Second black hole lurks at the centre of our Galaxy, according to astronomers who have watched a cluster of stars spinning around it, nature.com said.
Just three years ago, astronomers confirmed that the Milky Way revolves around a supermassive black hole, called Sagittarius A, which is about 2.6 million times more massive than the Sun.
But now a much smaller black hole, just 1,300 times our Sun's mass, has been found orbiting about three light years away from its supermassive cousin.
Jean-Pierre Maillard, an astronomer from the Institute of Astrophysics in Paris, France, led a team that looked at a very bright area of the galactic core called IRS 13, previously thought by astronomers to be a single object.
Using infrared observations from the Gemini Observatory at the summit of Mauna Kea on the Big Island of Hawaii, they discovered that IRS 13 is in fact a rotating cluster of seven stars, just 0.065 light years across.
Adding data from the Hubble Space Telescope and the Chandra X-ray Observatory, they calculated from the movement of the seven stars that they must be orbiting an intermediate-mass black hole, called IRS 13E, which spirals around Sagittarius A* at about 280 kilometres per second.
The IRS 13 cluster has also been seen emitting strong X-rays, another tell-tale sign that it hides a black hole. Smaller X-ray emissions throughout our Galaxy suggest that there may be many mini-black holes closer to Earth that are just one or two times the mass of our Sun, but this has yet to be confirmed.

Air pollution might still be important in summertime asthma.

Traffic fumes are not the reason why people's asthma gets worse in the winter, say US scientists.
According to BBC News Online, instead coughs and colds are to blame, according to their study of schoolchildren in Denver.
Respiratory infections doubled the chance of asthma worsening, the National Jewish Medical and Research Center team found.
But air pollution may still trigger summer asthma, they told the Journal of Allergy and Clinical Immunology.
Study author Dr Erwin Gelfand said, "It is well known that upper respiratory infections can cause problems for people with asthma, but the air pollution results were a surprise.
"We believe that careful monitoring of the children allowed us to filter out confounding factors that would have mistakenly suggested a significant impact of air pollution."
His team monitored 147 children aged six to 12 during three successive winters in Denver, Colorado, when air pollution was at its worst.
They recorded the number of times the children had asthma symptoms and were hospitalised for their asthma, as well as measuring their lung function, medication use and number of upper respiratory tract infections.
When they looked at the levels of six air pollutants, including particulate pollution, ozone and carbon monoxide, they found no obvious relation to asthma severity.
Higher carbon monoxide levels were marginally associated with increased use of asthma medication and daily asthma symptoms were marginally associated with ozone levels, but this was not significant.
Yet upper respiratory infections doubled the chances a child would suffer an asthma exacerbation and more than quadrupled the odds a child would suffer asthma symptoms.

A geology student on a field trip stumbled across the fossil of an oversized, salamander-like creature with vicious crocodile-like teeth that lived about 300 million years ago, AP said.
Scientists say the find is both a new species and a new genus, a broader category in the classification of plants and animals. Talks are under way about what to call the new species, starting with "Striegeli"--after the University of Pittsburgh student who discovered it.
Initially, Adam Striegel picked up the softball-sized rock along a fresh road cut near Pittsburgh International Airport, and thinking it was of little interest, threw it aside. Walking back through the same area, he retrieved the stone and showed it to class lecturer Charles Jones.
Paleontologists with the Carnegie Museum of Natural History also were stunned when the impeccably preserved fossil from a trematopid amphibian was unearthed this past spring in their own back yard.
The creature, believed to have been 3 to 4 feet (0.9 to 1.2 meters) long, is "new to science but we know it belongs to fairly terrestrial-adapted amphibians living in the Pennsylvanian Period, about 300 million years ago," said Christopher Beard, curator of vertebrate paleontology at the museum.
Carnegie paleontologist Dave Berman knew exactly what the stone-encased skull fossil was because only two others of the same family are known to exist. He found one of them more than a decade ago in New Mexico.
The species has some characteristics of a crocodile, but is closer to a massive salamander--one that could tear its prey to shreds.