We have analyzed 200 Rossi X-ray Timing Explorer observations of the black hole candidate GX 339-4, all from the bright hard state periods between 1996 and 2005. Purpose of our study is to investigate the radiation mechanisms in the hard state of GX 339-4. The broadband 3-200 keV spectra were successfully modeled by a simple analytic model, power-law with an exponential cut-off modified with a smeared edge. The obtained energy cut-off (E cut ) was distributed over 50-200 keV, and the photon index over 1.4-1.7. We found a clear anti-correlation (E cut ∝ L −0.70±0.06 ) between the X-ray luminosity (L) in 2-200 keV and E cut , when L is larger than 7 × 10 37 erg s −1 (assuming a distance of 8 kpc), while E cut is roughly constant at around 200 keV when L is smaller than 7 × 10 37 erg s −1 . This anti-correlation remained unchanged by adopting a more physical thermal Comptonization model, which resulted in the anti-correlation that can be expressed as kT e ∝ L −0.24±0.06 . These anti-correlations can be quantitatively explained by a picture in which the energy-flow rate from protons to electrons balances with the inverse Compton cooling.

We propose a simple spectral model for the Seyfert 1 Galaxy MCG-6-30-15 that can explain most of the 1 -40 keV spectral variation by change of the partial covering fraction, similar to the one proposed by Miller et al. (2008). Our spectral model is composed of three continuum components; (1) a direct power-law component, (2) a heavily absorbed power-law component by mildly ionized intervening matter, and (3) a cold disk reflection component far from the black hole with moderate solid-angle (Ω/2π ≈ 0.3) accompanying a narrow fluorescent iron line. The first two components are affected by the surrounding highly ionized thin absorber with N H ≈ 10 23.4 cm −2 and log ξ ≈ 3.4. The heavy absorber in the second component is fragmented into many clouds, each of which is composed of radial zones with different ionization states and column densities, the main body (N H ≈ 10 24.2 cm −2 , log ξ ≈ 1.6), the envelope (N H ≈ 10 22.1 cm −2 , log ξ ≈ 1.9) and presumably a completely opaque core. These parameters of the ionized absorbers, as well as the intrinsic spectral shape of the X-ray source, are unchanged at all. The central X-ray source is moderately extended, and its luminosity is not significantly variable. The observed flux and spectral variations are mostly explained by variation of the geometrical partial covering fraction of the central source from 0 (uncovered) to ∼0.63 by the intervening ionized clouds in the line of sight. The ionized iron K-edge of the heavily absorbed component explains most of the seemingly broad line-like feature, a well-known spectral characteristic of MCG-6-30-15. The direct component and the absorbed component anti-correlate, cancelling their variations each other, so that the fractional spectral variation becomes the minimum at the iron energy band; another observational characteristic of MCG-6-30-15 is thus explained.

Many Seyfert galaxies are known to exhibit significant X-ray spectral variations and seemingly broad iron K-emission line features. In this paper, we show that the "variable partial covering model", which has been successfully proposed for MCG-6-30-15 (Miyakawa, Ebisawa & Inoue 2012) and 1H0707-495 (Mizumoto, Ebisawa & Sameshima 2014), can also explain the spectral variations in 2-10 keV as well as the broad iron line features in 20 other Seyfert galaxies observed with Suzaku. In this model, the absorbed spectral component through the optically-thick absorbing clouds has a significant iron K-edge, which primarily accounts for the observed seemingly broad iron line feature. Fluctuation of the absorbing clouds in the line of sight of the extended X-ray source results in variation of the partial covering fraction, which causes an anti-correlation between the direct (not covered) spectral component and the absorbed (covered) spectral component below ∼10 keV. Observed spectral variation in 2-10 keV in a timescale of less than ∼day is primarily explained by such variations of the partial covering fraction, while the intrinsic soft X-ray luminosity is hardly variable.