A black hole, located within the ULX-1 system of the Pinwheel Galaxy (a.k.a. Messier 101), has left scientists scratching their heads, as they struggle to fathom the basis for its incredible luminosity. The system in question is some 22 million light-years from Earth and is forcing astronomers to reconsider our current understanding of black holes and how they consume matter.
The research team was led by Jifeng Liu of the Chinese Academy of Sciences in Beijing, with the results published in the Nov. 28 issue of the journal Nature.
A Lightweight Black Hole Outputting Soft X-Rays?
The ULX-1 system – abbreviated from ultraluminous X-ray source – comprises of a black hole and companion star that orbit one another. Using observations from NASA’s Hubble Space Telescope and Chandra X-ray
Observatory, as well as the Gemini Observatory in Hawaii, the team discovered that ULX-1 outputs enormous amounts of high-energy X-ray light.
The black hole, conjectured to be the X-ray source, was unusually “lightweight.” The group were puzzled by how such a small black hole, weighing in at just 20 to 30 times the mass of our sun, could snaffle up such a vast amount of material.
Stephen Justham, based at the National Astronomical Observatories of China, Chinese Academy of Sciences, argues that the black hole is likely to be consuming material at the upper end of its theoretical limits, principally based upon the high energy output observed:
“We thought that when small black holes were pushed to these limits, they would not be able to maintain such refined ways of consuming matter… We expected them to display more complicated behavior when eating so quickly. Apparently we were wrong.”
X-ray sources can emit high and low energy X-rays, which are dubbed hard and soft X-rays, respectively. Larger black holes tend to generate more soft X-rays, whereas smaller black holes give off hard X-rays.
Based upon the intensity of the X-ray emissions, the astronomers anticipated the discovery of an intermediate-mass black hole, around 100 to 1,000 times the mass of the sun. The group were shocked, however, to find that the black hole was not only very small, but was also producing soft X-rays; this finding continues to baffle researchers.
Do Astronomers Need to Rethink Existing Black Hole Models?
Theoretical models, that describe how matter is consumed by black holes and radiates energy, suggest that soft X-rays primarily originate from the accretion disk; hard X-rays are thought to derive from the corona around the disk, meanwhile. In addition, these models work on the premise that the corona’s emission intensity increases as the amount of accretion nears the black hole’s theoretical limit of consumption.
However, based upon the relatively tiny size of the ULX-1 black hole, alongside the finding that it emits predominately soft X-rays, existing models appear to conflict with recent findings.
Liu, the study’s lead author, ruminates over these curious observations and the need to better understand the unusual nature of this relatively small black hole:
“Theories have been suggested which allow such low-mass black holes to eat this quickly and shine this brightly in X-rays. But those mechanisms leave signatures in the emitted X-ray spectrum, which this system does not display.”
Liu goes on to articulate the almost incredulous finding that such a modestly-sized black hole could be responsible for eating material, at a rate that appears to broach its theoretical maximum, whilst remaining quite “placid.” He also expresses his keen interest in finding theory that stacks up with observation.
The team employed the Gemini Multi-Object Spectrograph at the Gemini North telescope on Mauna Kea, Hawaii, to follow the companion star’s orbit. Using the spectroscopic analysis, they were also able to confirm that the companion star was an extremely hot, massive Wolf-Rayet star, from which the black hole was feeding.
Wolf-Rayet stars show strong stellar winds that are ideal sources of material for famished black holes. However, it was originally thought that these winds – comprising of gales of charged particles, billowing from the star’s atmosphere – would have been too weak and inefficient to explain the ultraluminous X-ray source detected.
Ultimately, although the study has proved insightful, the researchers remain somewhat disappointed that they found no evidence of the elusive intermediate-mass black hole. Some astronomers have suggested that these middleweight black holes could prove key to understanding the birth of supermassive black holes, typically in the order of hundreds of thousands to billions of solar masses; it is thought that these monstrous enigmas lie at the heart of most, if not all, galaxies.
By James Fenner