“Although we know that supermassive black holes are hiding in the nuclei of galaxies, we do not understand how these giants are able to grow so much within the age of the Universe,” said Paynter, lead author of the study now published in the journal Nature Astronomy.
The importance of the discovery, considered the missing link in the study of black holes, is measured by an unusual fact: astronomers have never confirmed that it is a “medium” size black hole. The small black holes (or stellar masses) are what remains of supermassive stars. They are common in the Universe, as much as the stars that originate them.
Less common are those that inhabit the center of galaxies, with a hundred billion solar masses (the Milky Way has one, called Sagittarius A *). They are thought to originate from the first black holes, which arose when the cosmos was less than a billion years old. In common, the two have the way they grow: joining other black holes, feeding on anything that falls beyond their event horizon and, finally, becoming supermassive.
The problem is that, by equalizing the estimated age of the Universe and the size of the largest existing black holes, the account does not match. According to black hole physics, there is a limit to the speed with which they devour matter, and how much a black hole can grow. In addition, merging with others assumes that there are black holes available to be absorbed.
For this reason, astronomers were looking for a black hole that was neither so small (with size ranging from 100 to 100,000 times the mass of the Sun) nor monstrously large, such as J2157-3602, the uniqueness of 34 billion solar masses that faster grows, swallowing the equivalent of one sun a day.
“This black hole could be a relic, a primordial black hole created at the beginning of the Universe even before the formation of the first stars and galaxies. These first singularities may be the seeds of the supermassive black holes that live in the heart of galaxies today,” said the astrophysicist Eric Thrane, from the School of Physics at Monash University and co-author of the study.
Paynter reviewed the more than 2,700 explosion records that BATSE (acronym for Burst And Transient Source Experiment, or Experiment in Gust and Transient Source) captured between 1990 and 1999, looking for one that would reveal, through gravitational lenses, the presence of an IMBH.
Gravitational lenses are an effect that astronomers use to locate celestial bodies and measure the Universe. It is caused by powerful gravity-generating objects, such as galaxies and black holes.
The light generated behind them is deflected and multiplied – but, despite this distortion, gravitational lenses provide more accurate observations, as the light reaches the amplified Earth, making objects hitherto invisible, observable.
The GRB 950830 gamma ray burst, captured in 1995, was what Paynter was looking for. The blast could be picked up on Earth because its light was deflected by an object of neither very small nor very large mass – a black hole of intermediate mass.
“Finding the first IMBH via gravitational lenses is exciting: in 30 years of research, no statistically robust candidate has been found,” he says.
There are two possible IMBH, found in 2020: one was spotted in March swallowing a star (with 50 thousand solar masses, it is the 3XMM J215022.4-055108) and the other, in September, has 150 solar masses, probably originated from the fusion of two minor singularities.
For astrophysicist and pioneer in the use of gravitational lenses Rachel Webster, co-author of the article, “with IMBH we will be able to estimate the total number of these objects in the Universe”. It is estimated that there are 46 IMBH only in the vicinity of the Milky Way.
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