The X-Ray Imaging and Spectroscopy Mission (XRISM), a joint mission between the Japanese Aerospace Exploration Agency (JAXA) and NASA, launched on Sept. 7th, 2023. Its advanced imaging filters and spectrometers were designed to study black holes and neutron stars and detect the hot plasma in the intergalactic medium. Alongside the European Space Agency’s (ESA) X-ray Multi-Mirror Mission Newton (XMM-Newton) and NASA’s Nuclear Spectroscopic Telescope Array (NuSTAR), XRISM has provided the sharpest-ever X-ray spectrum of the iconic MCG–6-30-15.
Located 120.7 million light-years from Earth, this Type 1 Seyfert galaxy is noted for its variable X-ray spectrum and a central supermassive black hole (SMBH), estimated to be about 2 million solar masses. The research team, led by Laura Brenneman of the Harvard & Smithsonian Center for Astrophysics (CfA), has managed to isolate the broad iron emission line and the associated “reflection” that are indicative of a rapidly-spinning SMBH. Thanks to XRISM's unmatched spectral resolution, the team was able to study the black hole's immediate environment (which includes an accretion disk that extends close to its event horizon.
For some time, astronomers have suspected that a large fraction of the X-ray emissions coming from this galaxy originates from matter very close to the galaxy’s SMBH. However, previous X-ray telescopes have lacked the resolution to separate the various emission and absorption lines in this energy range, preventing them from investigating this theory. In regions that are close to an SMBH's event horizon, gravity radically alters the curvature of spacetime (consistent with Einstein's Theory of General Relativity), making it hard to separate light signals originating near the event horizon from more distant gas clouds.
By combining ultra-high resolution data from XRISM’s “Resolve” X-ray instrument with the broadband power of XMM-Newton and NuSTAR, the team obtained data that allowed them to separate the emission and absorption lines from these two sources. As Brenneman explained in a CfA press release:
Astrophysical black holes have only two properties: mass and spin. We can estimate their masses by several different means, but measuring their spins is much harder and requires collecting data from gas that is orbiting the black hole immediately outside the event horizon. For supermassive black holes in active galactic nuclei, this is best accomplished by obtaining X-ray spectra with high signal-to-noise and spectral resolution.
The study, which recently appeared in *The Astrophysical Journal*, confirmed the presence of a warped iron emission line in the X-ray spectrum. This has provided the first evidence of material orbiting at close to the speed of light near the event horizon rather than outflowing winds between Earth and the galaxy. This region, they suggest, produces about 50 times as much X-ray reflection as more distant gas clouds. A companion study led by co-author Daniel R. Wilkins of Ohio State University, recently submitted to The Astrophysical Journal, builds on these results by analyzing the spectra at different times of observation.
According to Brennerman, these results show how astronomers can use XRISM to confirm and refine previous measurements of black hole spin rates obtained from lower-resolution X-ray spectra. Their study also revealed crucial data about the SMBH's corona, the billion-degree region that extends above and below the accretion disk. This region is responsible for producing most of an SMBH's X-ray emission and has long been a mystery in astrophysics. Their study has also revealed at least five distinct zones of a wind created and driven by accretion onto the black hole. Said Brennerman:
We want to go back and look at all of the sources for which we have lower-resolution spectra and observe them with XRISM, and say, ‘Okay, now that we're confident we can separate out the narrow and the broad features, how accurate were our previous spin measurements?’ Understanding these winds in addition to the black hole’s spin is important because they can tell us how galaxies grow and evolve, either primarily by collecting gas or by mergers with other galaxies and black holes. So measuring these two quantities accurately gives us a holistic view of the symbiotic relationship between supermassive black holes and their host galaxies.
Further Reading: Harvard & Smithsonian Center for Astrophysics (CfA), The Astrophysical Journal
