Volume 600, April 2017
|Number of page(s)||10|
|Section||Stellar structure and evolution|
|Published online||20 March 2017|
ALMA observations of 4U 1728−34 and 4U 1820−30: first detection of neutron star X-ray binaries at 300 GHz
1 ESO, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
2 XMM-Newton Science Operations Centre, ESAC/ESA, Camino Bajo del Castillo s/n, Urb. Villafranca del Castillo, 28691 Villanueva de la Cañada, Madrid, Spain
3 Department of Quantum Physics and Astrophysics & Institute of Cosmos Sciences, University of Barcelona, Martí i Franquès 1, 08028 Barcelona, Spain
4 International Centre for Radio Astronomy Research, Curtin University, GPO Box U1987, Perth, WA 6845, Australia
5 Department of Astronomy, Harvard university, 60 Garden street, Cambridge, MA 02138, USA
6 New York University Abu Dhabi, PO Box 129188, Abu Dhabi, UAE
Received: 2 August 2016
Accepted: 19 November 2016
We report on the first observations of neutron star low-mass X-ray binaries with the Atacama Large Millimeter/submillimeter Array (ALMA) at ~300 GHz. Quasi-simultaneous observations of 4U 1728−34 and 4U 1820−30 were performed at radio (ATCA), infrared (VLT) and X-ray (Swift) frequencies, spanning more than eight decades in frequency coverage. Both sources are detected at high significance with ALMA. The spectral energy distribution of 4U 1728−34 is consistent with synchrotron emission from a jet with a break from optically thick to optically thin emission at 1.3–11.0 × 1013 Hz. This is the third time a jet spectral break has been reported for a neutron star X-ray binary. The radio to mm spectral energy distribution of 4U 1820−30 has significant detections at 5 and 300 GHz. This confirms the presence of radio emission during a soft state for this neutron star and represents the first detection of mm emission during such a state, unambiguously pointing to the presence of a jet. We also report on three additional unrelated sources – showing mm emission – in the ALMA fields of view of 4U 1728−34 and 4U 1820−30.
Key words: X-rays: binaries / accretion, accretion disks / stars: individual: 4U 1728-34 / stars: individual: 4U 1820-30 / ISM: jets and outflows
© ESO, 2017
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