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J. Homan (MIT), M. Buxton (Yale), S. Markoff (MIT), C. Bailyn (Yale), E. Nespoli, T. Belloni (INAF/OAB - Italy)
We have made quasi-simultaneous X-ray and optical/near-infrared (nIR) observations of the black hole candidate X-ray transient GX 339-4. Our observations were made over a time span of more than eight months in 2002 and cover the initial rise and transition from the hard to the soft state in X-rays. Two distinct patterns of correlated X-ray-optical/nIR behavior are observed. In the hard state the optical/nIR and X-ray fluxes correlate well over three orders of magnitude in X-ray flux. The slope of the relation between nIR and X-ray fluxes is similar to that of radio and X-ray flux relations found in several black hole systems. In the softer X-ray states the optical/nIR spectrum of GX 339-4 is much bluer and the ratio of X-ray to nIR flux increases by a factor of more than 10. During this time changes in the nIR precede those in the soft X-rays by more than two weeks, indicating a disk origin of the nIR emission. We present spectral energy distributions, including radio data, and discuss possible sources for the optical/nIR emission. We conclude that 1) in the hard state the optical/nIR emission probably originates in the optically thin part of a jet and that 2) in none of the X-ray states X-ray reprocessing is the main source of optical/nIR emission. Finally, we argue that previous measurements of X-ray-optical/nIR lags at the start of transient outbursts were most likely due to the insensitivity of the X-ray instruments.
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Bulletin of the American Astronomical Society, 36 #3
© 2004. The American Astronomical Soceity.