Astronomers have obtained the sharpest-ever Xray spectrum of an iconic active galaxy, providing the most accurate, precise view ever obtained of the extreme relativistic effects imprinted onto the spacetime around a supermassive black hole.
Using a powerful trio of telescopes headlined by the Japan Aerospace Exploration Agency (JAXA)/NASA XRISM mission, the research team, led by high-energy astrophysicist Laura Brenneman of the Center for Astrophysics | Harvard & Smithsonian, isolated the broad iron emission line and associated “reflection” that signify a rapidly-spinning black hole. The findings are published in The Astrophysical Journal.
Previous X-ray spectral observations have lacked the resolution to separate the various emission and absorption features in this energy range, but XRISM’s unmatched spectral resolution allows scientists to examine the black hole’s immediate environment with unprecedented accuracy, the authors explained.
“Astrophysical black holes have only two properties: mass and spin,” Brenneman said. “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.”
New insights into MCG–6-30-15 galaxy
Astronomers have long suspected that a large fraction of the X-ray signals from the galaxy known as MCG–6-30-15 come from matter very close to the galaxy’s central supermassive black hole, which is estimated to be about 2 million times the mass of our sun.
But in regions this close to a supermassive black hole’s event horizon, gravity stretches and warps light in line with Einstein’s theories of relativity, explains Brenneman, making it hard to separate these light signals.
Combining results from XRISM’s newly commissioned, ultra-high-resolution “Resolve” X-ray instrument with the broadband power of the European Space Agency’s XMM-Newton and NASA’s NuSTAR produced data that allowed the scientists to separate the spectrum of matter near the event horizon from emission and absorption lines originating in more distant gas.
The study confirms a distinctive, warped iron emission line in the X-ray spectrum, which is evidence of material orbiting at extreme speeds near the event horizon of the black hole rather than being created by outflowing winds along the line of sight to the galaxy. The fast-moving inner disk produces about 50 times as much X-ray reflection as the gas clouds further away, the study suggests.
Further discoveries and future research
A companion publication led by Dan Wilkins of Ohio State University, recently submitted to The Astrophysical Journal, builds on this research by analyzing the spectra at different times throughout the observation. The results confirm and refine the measurement of the rapid black hole spin and provide new insight into the properties of the corona, the highly-energized region just outside the black hole that is producing most of the X-ray emission we observe. The exact nature of the corona remains one of the most intriguing mysteries in astrophysics.
Brenneman said that astronomers will now be able to use XRISM to check the accuracy of previous black hole spin measurements made with lower-resolution X-ray spectra.
“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?'” she explained.
The data have also revealed at least five different “zones” of a wind being driven outward as a byproduct of accretion onto the black hole, said Brenneman.
“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,” said Brenneman. “So measuring these two quantities accurately gives us a holistic view of the symbiotic relationship between supermassive black holes and their host galaxies.” https://www.cfa.harvard.edu/news/new-x-ray-space-telescope-gives-sharpest-ever-glimpse-growth-rapidly-spinning-black-hole
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