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    The MAVERIC survey: a hidden pulsar and a black hole candidate in ATCA radio imaging of the globular cluster NGC 6397

    80260.pdf (4.989Mb)
    Access Status
    Open access
    Authors
    Zhao, Yue
    Heinke, Craig O
    Tudor, Vlad
    Bahramian, Arash
    Miller-Jones, James
    Sivakoff, Gregory R
    Strader, Jay
    Chomiuk, Laura
    Shishkovsky, Laura
    Maccarone, Thomas J
    Marcano, Manuel Pichardo
    Gelfand, Joseph D
    Date
    2020
    Type
    Journal Article
    
    Metadata
    Show full item record
    Citation
    Zhao, Y. and Heinke, C.O. and Tudor, V. and Bahramian, A. and Miller-Jones, J.C.A. and Sivakoff, G.R. and Strader, J. et al. 2020. The MAVERIC survey: a hidden pulsar and a black hole candidate in ATCA radio imaging of the globular cluster NGC 6397. Monthly Notices of the Royal Astronomical Society. 493 (4): pp. 6033-6049.
    Source Title
    Monthly Notices of the Royal Astronomical Society
    DOI
    10.1093/mnras/staa631
    ISSN
    0035-8711
    Faculty
    Faculty of Science and Engineering
    School
    School of Elec Eng, Comp and Math Sci (EECMS)
    Funding and Sponsorship
    http://purl.org/au-research/grants/arc/FT140101082
    Remarks

    Copyright © 2020 The American Astronomical Society. All rights reserved.

    URI
    http://hdl.handle.net/20.500.11937/80178
    Collection
    • Curtin Research Publications
    Abstract

    Using a 16.2 hr radio observation by the Australia Telescope Compact Array (ATCA) and archival Chandra data, we found $>5\sigma$ radio counterparts to 4 known and 3 new X-ray sources within the half-light radius ($r_\mathrm{h}$) of the Galactic globular cluster NGC 6397. The previously suggested millisecond pulsar (MSP) candidate, U18, is a steep-spectrum ($S_\nu \propto \nu^\alpha$; $\alpha=-2.0^{+0.4}_{-0.5}$) radio source with a 5.5 GHz flux density of $54.7\pm 4.3~\mathrm{\mu Jy}$. We argue that U18 is most likely a "hidden" MSP that is continuously hidden by plasma shocked at the collision between the winds from the pulsar and companion star. The nondetection of radio pulsations so far is probably the result of enhanced scattering in this shocked wind. On the other hand, we observed 5.5 GHz flux of the known MSP PSR J1740-5340 (U12) to decrease by a factor of $>2.8$ during epochs of 1.4 GHz eclipse, indicating that the radio flux is absorbed in its shocked wind. If U18 is indeed a pulsar whose pulsations are scattered, we note the contrast with U12's flux decrease in eclipse, which argues for two different eclipse mechanisms at the same radio frequency. In addition to U12 and U18, we also found radio associations for 5 other Chandra X-ray sources, four of which are likely background galaxies. The last, U97, which shows strong H$\alpha$ variability, is mysterious; it may be either a quiescent black hole low-mass X-ray binary, or something more unusual.

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