the highest predicted rates of stellar interactions among globular clusters, owing to its very large core. Cen. The nine fields observed with ACS form a mosaic that encompasses more than 100 of the Chandra sources identified with ACIS-I in the direction of ! We report the first detection of an optical counterpart for a Chandra source using the ACS data. While more than two dozen qLMXBs have been identified in 10 different globular clusters from their X-ray spectra (Heinke et al 2003b), this is only the second optical counterpart found for one of these objects during quiescence (the other being X5 in 47 Tuc; Edmonds et al 2002). Results of our search for additional optical counterparts to Chandra sources, as well as our study of stellar populations in ! Results of our search for additional optical counterparts to Chandra sources, as well as our study of stellar populations in ! Cen using the ACS data, will appear in subsequent papers
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
