The researchers reanalysed observations from the XRISM X-ray mission together with simultaneous measurements from the NICER and NuSTAR telescopes. While a simple model produced a spin value of about 0.99, more detailed models gave values between zero and 0.17. When data from all three telescopes were combined, virtually any spin value remained possible.

The finding highlights a fundamental problem in determining black-hole spin: increasingly precise observations do not necessarily produce a more precise answer if the physical model used to interpret the data is uncertain.

Light does not reach us from a black hole, so its spin cannot be observed directly. Instead, astronomers infer it from radiation emitted by material orbiting the black hole. The faster the black hole spins, the closer the inner edge of the surrounding disk can approach it.

Near the black hole, the motion of the material and its strong gravitational field alter the energy of the radiation, leaving a characteristic signature in the X-ray spectrum.

One of the most important such signatures is an iron line. It is formed when radiation from hot plasma above the disk strikes its surface and is partially reflected.

The plasma can be thought of as a lamp and the disk as an uneven mirror, with the reflected radiation carrying information about the strong gravitational field around the black hole. By analysing the shape of this reflection, astronomers try to determine how close the disk extends to the black hole and then estimate its spin.

Cygnus X-1 is one of the best-studied systems containing a black hole. Previous analyses indicated that its black hole spins at nearly the fastest physically possible speed.

However, this result is difficult to reconcile with gravitational-wave observations, which indicate that black holes in many colliding systems spin relatively slowly. Stellar evolution models also predict that black holes are often born with a small spin rate.

Resolving this dispute is important for understanding the evolution of massive stars and binary systems.

A team including Professor Andrzej Zdziarski from the Nicolaus Copernicus Astronomical Center of the Polish Academy of Sciences and Michał Szanecki from the University of Lodz reanalysed observations of Cygnus X-1 made by the XRISM X-ray mission. They also included simultaneous measurements from the NICER and NuSTAR telescopes, covering a wider energy range.

The simplest model confirmed the previous result: the spin parameter was approximately 0.99 on a scale where zero indicates no spin and one represents the maximum angular momentum that a black hole of a given mass can have, according to general relativity.

However, when the researchers applied a more precise description of the hot plasma, the result changed to a range between zero and 0.17. Another advanced model also indicated a small spin.

When the researchers analysed combined data from all three telescopes, however, virtually any value of the parameter was possible.

The discrepancy results from the fact that before calculating the spin, researchers must describe the radiation illuminating the disk.

The simpler model assumes a certain mathematical form for it. More precise models account for photons repeatedly colliding with hot electrons and gaining energy.

Changing the description of this “lamp” also changes the predicted shape of the reflection. Some of the features previously attributed to rapid spin may therefore result from the structure of the plasma or the geometry of the disk.

The study also reveals that the inner edge of the disk is at most about 10 gravitational radii from the centre of the system. In the case of Cygnus X-1, this corresponds to approximately 200-300 km.

This result is consistent with a disk approaching a low-spin black hole. If the black hole were spinning very rapidly, the disk would have to end sooner than previously assumed.

Therefore, the data better determine the location of the disk's edge than the black hole's spin itself.

There may also be at least two regions of hot plasma above the disk, with some of this plasma flowing away from the disk. This structure helps explain both the weak reflection of radiation and the previously observed X-ray polarization.

However, this is a proposed image of the source, not a definitively confirmed model.

The work demonstrates a limitation typical of astronomy: increasingly precise data do not always yield equally precise answers. Sometimes the result turns out to depend more strongly on the assumptions used to interpret them.

The results were published in The Astrophysical Journal Letters (https://doi.org/10.3847/2041-8213/ae82fa). (PAP)

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