X-Ray and Near-Infrared Observations of GX 339-4 in the Low/Hard State with Suzaku and IRSF

Kavli Affiliate: Ronald Remillard

| First 5 Authors: Megumi Shidatsu, Yoshihiro Ueda, Fumie Tazaki, Tatsuhito Yoshikawa, Takahiro Nagayama

| Summary:

X-ray and near-infrared ($J$-$H$-$K_{rm s}$) observations of the Galactic
black hole binary GX 339–4 in the low/hard state were performed with Suzaku
and IRSF in 2009 March. The spectrum in the 0.5–300 keV band is dominated by
thermal Comptonization of multicolor disk photons, with a small contribution
from a direct disk component, indicating that the inner disk is almost fully
covered by hot corona with an electron temperature of $approx$175 keV. The
Comptonizing corona has at least two optical depths, $tau approx 1,0.4$.
Analysis of the iron-K line profile yields an inner disk radius of
$(13.3^{+6.4}_{-6.0}) R_{rm g}$ ($R_{rm g} $ represents the gravitational
radius $GM/c^2$), with the best-fit inclination angle of $approx50^circ$.
This radius is consistent with that estimated from the continuum fit by
assuming the conservation of photon numbers in Comptonization. Our results
suggest that the standard disk of GX 339–4 is likely truncated before reaching
the innermost stable circular orbit (for a non rotating black hole) in the
low/hard state at $sim$1% of the Eddington luminosity. The one-day averaged
near-infrared light curves are found to be correlated with hard X-ray flux with
$F_{rm Ks} propto F_{rm X}^{0.45}$. The flatter near infrared $nu F_{nu}$
spectrum than the radio one suggests that the optically thin synchrotron
radiation from the compact jets dominates the near-infrared flux. Based on a
simple analysis, we estimate the magnetic field and size of the jet base to be
$5times10^4$ G and $6times 10^8$ cm, respectively. The synchrotron self
Compton component is estimated to be approximately 0.4% of the total X-ray
flux.

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