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Paduano et al.
present the results of an ultra-deep radio continuum survey,
containing ∼480 hours of observations, of the Galactic globular
cluster 47 Tucanae with the Australia Telescope Compact Array.
This is the deepest
radio image of a globular cluster, and the deepest image ever made
with the Australia Telescope Compact Array.
They identify ATCA J002405.702-720452.361, a faint (6 micro-jansky at 7.25
GHz), flat-spectrum radio
source that is positionally coincident with the cluster centre and
potentially associated with a faint X-ray source. No convincing
optical counterpart was identified. The team used radio, X-ray, optical, and
ultraviolet data to show that explanations involving a background active
galactic nucleus, a chromospherically active binary, or a binary
involving a white dwarf are unlikely.
The most plausible explanations are that the source is an undiscovered
millisecond pulsar or a weakly accreting black hole. If the X-ray
source is associated with the radio source, the fundamental plane of
black hole activity suggests a black hole mass in the range 54--6000 solar masses,
indicating an intermediate-mass black hole or a heavy stellar-mass
black hole.
The image above is the 7.25 GHz radio image of 47 Tuc.
The RMS noise level in this image is ∼790 nJy/beam.
The core radius of 47 Tuc is shown by the dashed red circle.
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