Strong disk winds traced throughout outbursts in black-hole X-ray binaries

• Published in: Nature (London) Vol. 554; no. 7690; pp. 69 - 72
• Main Authors: Tetarenko, B. E., Lasota, J.-P., Heinke, C. O., Dubus, G., Sivakoff, G. R.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 01.02.2018; Nature Publishing Group
• Subjects: 639/33/34/864; 639/766/34/864; Accretion; Accretion disks; Angular momentum; Astrophysics; Bayesian analysis; Binary stars; Black holes; Black holes (Astronomy); Humanities and Social Sciences; Instability; letter; Light; Magnetic fields; Mass transport; Mathematical models; Momentum transport; multidisciplinary; Neutron stars; Numerical simulations; Observations; Parameter estimation; Physics; Science; Stars & galaxies; Stellar evolution; Viscosity; White dwarf stars; Wind; Winds; X ray binaries; X ray stars
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature25159

Analysis of the light curves of outbursts in black-hole X-ray binaries suggests that throughout the accretion process mass is lost from the accretion disks through strong, magnetically driven disk winds. Windy disks Black holes, neutron stars and white dwarfs in close binary systems where mass is flowing from the companion onto the compact object have recurring outbursts that could illuminate the poorly understood accretion process. Bailey Tetarenko and colleagues have analysed 21 archival X-ray outbursts from black-hole X-ray binaries. They conclude that either there is a large rate of angular-momentum transport in the accretion disk, helped by a large-scale magnetic field that permeates the disk, or mass is being lost from the disk in outflows that control the X-ray outburst. Recurring outbursts associated with matter flowing onto compact stellar remnants (such as black holes, neutron stars and white dwarfs) in close binary systems provide a way of constraining the poorly understood accretion process. The light curves of these outbursts are shaped by the efficiency of angular-momentum (and thus mass) transport in the accretion disks, which has traditionally been encoded in a viscosity parameter, α . Numerical simulations 1 , 2 , 3 of the magneto-rotational instability that is believed to be the physical mechanism behind this transport yield values of α of roughly 0.1–0.2, consistent with values determined from observations of accreting white dwarfs 4 . Equivalent viscosity parameters have hitherto not been estimated for disks around neutron stars or black holes. Here we report the results of an analysis of archival X-ray light curves of 21 outbursts in black-hole X-ray binaries. By applying a Bayesian approach to a model of accretion, we determine corresponding values of α of around 0.2–1.0. These high values may be interpreted as an indication either of a very high intrinsic rate of angular-momentum transport in the disk, which could be sustained by the magneto-rotational instability only if a large-scale magnetic field threads the disk 5 , 6 , 7 , or that mass is being lost from the disk through substantial outflows, which strongly shape the outburst in the black-hole X-ray binary. The lack of correlation between our estimates of α and the accretion state of the binaries implies that such outflows can remove a substantial fraction of the disk mass in all accretion states and therefore suggests that the outflows correspond to magnetically driven disk winds rather than thermally driven ones, which require specific radiative conditions 8 .