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25th of July 2017
AR Sco
Marcote et al.
The binary system AR Sco contains an M star and the only known radio-pulsing white dwarf. The system shows emission from radio to X-rays, likely dominated by synchrotron radiation. The mechanism that produces most of this emission remains unclear. Two competing scenarios have been proposed: collimated outflows, and direct interaction between the magnetospheres of the white dwarf and the M star. Marcote et al. conducted a radio observation with the Australian Long Baseline Array (LBA) in October 2016 at 8.5 GHz to study the compactness of the radio emission. Simultaneous data were recorded with the Australia Telescope Compact Array (ATCA) for a direct comparison of the obtained flux densities. AR Sco was found to show radio emission compact on milliarcsecond angular scales (<0.02 Astronomical Units). The emission is orbitally modulated, with an average flux density of ~6.5 mJy. A comparison with the simultaneous ATCA data shows that no flux is resolved out on mas scales, implying that the radio emission is produced in this compact region. Additionally, the obtained radio light curves on hour timescales are consistent with the optical light curve. The team conclude that the radio emission in AR Sco is likely produced in the magnetosphere of the M star or the white dwarf, with no evidence for a radio outflow or collimated jets significantly contributing to the radio emission.

The images above of AR Sco were obtained with the LBA on 20 October 2016 at 8.4 GHz. The left-hand image was obtained with a natural weighting and no self-calibration. The right-hand image was obtained with a Briggs robust weighting of zero after self-calibration in phase. Contours start at three times the noise level of 0.4 mJy in both cases and increase by factors of sqrt(2). The synthesized beams, represented by the white ellipses in the bottom left corners, are 1.4 mas × 2.6 mas at a position angle of 39 degrees, and 1.8 mas × 2.9 mas at a position angle of 40 degrees, respectively.

The paper describing these results has been published in Astronomy & Astrophysics, vol. 601, L7.

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