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28th of May 2021
ATCA detection of a quiescent black hole X-ray binary
by Plotkin et al.
Quiescent black hole X-ray binaries are believed to be fed by hot accretion flows that launch compact, relativistic jets. However, due to their low luminosities, quiescent jets had been detected in the radio waveband from only five systems. Plotkin et al. present the radio detection of a new quiescent black hole X-ray binary, BW Cir, detected with the Australia Telescope Compact Array. BW Cir was detected over two different epochs in 2018 and 2020, with the ATCA observations paired with quasi-simultaneous X-ray detections with Chandra and Swift. BW Cir is now the sixth quiescent X-ray binary with a confirmed radio jet. However, the distance to BW Cir is uncertain, and BW Cir shows different behaviour in the radio/X-ray luminosity plane depending on the distance assumed. Estimates based on the properties of the G-type stellar companion to the black hole placed BW Cir beyond 25 kpc (80,000 light years) away, while initial optical astrometric measurements from the Gaia satellite suggested a distance of just a few kpc. The most recent measurements from Gaia indicate an intermmediate distance of ~7 kpc (23,000 light years), though still with a large uncertainty in that value. Even though there is now less tension between the parallax and donor-star based distance measurements, it remains an unresolved matter, and Plotkin et al. conclude with suggestions on how to reconcile the two measurements. The image above shows the ATCA radio images of BW Cir from 2020 at a central frequency of 7.25 GHz after stacking the 5.5 and 9.0 GHz basebands. The white square, which is 2.5 arcsec on each side, is centred on the optical Gaia position of BW Cir. The source was detected at a flux density of 23.6 ± 3.6 𝜇Jy.

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