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An energetic hot wind from the low-luminosity active galactic nucleus M81*

Abstract

For most of their lifetime, super-massive black holes (SMBHs) commonly found in galactic nuclei obtain mass from the ambient medium at a rate well below the Eddington limit1, which is mediated by a radiatively inefficient, hot accretion flow2. Both theory and numerical simulations predict that a strong wind must exist in such hot accretion flows3,4,5,6. The wind is of special interest not only because it is an indispensable ingredient of accretion but also, perhaps more importantly, because it is believed to play a crucial role in the evolution of the host galaxy via the so-called kinetic mode active galactic nucleus feedback7,8. Observational evidence for this wind, however, remains scarce and indirect9,10,11,12. Here we report the detection of a hot outflow from the low-luminosity active galactic nucleus in M81, based on Chandra high-resolution X-ray spectroscopy. The outflow is evidenced by a pair of Fe xxvi Lyα lines redshifted and blueshifted at a bulk line-of-sight velocity of ±2.8 × 103 km s−1 and a high line ratio of Fe xxvi Lyα to Fe xxv Kα implying a plasma temperature of 1.3 × 108 K. This high-velocity, hot plasma cannot be produced by stellar activity or the accretion inflow onto the SMBH. Our magnetohydrodynamical simulations show that, instead, it is naturally explained by a wind from the hot accretion flow, propagating out to 106 times the gravitational radius of the SMBH. The kinetic energy and momentum of this wind can significantly affect the evolution of the circumnuclear environment and beyond.

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Fig. 1: Detection and characterization of highly ionized Fe lines.
Fig. 2: The hot wind model.

Data availability

Source data are provided with this paper. The original X-ray data used in this work are publicly available in the online HEASARC archive at https://heasarc.gsfc.nasa.gov/cgi-bin/W3Browse/w3browse.pl. Reduced X-ray spectra are available in Supplementary Data 15.

Code availability

Spectral analysis is conducted using Xspec (https://heasarc.gsfc.nasa. gov/docs/xanadu/xspec/), which employs ATOMDB for the modelling of atomic lines. The ZEUS-MP/2 and ATHENA++ codes used in this work are publicly available at https://github.com/bwoshea/ZEUS-MP_2 and at https://github.com/PrincetonUniversity/athena-public-version. The wind simulation data for Fig. 2 are provided with this paper.

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Acknowledgements

This research made use of observations taken at the Chandra X-ray Observatory, the software package CIAO provided by the Chandra X-ray Center, the spectral fitting package Xspec, and the High Performance Computing Resource at the Core Facility for Advanced Research Computing at the Shanghai Astronomical Observatory. Z.L. and F.S. acknowledges support by the National Key Research and Development Program of China (grant no. 2017YFA0402703) and Natural Science Foundation of China (grant no. 11873028). F.Y. and B.Z. are supported in part by the National Key Research and Development Program of China (grant no. 2016YFA0400704), the Natural Science Foundation of China (grant no. 11633006) and the Key Research Program of Frontier Sciences of the Chinese Academy of Sciences (grant no. QYZDJSSW-SYS008). We thank C. Cui and H. Yang for help with initial model tests and GRMHD simulation of hot accretion flows, and J. Wang for helpful discussions.

Author information

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Authors

Contributions

This research programme was designed and framed by Z.L. and F.Y. Analysis and modelling of the X-ray data were performed by F.S. with the help of Z.L. Numerical simulations of the accretion flow and wind were performed by B.Z. and F.Y. All authors were involved in the discussion and interpretation of the results presented, and all contributed to writing the paper.

Corresponding authors

Correspondence to Zhiyuan Li or Feng Yuan.

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The authors declare no competing interests.

Additional information

Peer review information Nature Astronomy thanks Andrew Young and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Extended data

Extended Data Fig. 1 The combined Chandra/HETG images highlighting the 1st-order MEG/HEG arms.

