A connection between the X-ray spectral branches and the radio brightness in GX17 + 2

Abstract

GX17 + 2 (4U 1813 – 14) is a bright X-ray binary1,2 in which matter is accreting on to a neutron star from a nearby companion. X-ray bursts, which are due to thermonuclear flashes on the surface of the neutron star3, are sometimes observed4–6, as well as quasiperiodic oscillations in the X-ray flux. The frequencies of the quasiperiodic oscillations depend on the spectral state of the source7–13, which manifests itself as three distinct spectral 'branches' in an X-ray colour–colour diagram. GX17 + 2 is also a variable radio source14–20; there is no believable optical counterpart21–23. We report here on simultaneous X-ray and radio observations which showed a connection between the spectral branches and the radio brightness. The 6-cm and 20-cm flux density increased by factors of 30 ± 5 and 40 ± 10, respectively, as the X-ray state changed from the so-called 'flaring branch' to the 'horizontal branch'. We suspect that this X-ray/radio connection is common among bright low-mass X-ray binaries.