Reverberation lags viewed in hard X-rays from an accreting stellar-mass black hole

The X-ray-emitting corona near a black hole (BH) is too small to be directly imaged, but the rapid variability is used to infer the geometry by measuring time lags caused by coronal X-rays reflecting off the disk, known as reverberation lags. Though reverberation lags have previously been detected for some supermassive BHs in active galactic nuclei (AGNs), detecting them from stellar-mass BHs poses much greater challenges due to their size being over a million times smaller. Previous measurements of reverberation lags for stellar-mass BHs were limited to energies below 10 keV. Here, we report the detection of the Compton hump reverberation, peaking at about 30 keV, from an X-ray binary. The accompanying detection of an iron line feature at about 6.4 keV confirms the X-ray reverberation scenario and provides strong evidence that accretion flows in AGNs and X-ray binaries are governed by an ubiquitous process. High-energy X-ray reverberation has been observed in active galactic nuclei but remained undetected in stellar-mass black holes. Here, the authors show detection of Compton hump reverberation in an X-ray binary, supporting a unified accretion picture across black hole masses.