An international research team has reported XRISM observations of the black hole X-ray binary 4U 1630-472 located in our Milky Way. XRISM is an X-ray astronomy satellite developed through collaboration between Japan, the United States, and Europe, and was launched on September 7, 2023, from the Tanegashima Space Center.

The observations were conducted during the gradual fading phase of the outburst, successfully capturing highly ionized iron absorption lines in the system's darkest X-ray state. These results provide a rare view of the structure and motion of the hot gas surrounding the black hole in its darkest X-ray phase, offering new insights into how these extreme systems evolve and interact with their surrounding environment.

The research results were published in The Astrophysical Journal Letters. The research team was led by Professor Jon M. Miller of the University of Michigan, Dr. Misaki Mizumoto of Fukuoka Normal University, and Dr. Megumi Shidatsu of Ehime University.

Black holes range in mass from a few times the mass of the Sun to billions of times the mass of the Sun. Black hole X-ray binaries contain a stellar-mass black hole, typically with a mass of less than a few tens of times that of the Sun, orbiting an ordinary star. Gas pulled in 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, producing intense X-ray radiation.

Approximately 100 confirmed or candidate black hole X-ray binaries are currently known, including the famous Cygnus X-1. These systems spend most of their time in a faint state but occasionally erupt, with their X-ray brightness increasing by 10,000 times in as little as one week. During such events, some systems launch powerful winds from their accretion disks, but the conditions that trigger such strong outbursts and winds remain unclear. Studying these stellar-mass black holes can also provide valuable insights into the behavior of supermassive black holes at the centers of galaxies, which may have profound effects on star formation and galaxy evolution. By observing stellar-mass black holes up close, astronomers aim to uncover the universal processes that shape the cosmic environment.

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

Observing transient phenomena requires rapid coordination. The team monitored the black hole X-ray binary daily with a wide-angle X-ray instrument and worked closely with the XRISM operations team to adjust the observation schedule in a short time, ultimately making this observation possible.

Even in such a faint phase, the obtained spectrum showed clear absorption lines from highly ionized iron. Notably, in the latter half of the observation, the absorption strengthened even though the X-ray brightness showed almost no change.

Analysis indicates that the absorbing gas is located in the outer accretion disk and moves at a speed of less than about 200km/s — much slower than the wind speeds of about 1000km/s observed in brighter phases. At such low speeds, the gas remains gravitationally bound to the black hole. The increase in absorption in the latter half of the observation is likely due to a localized gas cloud at the outer edge of the accretion disk, possibly formed where the infalling gas stream from the companion star collides with the accretion disk. These observations mark the first time that detailed absorption features of a black hole X-ray binary have been resolved at such low luminosity. Thanks to XRISM's excellent spectral capability, astronomers have been able to map the motion and distribution of hot gas near the black hole, a region that was previously unobservable.

The results suggest that even when X-ray output is weak, highly ionized gas may still exist around the black hole and may even be in motion. This provides valuable insights into the inflow and outflow of gas in the accretion disk, as well as the physical conditions that may trigger wind formation.

The results indicate that in the observed faint state, the high-temperature gas did not escape the system in the form of a wind. However, in brighter states, 4U 1630-472 has been found to launch powerful high-speed outflows, raising key questions:

What specific conditions in luminosity and disk structure trigger the acceleration of gas to form fast winds?

How much mass and energy does this wind inject into the surrounding environment?

The team's next goal is to use the XRISM telescope to capture future outbursts at different brightness levels, thereby tracking changes in gas properties over time. They are currently on standby, ready to respond quickly to the next black hole X-ray binary outburst.