Ministerio de Ciencia, Innovación y Universidades Gobierno de Canarias Universidad de La Laguna CSIC Centro de Excelencia Severo Ochoa

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An elevation of 0.1light‐seconds for the optical jet base in an accreting Galactic black hole system

Author/s: P. Gandhi, M. Bachetti, V.S. Dhillon, R.P. Fender, L.K. Hardy, F.A. Harrison, S.P. Littlefair, J. Malzac, S. Markoff, T.R. Marsh, K. Mooley, D. Stern, J.A. Tomsick, D.J. Walton, P. Casella, F. Vincentelli, D. Altamirano, J. Casares, C. Ceccobello, P.A. Charles, C. Ferrigno, R.I. Hynes, C. Knigge, E. Kuulkers, M. Pahari, F. Rahoui, D.M. Russell, A.W. Shaw

Reference: 2017 Nature Astronomy 1 859 | Link

Schematic of the accretion and jet geometry of V404 Cygni. The black hole and accretion disc are situated off to the right. X‐rays originate in a compact region within ~5 R<sub>S</sub> of the black hole due to Comptonisation in the jet itself or a coronal. Optical photons are delayed with respect to X‐rays by ~0.1 s. This time delay limits the extension of the putative acceleration and collimation zone (ACZ), site of broadband synchrotron optical emission. Beyond the optical base, shocks in the longitudinally and laterally expanding jet inject energy that power the broadband lower frequency radiation down to the radio.
Schematic of the accretion and jet geometry of V404 Cygni. The black hole and accretion disc are situated off to the right. X‐rays originate in a compact region within ~5 RS of the black hole due to Comptonisation in the jet itself or a coronal. Optical photons are delayed with respect to X‐rays by ~0.1 s. This time delay limits the extension of the putative acceleration and collimation zone (ACZ), site of broadband synchrotron optical emission. Beyond the optical base, shocks in the longitudinally and laterally expanding jet inject energy that power the broadband lower frequency radiation down to the radio.

Relativistic plasma jets are observed in many accreting black holes. According to theory, coiled magnetic fields close to the black hole accelerate and collimate the plasma, leading to a jet being launched. Isolating emission from this acceleration and collimation zone is key to measuring its size and understanding jet formation physics. But this is challenging because emission from the jet base cannot be easily disentangled from other accreting components. Here, we show that rapid optical flux variations from the Galactic black‐hole binary V404 Cyg during the peak of its 2015 outburst are delayed with respect to X‐rays radiated from close to the black hole by ~0.1 seconds. Furthermore, this delayed signal appears together with a brightening radio jet. This result not only support a jet origin for the optical variations, but it also sets a characteristic elevation of ≲103 Schwarzschild radii for the main inner optical emission zone above the black hole, constraining both internal shock and magnetohydrodynamic models. Similarities with blazars suggest that jet structure and launching physics could potentially be unified under mass‐invariant models. Two of the best‐studied jetted black hole binaries show very similar optical lags, so this size scale may be a defining feature of such systems.

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