The accretion of matter onto black holes and neutron stars often leads to the launching of outflows that can greatly affect the environments surrounding the compact object. An important means of studying these winds is through X-ray absorption line spectroscopy, which allows us to probe their properties along a single sightline, but usually provides little information about the global three-dimensional wind structure, which is vital for understanding the launching mechanism and total wind energy budget. Here, we study Hercules X-1, a nearly edge-on X-ray binary with a warped accretion disk precessing with a period of about 35 d. This disk precession results in changing sightlines towards the neutron star, through the ionized outflow. We perform time-resolved X-ray spectroscopy over the precession phase and detect a strong decrease in the wind column density by three orders of magnitude as our sightline progressively samples the wind at greater heights above the accretion disk. The wind becomes clumpier as it rises upwards and expands away from the neutron star. Modelling the warped disk shape, we create a two-dimensional map of wind properties. This measurement of the vertical structure of an accretion disk wind allows direct comparisons with three-dimensional global simulations to reveal the outflow launching mechanism.
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XMM-Newton data were reduced using the XMM SAS software and Chandra data were reduced using the CIAO software. All spectra were analysed using the spectral fitting package SPEX. All figures except Fig. 1 and Supplementary Fig. 3 were made in VEUSZ, a Python-based scientific plotting package, developed by Jeremy Sanders and available at veusz.github.io.
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P.K. thanks M. Parker for helpful discussions about photoionization absorption spectral models in XSPEC and SPEX fitting packages. Support for this work was provided by the National Aeronautics and Space Administration through the Smithsonian Astrophysical Observatory (SAO) contract SV3-73016 to MIT for Support of the Chandra X-Ray Center and Science Instruments. P.K. and E.K. acknowledge support from NASA grants 80NSSC21K0872 and DD0-21125X. C.S.R. thanks the STFC for support under consolidated grant ST/S000623/1, as well as the European Research Council (ERC) for support under the European Union’s Horizon 2020 research and innovation programme (grant 834203). R.B. acknowledges support by NASA under award number 80GSFC21M0002. This work is based on observations obtained with XMM-Newton, an ESA science mission funded by ESA member states and the USA (NASA). This research has also made use of data obtained from NASA’s Chandra mission.
The authors declare no competing interests.
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Variation in optical depths of the disk wind absorption lines during the three full-orbit, back-to-back XMM-Newton observations from August 2020. The 4 panels focus on the strongest elemental transitions of N VII, O VIII, Ne X and Fe XXV/XXVI. The Y-axis is in the units of Counts/cm2/s/Å, but the data from different observations were shifted vertically by linear amounts for visual purposes. Observation 0865440101, taken at the beginning of the precession cycle (phase 0-0.04), shows the strongest wind absorption. The lines get weaker at later precession phases during the second observation (0865440401, phase 0.06-0.10) and nearly disappear by the end of the third observation (0865440501, phase 0.12-0.15). Error bars indicate 1σ confidence intervals.
Variation of wind column density with precession phase (left panel), observed Her X-1 luminosity (middle panel) and orbital phase (right panel). Observations from the August 2020 and archival XMM-Newton observations are in black and blue colors, Chandra observations are in red. Error bars indicate 1σ confidence intervals.
Variation of wind ionization parameter with precession phase (left panel), observed Her X-1 luminosity (middle panel) and orbital phase (right panel). Observations from the August 2020 and archival XMM-Newton observations are in black and blue colors, Chandra observations are in red. Error bars indicate 1σ confidence intervals.
Variation of the disk wind outflow velocity (top panel) and velocity width (bottom panel) versus the disk precession phase. Observations from the August 2020 XMM-Newton campaign are in black, archival XMM-Newton observations are in blue, and Chandra observations are in red color. Error bars indicate 1σ confidence intervals.
The wind mass outflow rate calculated by assuming an outflow with a large relative thickness (ΔR/R=1), with a launch solid angle of 4π. Observations from the August 2020 and archival XMM-Newton observations are in black and blue colors, Chandra observations are in red. The green horizontal line shows the measured mass accretion rate through the outer accretion disk50. Error bars indicate 1σ confidence intervals.
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Kosec, P., Kara, E., Fabian, A.C. et al. Vertical wind structure in an X-ray binary revealed by a precessing accretion disk. Nat Astron 7, 715–723 (2023). https://doi.org/10.1038/s41550-023-01929-7