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Correlated fast X-ray and optical variability in the black-hole candidate XTE J1118+480


Black holes become visible when they accrete gas, a common source of which is a close stellar companion. The standard theory for this process (invoking a ‘thin accretion disk’) does not explain some spectacular phenomena associated with these systems, such as their X-ray variability1 and relativistic outflows2, indicating some lack of understanding of the actual physical conditions. Simultaneous observations at multiple wavelengths can provide strong constraints on these conditions. Here we report simultaneous high-time-resolution X-ray and optical observations of the transient source XTE J1118+480, which show a strong but puzzling correlation between the emissions. The optical emission rises suddenly following an increase in the X-ray output, but with a dip 2–5 seconds in advance of the X-rays. This result is not easy to understand within the simplest model of the optical emission, where the light comes from reprocessed X-rays. It is probably more consistent with an earlier suggestion3 that the optical light is cyclosynchrotron emission that originates in a region about 20,000 km from the black hole. We propose that the time dependence is evidence for a relatively slow (<0.1c), magnetically controlled outflow.

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Figure 1: Cross-correlation of the X-ray and optical time series of XTE J1118+480, showing onset of an optical response within 30 ms.
Figure 2: Autocorrelations of the X-ray and optical time series, showing that the optical variability is not due to reprocessing of the X-rays.
Figure 3: Correlation on timescales up to 30 s, showing the enigmatic ‘precognition dip’ at negative lags of 2–5 s.


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We thank the XTE time-allocation team and Skinakas observatory for their support and flexibility in making these observations possible, and F. Schrey for technical support. We thank A. Zdziarski, K. Horne and R. Hynes for discussions. This work was done with support from the European Commission, the Training and Mobility of Researchers (TMR) research network programme.

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Correspondence to H. C. Spruit.

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Kanbach, G., Straubmeier, C., Spruit, H. et al. Correlated fast X-ray and optical variability in the black-hole candidate XTE J1118+480. Nature 414, 180–182 (2001).

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