When a recently-discovered black hole was discovered which emits brilliant light, scientists were baffled, and scrambled to explain how a black hole could shine with such brilliance. According to a new study, the black-hole system called ULX-1 in the nearby Pinwheel Galaxy is twice as bright as astronomers had believed one could ever be.
The Pinwheel Galaxy (or M101) is 22 million light-years from Earth. The amazing brightness of the black hole there might force astronomers to rethink theories about black holes and how they radiate energy.
According to the study’s coauthor, Joel Bregman of the University of Michigan, the black hole “is an extremely luminous one that is shining as brightly as it possibly can.”
The black-hole system in the Pinwheel Galaxy is made up of a star and a black hole which orbit each other. ULX stands for “ultraluminous X-ray source.” The black hole generates a massive amount of extremely bright x-ray light. The light comes from material that the black hole is consuming, as it spirals down into its depths.
The black hole is behaving somewhat like an intermediate black hole, but intermediate black holes are between 100-1,000 times the mass of the sun. However, the new study, which appears in the journal Nature, suggests that the black hole is just 20 times the mass of the sun.This mass would put it on rather the small side for a black hole, yet it consumes matter at the rate a larger one would.
Jifeng Lui of the Chinese Academy of Sciences in Beijing led the research team which studied ULX-1. They used two NASA spacecraft, the chandra X-ray Observatory and the Hubble Space Telescope, and the Gemini Observatory in Hawaii to conduct their research.
By knowing through spectroscopic analysis using the Gemini Multi-Object Spectrograph in Hawaii that the companion star in the ULX-1 black hole system is a large, hot type of star known as a Wolf-Rayet, the research team was able to infer the mass of the star from its luminosity. They inferred that it was 19 times the sun’s mass.
Lui and his team learned that the black hole and the Wolf-Rayet star orbited each other every 8.2 days. This fact enabled the research team to estimate that the mass of the black hole was somewhere between 20-30 times the sun’s mass.
The low mass of the black hole caused Lui and the other researchers to theorize that the black hole was a stellar one. Stellar black holes form after a star dies and collapses in upon itself. A middleweight black hole has yet to be definitely discovered, though it’s theorized that they exist at the heart of most — and possibly all — galaxies.
Ultraluminous X-ray sources have often been taken to be evidence of intermediate-sized black holes, but Lui said in a statement that their findings might “turn the trend” of this assumption by many astronomers occurring so much in the future.
How is the black hole in ULX-1 system emitting so much light?
Lui and his research team theorize that the black hole is emitting light from the stellar wind (stream of charged particles) that it’s consuming from its companion star.
In concluding that the black hole in the ULX-1 system is a stellar one, Lui and his team has demonstrated that “our understanding of the black hole radiation mechanism is incomplete and needs revision.”
According to another member of the research team, Stephen Justham — also of the National Astronomical Observatories of China — low mass black holes like the one in the ULX-1 system must consume energy at close to the theoretical limits that are possible in order for it to release as much energy as has been detected.
Theoretical models of how matter falls into and is consumed by black holes which radiate forth energy show that soft X-rays come mostly from what’s known the “accretion disk a disk which encircles the black hole).” Hard X-rays, on the other hand, are emitted by a high-energy corona which is around the disk.
One of the many puzzling aspects about ULX-1 is that, based upon the size of the black hole, that the region around it should be marked by hard X-rays and be more complex in its structure — but, that isn’t the case with the black hole in the ULX-1 system.
According to Jifeng Lui, current theories don’t adequately explain how the black hole in the ULX-1 system is behaving. Mechanisms mentioned in these theories that “allow such low-mass black holes to eat this quickly and shine this brightly” also “leave signatures in the emitted X-ray spectrum.” But, these sorts of signatures aren’t displayed by the black hole in ULX-1.
The study’s coauthor, Paul Crowther of the Univerity of Sheffield in the UK, said that as it takes so long for us to get light and images from a galaxy as far away as the Pinwheel Galaxy, the Wolf-Rayet star is also probably dead, “so this system is now likely a double black hole binary.”
Written by: Douglas Cobb