Scientists have shown that explosions from the early universe might help in solving black holes' middle sibling problem.
A statistical analysis of thousands of gamma-ray bursts, the luminous energetic explosions that follow the violent collapse or merging of stars, show a time delay to some of the signals, a tell-tale sign of gravitational lensing.
In a letter published in Nature Astronomy, researchers from Melbourne University and Monash University have said that this bending of electro-magnetic radiation points to evidence of an intermediate-mass black hole, defined as having a mass between 100 and 100,000 times that of the Sun.
Intermediate-mass black holes have proved something of a troublesome middle child in this class of very dense celestial objects.
While researchers have detected gravitational waves from smaller black holes through the LIGO-Virgo collaboration and super-massive black holes have been detected by the Event Horizon Telescope, intermediate-mass black holes have been difficult to observe, the authors argued.
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They have long been supposed to live in the middle of globular clusters – a spherical blob of stars – as friction and gravity cause the biggest stars to sink to the bottom of the gravitational well. Since 2004, simulations have indicated that, for small compact clusters, stellar mergers happen within the lifetime of these giant stars, the authors said.
"Critically, these mergers occur before the stars go supernova and disturb the system, leading to a runaway collision and the formation of a megastar [103 the mass of the Sun]. These short-lived monsters could seed intermediate-mass blackholes, which subsequently grow through accretion and mergers. Yet direct observational signatures of their existence are elusive," the letter said.
In gravitational lensing, the path of electro-magnetic photons from a background source is distorted due to curved spacetime, producing multiple images. The difference between arrival times for each image can be used to infer the gravitational structure of the lens, and hence the intermediary object. Although it is a result of Einstein's 1915 general theory of relativity, gravitational lensing was not observed until 1979.
The researchers used Bayesian analysis to identify gravitational lensing from a catalogue of gamma-ray bursts resulting from collapsing stars in the distant and early universe.
"The detection of lensed images with millisecond-to-second time delays provides evidence for intermediate-mass black holes, a population that has been difficult to observe," the authors said.
Further research and detections using these techniques could help understand how commonplace these elusive objects are in the universe, and show their role in the evolution of their more massive counterparts.
"If a significant intermediate-mass black hole population exists, it could provide the seeds for the growth of supermassive black holes in the early universe," the letter added. ®