XRISM Detects Iron in Accretion Disk around NGC 4151’s Supermassive Black Hole

NGC 4151 is a spiral galaxy located some 62 million light-years away in the northern constellation of Canes Venatici.

This artist’s concept shows the possible locations of iron revealed in XRISM’s X-ray spectrum of NGC 4151. Image credit: Conceptual Image Lab, NASA’s Goddard Space Flight Center.

The X-Ray Imaging and Spectroscopy Mission (XRISM) — a collaboration between JAXA and NASA, with significant participation from ESA — lifted off from the Tanegashima Space Center in Japan on September 6, 2023.

After starting science operations in February 2024, the spacecraft focused on the supermassive black hole at the center of NGC 4151.

“XRISM’s Resolve instrument captured a detailed spectrum of the area around the black hole,” said Dr. Brian Williams, a researcher at NASA’s Goddard Space Flight Center.

“The peaks and dips are like chemical fingerprints that can tell us what elements are present and reveal clues about the fate of matter as it nears the black hole.”

NGC 4151’s supermassive black hole holds more than 20 million times the Sun’s mass.

The galaxy is also active, which means its center is unusually bright and variable.

Gas and dust swirling toward the black hole form an accretion disk around it and heat up through gravitational and frictional forces, creating the variability.

Some of the matter on the brink of the black hole forms twin jets of particles that blast out from each side of the disk at nearly the speed of light.

A puffy donut-shaped cloud of material called a torus surrounds the accretion disk.

XRISM’s Resolve instrument captured data from the center of NGC 4151. The resulting spectrum reveals the presence of iron in the peak around 6.5 keV and the dips around 7 keV, light thousands of times more energetic that what our eyes can see. Image credit: JAXA / NASA / XRISM Resolve / CXC / CfA / Wang et al. / Isaac Newton Group of Telescopes, La Palma / Jacobus Kapteyn Telescope / NSF / NRAO / VLA.

“In fact, NGC 4151 is one of the closest-known active galaxies,” Dr. Williams and his colleagues said.

“Other missions, including NASA’s Chandra X-ray Observatory and the NASA/ESA Hubble Space Telescope, have studied it to learn more about the interaction between black holes and their surroundings, which can tell scientists how supermassive black holes in galactic centers grow over cosmic time.”

“The galaxy is uncommonly bright in X-rays, which made it an ideal early target for XRISM.”

“Resolve’s spectrum of NGC 4151 reveals a sharp peak at energies just under 6.5 keV — an emission line of iron.”

Astronomers think that much of the power of active galaxies comes from X-rays originating in hot, flaring regions close to the black hole.

X-rays bouncing off cooler gas in the disk causes iron there to fluoresce, producing a specific X-ray peak.

This allows them to paint a better picture of both the disk and erupting regions much closer to the black hole.

“The spectrum also shows several dips around 7 keV,” the astronomers said.

“Iron located in the torus caused these dips as well, although through absorption of X-rays, rather than emission, because the material there is much cooler than in the disk.”

“All this radiation is some 2,500 times more energetic than the light we can see with our eyes.”

“Iron is just one element XRISM can detect. The telescope can also spot sulfur, calcium, argon, and others, depending on the source.”

“Each tells us something different about the cosmic phenomena scattered across the X-ray sky.”

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