The evolving jet spectrum of the neutron star X-ray binary Aql X-1 in transitional states during its 2016 outburst

¹ ESO, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
e-mail: mdiaztri@eso.org
² Department of Physics & Astronomy, University of Southampton, Southampton, Hampshire SO17 1BJ, UK
³ Department of Astronomy, Yale University, PO Box 208101, New Haven CT 06520-8101, USA
⁴ International Centre for Radio Astronomy Research, Curtin University, GPO Box U1987 Perth, Western Australia 6845, Australia
⁵ New York University Abu Dhabi, PO Box 129188, Abu Dhabi, UAE
⁶ Dipartimento di Fisica, Università degli Studi di Cagliari, SP Monserrato-Sestu km 0.7, 09042 Monserrato, Italy
⁷ Faulkes Telescope Project, School of Physics and Astronomy, Cardiff University, 5 The Parade, Cardiff CF24 3AA, UK
⁸ Astrophysics Research Institute, Liverpool John Moores University, 146 Brownlow Hill, Liverpool L3 5RF, UK
⁹ XMM-Newton Science Operations Centre, ESAC/ESA, Camino Bajo del Castillo s/n, Urb. Villafranca del Castillo, 28691 Villanueva de la Cañada, Madrid, Spain
¹⁰ Institute of Cosmos Sciences, University of Barcelona, Martí i Franquès 1, 08028 Barcelona, Spain
¹¹ Department of Astronomy, Harvard University, 60 Garden street, Cambridge MA 02138, USA

Received: 24 January 2018
Accepted: 20 April 2018

Abstract

We report on quasi-simultaneous observations from radio to X-ray frequencies of the neutron star X-ray binary Aql X-1 over accretion state transitions during its 2016 outburst. All the observations show radio to millimetre spectra consistent with emission from a jet, with a spectral break from optically thick to optically thin synchrotron emission that decreases from ~100 GHz to <5.5 GHz during the transition from a hard to a soft accretion state. The 5.5 GHz radio flux density as the source reaches the soft state, 0.82 ± 0.03 mJy, is the highest recorded to date for this source. During the decay of the outburst, the jet spectral break is detected again at a frequency of ~30–100 GHz. The flux density is 0.75 ± 0.03 mJy at 97.5 GHz at this stage. This is the first time that a change in the frequency of the jet break of a neutron star X-ray binary has been measured, indicating that the processes at play in black holes are also present in neutron stars, supporting the idea that the internal properties of the jet rely most critically on the conditions of the accretion disc and corona around the compact object, rather than the black hole mass or spin or the neutron star surface or magnetic field.