The geometry around the supermassive black hole, as revealed by 15 years of X-ray observational data

Oct. 27, 2023 | GATEWAY to Academic Articles

MOCHIZUKI Yuto / Dept. of Astronomy, The University of Tokyo
Dept. of Space Astronomy and Astrophysics, ISAS

It is widely believed that supermassive black holes*1 exist at the centers of galaxies, including our Milky Way, and understanding their structures is one of the most critical challenges in modern astronomy. When matter spirals down into a supermassive black hole, it forms an accretion disk*2 around the black hole. The central region of the accretion disk, which shines brightly, is known as the active galactic nucleus*3, and its intense activity is suggested to influence star formation throughout the galaxy.

To unravel the central structure of the active galactic nucleus with complex X-ray spectral variations*4, observed from the active galactic nucleus known as Mrk 766, we reanalyzed 15 years of archival data*5 from X-ray astronomical satellites in Europe and the United States. As a result, by considering previously unaccounted-for X-ray scattering components in addition to materials partially covering the line of sight that absorb some X-rays, and X-ray absorption due to outflows from the center, we successfully explained the X-ray observational data over the entire observation period with a simplified model. This study is expected to refine the conventional understanding of the structure of active galactic nuclei, with the launch of the XRISM satellite in September 2023.

Research Summary

The outflows of matter from the center of an active galactic nucleus, which are transported broadly outward, are suggested to influence star formation throughout the entire galaxy. To understand this influence, it is necessary to know the structure and physical conditions of these outflows. In this study, we analyzed X-ray observational data from Mrk 766, an active galactic nucleus known for exhibiting complex X-ray spectra, to investigate the physics of outflows and central structures.

X-rays can penetrate thin materials but are absorbed by denser ones, allowing us to estimate the surrounding structure by studying the absorption of X-rays originating from the high-temperature plasma*6 in the central region of the active galactic nucleus. When observing X-rays from Mrk 766, it is known that its brightness varies with time, indicating the presence of outflows. However, previous study could not explain these X-ray spectra uniformly over the entire observation period or unravel the central structure.

Figure 1
Figure 1: Schematic diagram of the central structure of the active galactic nucleus Mrk 766 (Mochizuki et al. 2023, Fig.6). Various regions around the black hole are labeled with numbers W1 to W5. X-rays generated from the hot corona surrounding the black hole undergo warm absorption by W3, absorption by the outflow from W4, partial absorption due to the triple structure of W1, W2, and W5, and further scattering (indicated by the light blue lines) due to the outflow from W4. The diagram created by Midooka et al. 2022 (https://www.isas.jaxa.jp/home/research-portal/en/gateway/2022/0926/) for the supermassive black hole NGC 5548 was modified to account for the differences between the two objects.

In this study, we performed simulation calculations using JAXA's supercomputer (JSS3) to model X-ray absorption by surrounding materials. Applying this model to observational data revealed that all the data could be explained by considering three types of absorption. The first is partial absorbers that partially absorb X-rays by covering part of the line of sight. We discovered that partial absorbers with a three-layer structure, which vary in their covering percentage along the line of sight, can explain the seemingly complex variations in X-ray spectra (Figure1. W1, W2, W5). The second is a high-speed outflow with a velocity of approximately 10% of the speed of light (about 30,000 kilometers per second) (Figure1. W4). Considering this outflow allowed us to explain the blue-shifted absorption lines due to the Doppler effect*7. The third is absorption caused by the warm absorber*8, which is believed to exist at a relatively distant location (Figure1. W3). Furthermore, Mrk 766 exhibits a broadened structure of iron emission lines*9, the origin of which has been a subject of long-standing debate. This study revealed that, in addition to the narrow emission lines resulting from scattering by distant neutral matter and the broad emission lines from the accretion disk, there is a slightly broadened emission line structure. Comparing this structure to previous simulations of radiative fluid outflows*10, it became evident that this structure is due to X-ray scattering components caused by the outflow.

As a result, we proposed a model comprising a distant scattering component, partial absorbers, an accretion disk, outflows, and a warm absorber, which can explain all 15-year X-ray observational data from the active galactic nucleus Mrk 766. Furthermore, we successfully constrained the amount, velocity, and angle of the outflows and supported the notion that partial absorbers originate from the outflows. The scattering component of outflows suggested in this study is likely to be present in other active galactic nuclei as well. This could potentially revise the previously assumed structure, which was inferred from a broad range of iron emission lines. Furthermore, with the launch of the XRISM satellite in September 2023, it is anticipated that the understanding of the central structure of active galactic nuclei will advance by elucidating the driving mechanism of outflows and the internal conditions of these outflows.

Terminologies

  • *1 Supermassive black hole : A black hole with a mass exceeding one million times that of the Sun.
  • *2 Accretion disk : A disk formed as material falls onto a celestial body while rotating.
  • *3 Active Galactic Nucleus : The brightly shining region at various wavelengths in the center of a galaxy.
  • *4 X-ray spectrum : The distribution of energy intensity of X-rays.
  • *5 Archive data : Data collected by satellites from previous observations. After a certain period, this data is made public and can be analyzed by anyone.
  • *6 Hot corona : The source of X-rays near a black hole. It is believed to consist of extremely hot material with temperatures exceeding 10 million degrees.
  • *7 Doppler effect : A phenomenon in which the wavelength of electromagnetic waves changes when there is a relative velocity between the light source and the observer.
  • *8 Warm absorber : A slow-moving absorbing entity located at a distance from a supermassive black hole.
  • *9 Emission lines : The X-rays emitted when atoms return from an unstable state to a stable state.
  • *10 Radiative fluid simulation : Simulation that solves the physical processes in which material flowing emits radiation due to electromagnetic waves.

Information

Journal Title Monthly Notices of the Royal Astronomical Society
Full title of the paper Origin of the complex iron line structure and spectral variation in Mrk 766
DOI https://doi.org/10.1093/mnras/stad2329
Publish date August 4, 2023
Author(s) MOCHIZUKI Yuto, MIZUMOTO Misaki, EBISAWA Ken
ISAS or JAXA member(s) among author(s) MOCHIZUKI Yuto (Dept. of Astronomy, The University of Tokyo / Dept. of Space Astronomy and Astrophysics, ISAS), EBISAWA Ken (Dept. of Astronomy, The University of Tokyo / Dept. of Space Astronomy and Astrophysics, ISAS)

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Author

MOCHIZUKI Yuto
March 2022: Bachelor of Science, Faculty of Science, Division 1, Department of Physics, Tokyo University of Science.
April 2022-Present: Master's Program in Astronomy, Graduate School of Science, the University of Tokyo.
April 2022-Present: Ebisawa Laboratory, Department of Space Physics, the Institute of Space and Astronautical Science (ISAS).