Overview
The X-ray Imaging and Spectroscopy Mission XRISM has successfully observed X-rays emitted from the innermost regions of the accretion disk surrounding the supermassive black hole at the center of the active galaxy MCG–6-30-15. According to Einstein’s theory of general relativity, light originating close to a black hole is strongly affected by intense gravity, which distorts spacetime and alters the observed wavelengths of the radiation.
By fully exploiting the exceptional spectral resolution of XRISM’s onboard high-resolution spectrometer Resolve, the research team was able to accurately extract the X-ray spectrum originating from near the innermost edge of the accretion disk. This achievement allowed the relativistic effects caused by the black hole’s strong gravity to be clearly identified. The results also suggest that the black hole may be rapidly rotating.
Scientific Background
Our solar system resides in a galaxy known as the Milky Way, but the Universe contains countless other galaxies. Many of them are known to host supermassive black holes at their centers, with masses millions to billions of times that of the Sun. Strong gravity draws surrounding gas toward the black hole, forming a rotating structure known as an accretion disk.
Iron atoms within the disk emit a characteristic X-ray feature called the Fe Kα emission line. This line serves as an important probe of the environment near a black hole. When the accretion disk extends deep into the strong-gravity region, the combined effects of rapid orbital motion (special relativistic effects) and spacetime distortion caused by intense gravity (general relativistic effects) produce a strongly broadened and asymmetric Fe Kα line profile.
In the case of MCG–6-30-15, observations by the Japanese X-ray astronomy satellite ASCA, launched in 1993, previously reported the detection of such a broadened Fe Kα line. However, because earlier missions lacked sufficient spectral resolution, it was also suggested that absorption by winds flowing out from the accretion disk could make the line appear broadened, leaving the true origin of the feature unresolved.
Observational Results
In February 2024, XRISM observed the supermassive black hole in MCG–6-30-15, located approximately 120 million light-years away in the direction of the constellation Centaurus. To obtain a broad-band X-ray spectrum beyond XRISM’s own energy coverage, the observation was carried out jointly with NASA’s NuSTAR mission and ESA’s XMM-Newton observatory. Within this coordinated campaign, XRISM played a central role.
Thanks to its outstanding spectral resolution, the Resolve is particularly well suited to investigating the detailed spectral profile in the Fe Kα band. As shown in the figure, the data clearly reveal not only narrow emission and absorption lines, but also a genuinely broadened Fe Kα emission component originating from the innermost accretion disk near the black hole. This result effectively resolves a long-standing debate that has continued for more than three decades since the ASCA era.
The narrow emission lines are attributed to X-ray reflection in regions farther from the black hole, while the absorption lines are interpreted as signatures of winds launched from the accretion disk. The ability of XRISM to identify and separate these components for the first time was crucial for determining the true contribution of the relativistically broadened Fe Kα line.
Observations of the Fe Kα line also provide important information about black hole rotation. A black hole possesses a boundary known as the event horizon, within which even light cannot escape outward. According to general relativity, for a non-rotating black hole, the accretion disk cannot extend inward beyond three times the radius of the event horizon. In contrast, if the black hole is rapidly rotating in the same direction as the disk, the disk can extend much closer to the event horizon. Further analysis of the XRISM data is expected to yield additional evidence for rapid black hole rotation.
Future Prospects
The XRISM observation of MCG–6-30-15 demonstrates that high-resolution X-ray spectroscopy is a powerful means of directly probing the physics of strong gravity near black holes. In particular, the clear detection of iron-line broadening caused by general relativistic effects represents a major step forward in measuring spacetime distortion around black holes.
In the future, similar techniques will be applied to supermassive black holes in other active galaxies, as well as to stellar-mass black holes within our own Galaxy, to further investigate the structure of accretion disks and black hole rotation. In addition to X-rays emitted from the immediate vicinity of the black hole, the XRISM data also reveal absorption lines produced by fast outflows launched from the accretion disk. Such outflows are thought to influence the evolution of entire galaxies. Detailed studies of the relationship between black hole mass and disk-driven outflows are therefore expected to provide key insights into the co-evolution of galaxies and their central black holes.
Paper Information
Journal：Astrophysical Journals
Title： A Sharper View of the X-ray Spectrum of MCG–6-30-15 with XRISM, XMM-Newton and NuSTAR
Authors： Laura W. Brenneman, Daniel R. Wilkins, Anna Ogorzałek, Daniele Rogantini, Andrew C. Fabian, Javier A. García, Anna Juráňová, Misaki Mizumoto, Hirofumi Noda, Ehud Behar, Rozenn Boissay-Malaquin, Matteo Guainazzi, Takashi Okajima, Erika Hoffman, Noa Keshet, Jelle Kaastra, Erin Kara, and Makoto Yamauchi
DOI： https://doi.org/10.3847/1538-4357/ae1225
