Low-luminosity states of the black hole candidate GX 339-4. I. ASCA and simultaneous radio/RXTE observations

We discuss a series of observations of the black hole candidate GX 339 - 4 in low-luminosity, spectrally hard states. We present spectral analysis of three separate archival Advanced Satellite for Cosmology and Astrophysics ( ASCA) data sets and eight separate Rossi X-Ray Timing Explorer (RXTE) data sets. Three of the RXTE observations were strictly simultaneous with 843 MH z and 8.3-9.1 GHz radio observations. All of these observations have (3-9 k eV) flux less than or similar to 10(-9) ergs s(-1) cm(-2). The ASCA data sh ow evidence for an approximate to 6.4 keV Fe line with equivalent width app roximate to 40 eV, as well as evidence for a soft excess that is well model ed by a power law plus a multicolor blackbody spectrum with peak temperatur e approximate to 150-200 eV. The RXTE data sets also show evidence of an Fe line with equivalent widths approximate to 20-140 eV. Reflection models sh ow a hardening of the RXTE spectra with decreasing X-ray flux; however, the se models do not exhibit evidence of a correlation between the photon index of the incident power law flux and the solid angle subtended by the reflec tor. "Sphere+disk" Comptonization models and advection-dominated accretion how (ADAF) models also provide reasonable descriptions of the RXTE data. Th e former models yield coronal temperatures in the range 20-50 keV and optic al depths of tau approximate to 3. The model fits to the X-ray data, howeve r, do not simultaneously explain the observed radio properties. The most li kely source of the radio flux is synchrotron emission from an extended outf low of size greater than O(10(7)GM/c(2)).