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An international research team has reported remarkable findings from an XRISM observation of the black hole X-ray binary 4U 1630–472, located in our galaxy. XRISM is an X-ray astronomy satellite developed by Japan in collaboration with the United States and Europe and was launched from the Tanegashima Space Center on September 7, 2023.

This observation, conducted during the fading end of an outburst, successfully captured the highly ionized iron absorption lines at the system's faintest X-ray state. The results offer a rare glimpse into the structure and motion of hot gas around a black hole during its faintest X-ray phase, providing new insights into how these extreme systems evolve and interact with their surroundings.

The work is in The Astrophysical Journal Letters. The team was led by Professor Jon M. Miller (University of Michigan), Dr. Misaki Mizumoto (University of Teacher Education Fukuoka), and Dr. Megumi Shidatsu (Ehime University).

Black holes range in size from a few to billions of solar masses. A black hole X-ray binary contains a stellar-mass black hole, typically less than a few ten times the solar mass, orbiting a normal star. Gas drawn from the companion star spirals toward the black hole, forming an extremely hot accretion disk. In its inner regions, temperatures can reach nearly 10 million Kelvin, generating intense X-ray emission.

About 100 confirmed or candidate black hole X-ray binaries are known, including the notable Cygnus X-1. These systems spend most of their time in a dim state, but occasionally undergo outbursts, during which their X-ray brightness can increase by factors of 10,000 in just about a week. During such episodes, some systems launch powerful winds from their accretion disks, yet the conditions that trigger such large outbursts and launch winds remain poorly understood.

Studying these stellar-mass black holes also offers valuable insights into the behavior of supermassive black holes at the centers of galaxies, which can profoundly influence star formation and galactic evolution. By observing stellar-mass black holes up close, astronomers aim to reveal universal processes that shape cosmic environments.

XRISM carries Resolve, a cutting-edge soft X-ray spectrometer capable of measuring X-ray energies with unprecedented precision. Shortly after the start of the regular operations, the team observed 4U 1630–472, a black hole X-ray binary located in the constellation Norma. Over roughly 25 hours from February 16–17, 2024, XRISM caught the system just before it returned to quiescence at the tail end of an outburst, when its X-ray brightness had already dropped to about one-tenth of its peak.

Observing transient phenomena required rapid coordination. The team conducted daily monitoring of black hole X-ray binaries daily using wide-field X-ray instruments, then worked closely with XRISM's operations team to adjust the schedule at short notice, making this observation possible.

The resulting spectra revealed clear absorption lines from highly ionized iron, even at this dim stage. Notably, in the latter half of the observation, the absorption strengthened despite little change in the X-ray brightness.

Analysis showed that the absorbing gas resided in the outer accretion disk, moving at less than ~200 km/s—much slower than the ~1000 km/s winds observed in brighter phases. At such low speeds, the gas remains gravitationally bound to the black hole. The increase in absorption during the latter half of the observation likely came from a localized gas cloud at the disk's outer edge, possibly formed where the infalling stream from the companion star collided with the disk.

These observations mark the first time that detailed absorption features have been resolved in a black hole X-ray binary at such low luminosity. Thanks to XRISM's exceptional spectral capabilities, astronomers were able to map the motion and distribution of hot gas near the black hole in a regime that had previously been beyond reach.

The results show that even when the X-ray output is weak, highly ionized gas can be present—and maybe in motion—around the black hole. This provides valuable insights into the inflow and outflow of gas in the accretion disk and the physical conditions that could trigger wind formation.

These results indicate that in the faint state observed here, the high-temperature gas is not escaping the system as a wind. However, in brighter states, 4U 1630–472 has been seen launching powerful, high-speed outflows, raising key questions:

• What exact conditions in luminosity and disk structure trigger the acceleration of gas into fast winds?
• How much mass and energy do such winds inject into their surroundings?

The team's next goal is to catch future outbursts at different brightness levels with XRISM, enabling them to track how the gas properties change over time. They are now on standby, ready to respond swiftly when the next eruption from a black hole X-ray binary occurs.