The 'off' state of GX 339-4

Abstract

We report BeppoSAX and optical observations of the black hole candidate GX 339-4 during its X-ray 'off' state in 1999. The broad-band (0.8-50 keV) X-ray emission can be fitted by a single power law with spectral index, α ∼ 1.6. The observed luminosity is 6.6 × 1033 erg s-1 in the 0.5-10 keV band, which is at the higher end of the flux distribution of black hole soft X-ray transients in quiescence, comparable to that seen in GS 2023+338 and 4U 1630-47. An optical observation just before the BeppoSAX observation shows the source to be very faint at these wavelengths as well (B = 20.1, V = 19.2). By comparing with previously reported 'off' and low states (LS), we conclude that the 'off' state is actually an extension of the LS, i.e. an LS at lower intensities. We propose that accretion models such as the advection-dominated accretion flows are able to explain the observed properties in such a state.