Multi-wavelength Observations of the 2002 Outburst of Gx 339–4: Two Patterns of X-ray–optical/near-infrared Behavior

We report on quasi-simultaneous Rossi X-ray Timing Explorer 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 a hard to a soft spectral state in X-rays. Two distinct patterns of correlated X-ray–optical/nIR behavior were found. During the hard state the optical/nIR and X-ray fluxes correlated well, with a nIR vs. X-ray flux power-law slope similar to that of the correlation found between X-ray and radio fluxes in previous studies of GX 339–4 and other black hole binaries. As the source went through an intermediate state the optical/nIR fluxes decreased rapidly and once it had entered the spectrally soft state, the optical/nIR spectrum of GX 339–4 was much bluer and the ratio of X-ray to nIR flux was higher by a factor of more than 10 compared to the hard state. In the spectrally soft state changes in the nIR preceded those in the soft X-rays by more than two weeks, indicating a disk origin of the nIR emission, and providing a measure of the viscous time scale. A sudden onset of nIR flaring of ∼0.5 magnitudes on a time scale of a day was also observed during this period. We present spectral energy distributions, including radio data, and discuss possible sources for the optical/nIR emission. We conclude that in the hard state this emission probably originates in the optically thin part of a jet and that in none of the X-ray states X-ray reprocessing is the dominant source of optical/nIR emission. Finally, comparing the light curves from the ASM and PCA instruments, we find that the X-ray/nIR delay depends critically on the sensitivity of the X-ray detector, with the delay inferred from the PCA (if present at all) being a factor of 3-6 times shorter than the delay inferred from the ASM; this may be important in interpreting previously reported X-ray-optical/nIR lags. Subject headings: accretion, accretion disks — binaries (including multiple): close — black hole physics — ISM: jets and outflows — stars: individual (GX 339–4) — X-rays: stars