IDENTIFICATION OF FAINT CHANDRA X-RAY SOURCES IN THE CORE-COLLAPSED GLOBULAR CLUSTER NGC 6397: EVIDENCE FOR A BIMODAL CATACLYSMIC VARIABLE POPULATION

Journal Article · Sun Oct 10 04:00:00 EDT 2010 · Astrophysical Journal

DOI:https://doi.org/10.1088/0004-637X/722/1/20· OSTI ID:21464728

Cohn, Haldan N; Lugger, Phyllis M ^[1]; Couch, Sean M ^[2]; Anderson, Jay ^[3]; Cool, Adrienne M ^[4]; Van den Berg, Maureen; Grindlay, Jonathan E ^[5]; Bogdanov, Slavko ^[6]

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1. Department of Astronomy, Indiana University, 727 E. Third St., Bloomington, IN 47405 (United States)
2. Department of Astronomy, University of Texas, RLM 15.202A, Austin, TX 78712 (United States)
3. Space Telescope Science Institute, Baltimore, MD 21218 (United States)
4. Department of Physics and Astronomy, San Francisco State University, 1600 Holloway Avenue, San Francisco, CA 94132 (United States)
5. Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA 02138 (United States)
6. Department of Physics, McGill University, Ernest Rutherford Physics Building 226, Montreal, PQ H3G 1A9 (Canada)

We have searched for optical identifications for 79 Chandra X-ray sources that lie within the half-mass radius of the nearby, core-collapsed globular cluster NGC 6397, using deep Hubble Space Telescope Advanced Camera for Surveys Wide Field Channel imaging in H{alpha}, R, and B. Photometry of these images allows us to classify candidate counterparts based on color-magnitude diagram location. In addition to recovering nine previously detected cataclysmic variables (CVs), we have identified six additional faint CV candidates, a total of 42 active binaries (ABs), two millisecond pulsars, one candidate active galactic nucleus, and one candidate interacting galaxy pair. Of the 79 sources, 69 have a plausible optical counterpart. The 15 likely and possible CVs in NGC 6397 mostly fall into two groups: a brighter group of six for which the optical emission is dominated by contributions from the secondary and accretion disk and a fainter group of seven for which the white dwarf dominates the optical emission. There are two possible transitional objects that lie between these groups. The faintest CVs likely lie near the minimum of the CV period distribution, where an accumulation is expected. The spatial distribution of the brighter CVs is much more centrally concentrated than those of the fainter CVs and the ABs. This may represent the result of an evolutionary process in which CVs are produced by dynamical interactions, such as exchange reactions, near the cluster center and are scattered to larger orbital radii, over their lifetimes, as they age and become fainter.

• OSTI ID: 21464728
• Journal Information: Astrophysical Journal, Journal Name: Astrophysical Journal Journal Issue: 1 Vol. 722; ISSN ASJOAB; ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
ACCRETION DISKS
BINARY STARS
COLOR
COSMIC RADIO SOURCES
COSMIC RAY SOURCES
COSMIC X-RAY SOURCES
DWARF STARS
ERUPTIVE VARIABLE STARS
GALAXIES
NOVAE
OPTICAL PROPERTIES
ORGANOLEPTIC PROPERTIES
PHOTOMETRY
PHYSICAL PROPERTIES
PULSARS
STARS
TELESCOPES
VARIABLE STARS
WHITE DWARF STARS