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21st of September 2015
Deep ATCA imaging identifies a new black hole candidate in the globular cluster 47 Tucanae
by James Miller-Jones (ICRAR/Curtin University)
Recent observational and theoretical work has suggested that stellar-mass black holes may exist in globular clusters, in contrast to previous theoretical predictions that they should be expelled via mutual gravitational interactions early in the lifetime of a cluster. New black hole candidates are being identified as faint, flat-spectrum sources in deep radio observations, whose radio-to-X-ray luminosity ratios are too high to be explained as any other class of accreting compact object, and work is ongoing to verify their nature. Miller-Jones et al. (2015) have recently reported the detection of steady radio emission from the known X-ray source X9 in the globular cluster 47 Tuc. This rules out the previous interpretation of X9 as a cataclysmic variable, and suggests that it may instead be a quiescent black hole in an X-ray binary system. The nature of the donor star is still uncertain, and is likely to be either a low-mass main sequence star, or possibly a white dwarf.

Figure caption: Left: 5.5-GHz ATCA image of the core of 47 Tuc. The cluster centre and core radius are marked with a red cross and red circle, respectively, and the orange circle highlights the radio detection of X9, with a flux density of 42+/-4 microJy/beam. Right: The correlation between radio and X-ray luminosities of accreting stellar-mass compact objects. Black holes are shown in black, neutron stars and transitional millisecond pulsars in blue, and cataclysmic variables in green. 47 Tuc X9 is shown in purple, lying within the scatter of the black hole correlation.

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