Fast infrared variability from a relativistic jet in GX 339-4

• Published in: Monthly notices of the Royal Astronomical Society. Letters 2010-05, Vol.404 (1), p.L21-L25
• Main Authors: Casella, P., Maccarone, T. J., O'Brien, K., Fender, R. P., Russell, D. M., Van Der Klis, M., Pe'er, A., Maitra, D., Altamirano, D., Belloni, T., Kanbach, G., Klein-Wolt, M., Mason, E., Soleri, P., Stefanescu, A., Wiersema, K., Wijnands, R.
• Format: Article
• Language: English
• Subjects: black hole physics; outflows; stars: winds; stars: winds, outflows; X-rays: binaries; X-rays: individual: GX 339−4
• ISSN: 1745-3925; 1745-3933
• DOI: 10.1111/j.1745-3933.2010.00826.x

We present the discovery of fast infrared/X-ray correlated variability in the black hole transient GX 339−4. The source was observed with subsecond time resolution simultaneously with Very Large Telescope/Infrared Spectrometer And Array Camera and Rossi X-ray Timing Explorer/Proportional Counter Array in 2008 August, during its persistent low-flux highly variable hard state. The data show a strong correlated variability, with the infrared emission lagging the X-ray emission by 100ms. The short time delay and the nearly symmetric cross-correlation function, together with the measured brightness temperature of ∼2.5 × 106K, indicate that the bright and highly variable infrared emission most likely comes from a jet near the black hole. Under standard assumptions about jet physics, the measured time delay can provide us a lower limit of Γ > 2 for the Lorentz factor of the jet. This suggests that jets from stellar-mass black holes are at least mildly relativistic near their launching region. We discuss implications for future applications of this technique.