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 , \alpha \sim 1.6 .
The observed luminosity is 6.6 \times 10 ^ { 33 } 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 .
a 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 .