A Long, Hard Look at the Low–hard State in Accreting Black Holes

We present the first results of coordinated multi-wavelength observations of the Galactic black hole GX 339−4 in a canonical low–hard state, obtained during its 2004 outburst. XMM-Newton observed the source for 2 revolutions, or approximately 280 ksec; RXTE monitored the source throughout this long stare. The resulting data offer the best view yet obtained of the inner accretion flow geometry in the low–hard state, which is thought to be analogous to the geometry in low-luminosity active galactic nuclei. The XMM-Newton spectra clearly reveal the presence of a cool accretion disk component, and a relativistic Fe K emission line. The results of fits made to both components strongly suggest that a standard thin disk remains at or near to the innermost stable circular orbit, at least in bright phases of the low–hard state. These findings indicate that both potential links between the inner disk radius and the onset of a steady compact jet, and the paradigm of a radially–recessed disk in the low–hard state, do not hold universally. The results of our observations can best be explained if a standard thin accretion disk fuels a corona which is closely related to, or consistent with, the base of a compact jet. In a brief examination of archival data, we show that Cygnus X-1 supports this picture of the low/hard state. We discuss our results within the context of disk–jet connections and prevailing models for accretion onto black holes.