Speaker
Description
It is well accepted by our community that the hard X-ray spectra in black-hole X-ray binaries (BHXRBs) are produced by Comptonization of soft photons. It is also well known that Comptonization produces timelags. This is because the more the soft photons are scattered, the more their energy is increased, the larger the random walk that they do in the scattering medium, and the larger their timelag. Thus, harder photons lag softer ones, naturally. Yet, the majority in our community invoke propagating fluctuation to explain timelags. There is absolutely no doubt that propagating fluctuations occur in the inflow around BHXRBs, but unfortunately there is no theory describing them or even a model with which one could calculate timelags. By assuming that the propagating fluctuations can start anywhere and end anywhere, all timelags, small, medium, and large can be accounted for, but this does not constitute an explanation. On the other hand, I will demonstrate that Comptonization alone can explain quantitatively not only the observed timelags in BHXRBs, but also their correlations with other quantities. These are: 1) timelag vs. Fourier frequency, 2) timelag vs. photon energy, 3) timelag vs. spectral index, and 4) timelag vs. cutoff energy. In my opinion, at least the last two correlations cannot possibly be explained by the propagating-fluctuations idea in its present form. I remark that the Comptonization model explains in addition, quantitatively, the radio – X-ray correlation in BHXRBs and makes predictions.