XRISM reveals the hot gas in slow motion near the black hole during the low X -ray phase

An international research team reported remarkable results of an XRISM observation of the radiography of the black hole 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 has been launched from the Tanegashima Space Center on September 7, 2023.

This observation, carried out during the ending of the discoloration of an explosion, successfully succeeded in the highly ionized iron absorption lines in the lowest X -rays of the system. The results offer a rare overview of the structure and movement of hot gas around a black hole during its lowest X -ray phase, providing new information on how these extreme systems evolve and interact with their environment.

The work is published in Astrophysical newspaper letters. The team was led by Professor Jon Mr. Miller (University of Michigan), Dr. Misaki Mizumoto (University of Education for Fukuoka teachers) and Dr. Megumi Shidatsu (Ehime University).

Black holes vary in size from a few to billions of solar masses. An X -ray binary of black hole contains a black hole in the stellar mass, generally less than sometimes the solar mass, in orbit of a normal star. Gas from the spirals’s companion star towards the black hole, forming an extremely hot accretion disc. In its interior regions, temperatures can reach nearly 10 million Kelvin, generating intense X -ray emissions.

About 100 binary of X-ray with confirmed black holes or candidates are known, including the notable Cygnus X-1. These systems spend most of their time in a dark state, but sometimes undergo explosions, during which their brightness of X -rays can increase by factors of 10,000 in a little a week. During these episodes, some systems launch powerful winds from their accretion discs, but the conditions that trigger such large explosions and launch winds remain poorly understood.

The study of these black holes of stellar mass also offers valuable information on the behavior of supermassive black holes in the centers of galaxies, which can deeply influence the formation of stars and galactic evolution. By closely observing black holes of stellar mass, astronomers aim to reveal universal processes that shape cosmic environments.

The XRISM carries the resolution, a gentle X -ray spectrometer capable of measuring X -ray energies with unprecedented precision. Shortly after the start of regular operations, the team observed 4U 1630–472, an X -ray binary of black hole located in the Norma constellation. In about 25 hours of February 16 to 17, 2024, Xrism caught the system just before its return to quiescence at the end of an explosion, when its brightness of the X -rays had already fallen to about a tenth of its peak.

The observation of transient phenomena required rapid coordination. The team carried out daily surveillance of the binary of X Black Hole X -rays daily using large X -ray instruments, then worked in close collaboration with the XRISM operations team to adjust the calendar in the short term, which makes this observation possible.

The resulting spectra have revealed clear absorption lines from highly ionized iron, even at this low stage. In particular, in the second half of the observation, absorption has strengthened despite little change in the brightness of X -rays.

The analysis has shown that absorbing gas was resided in the external accretion disc, moving at less than ~ 200 km / s – much slower than the winds of around 1000 km / s observed in brighter phases. At such low speeds, the gas remains linked to gravitation to the black hole. The increase in absorption during the second half of the observation probably comes from a gas cloud located at the outer edge of the disc, perhaps formed where the watercourse at the influence of the complementary star has collided with the disc.

These observations mark the first time that detailed absorption characteristics have been resolved in an X -ray binary of black hole with such a low light. Thanks to Xrism’s exceptional spectral capacities, astronomers were able to map the movement and distribution of hot gas near the black hole in a diet that was previously out of reach.

The results show that even when the X -ray outlet is low, highly ionized gas can be present – and perhaps in motion – in the black hole. This provides valuable information on the influx and gas flow in the accretion disk and the physical conditions that could trigger wind formation.

These results indicate that in the low condition observed here, high temperature gas does not escape the system as a wind. However, in brighter states, 4U 1630–472 was seen launching powerful outings at high speed, raising key questions:

• What exact conditions in the brightness and disk structure trigger the acceleration of fast wind gas?
• How much mass and energy do these winds inject into their environment?

The team’s next goal is to catch future explosions at different levels of brightness with XRISM, allowing them to follow how gas properties change over time. They are now on standby, ready to respond quickly when the next eruption of an X -ray binary of black hole occurs.