Spectra are extracted from the individual observations of different roll angles and then combined to form the final spectrum. The green rectangles illustrate the spectral extraction region (solid rectangles for the source and the adjacent dashed rectangles for the background) for one of the 15 observations. The NuSTAR source region is marked by the vermillion solid circle, while the corresponding background region is marked by the vermillion dashed annulus. Discrete sources falling within the NuSTAR spectral extraction regions are not excluded, but their collective flux contribution is negligible.

Extended Data Fig. 2 Observed X-ray spectra of M81*.

Red: coadded Chandra/MEG 1st-order spectrum; Black: coadded Chandra/HEG 1st-order spectrum; Green: NuSTAR/FPMA spectrum; Blue: NuSTAR/FPMB spectrum. The error bars are of 1σ. The best-fit absorbed power-law model is shown by the solid lines. The ratio of residual/error is shown in the bottom panel. Significant excess is seen between 6–7 keV due to the presence of Fe lines. The spectra shown here are binned to achieve a S/N greater than 3 for better illustration, while in the actual spectral fit throughout this work the spectra are binned to have at least one count per bin to optimize the spectral resolution.

Extended Data Fig. 3 Blind line search of the MEG and HEG spectra over 1–3 keV.

Blue, green and orange contours indicate confidence level of 99%, 90% and 68%, respectively, of a test line according to the differential C-stat value against the baseline continuum model. The black contour denotes where ΔC = + 0.5. Significant lines with an identified atomic transition are denoted with the pink vertical dashed lines.

Extended Data Fig. 4 Coadded HETG zeroth-order spectrum from an annulus of inner-to-outer radii of 2.5 –5.0 around M81*.

The spectrum can be well fitted by an absorbed power-law, shown as the magenta line. The error bars are of 1σ. The ratio of residual/error is shown in the bottom panel. An additional thermal component, represent by an apec model with a plasma temperature of 0.9 keV, is allowed by the data and shown as the blue line. The sum of the power-law and apec is plotted as the black line.

Extended Data Fig. 5 Predicted Fe line luminosity of an isotropic and uniform gas cloud photonionized by a central AGN.

The upper (lower) panel is for Fe XXVI Lyα (XXV Kα). The ionization parameter is evaluated for an intrinsic X-ray spectrum same as M81* and over photon energy of 2–10 keV. The cloud has an equivalent hydrogen column density of NH = 1023cm−2 (black solid line), 1022cm−2 (red dash-dotted line) and 1021cm−2 (blue dashed line). The black dotted horizontal line in each panel marks the observed line luminosity, which is substantially higher than the predicted values.

Extended Data Fig. 6 Predicted 6.5–7.3 keV spectrum from simulation of the hot accretion flow.

The viewing angle is set to be 45 with respect to the jet axis. The blue, red and black curves show the blueshifted, redshifted and total spectrum, respectively. The spectra have been convolved with the HEG instrumental response. The black crosses mark the observed spectrum as a reference. The error bars are of 1σ. The observed Fe XXVI and XXV lines have an equivalent width substantially higher than predicted by the hot accretion flow.

Extended Data Table 1 Log of X-ray observations
Extended Data Table 2 Additional Emission lines in the HETG spectrum

Supplementary information

Supplementary Data 1

The stacked Chandra HEG source and background spectra with the instrumental response files.

Supplementary Data 2

The stacked Chandra MEG source and background spectra with the instrumental response files.

Supplementary Data 3

The NuSTAR FPMA source and background spectra with instrumental response files.

Supplementary Data 4

The NuSTAR FPMB source and background spectra with instrumental response files.

Supplementary Data 5

The stacked Chandra HETG zero-order spectrum of an off-nuclear region with instrumental response files.

Source data

Source Data Fig. 2

Source data for the simulation results that can reproduce Fig. 2.

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Shi, F., Li, Z., Yuan, F. et al. An energetic hot wind from the low-luminosity active galactic nucleus M81*. Nat Astron 5, 928–935 (2021). https://doi.org/10.1038/s41550-021-01394-0

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