NASA’s Hubble Space Telescope and Chandra X-Ray Observatory may be relatively old space observatories, but that doesn’t stop them from making groundbreaking cosmological discoveries. The two telescopes just spotted an extremely rare type of black hole currently eating a star in space.
Hubble and Chandra “teamed up” to observe NGC 6099 HLX-1, a “bright” X-ray source that appears to emanate from a compact star cluster in NGC 6099, a vast elliptical galaxy located about 450 million light-years from Earth.
The Hubble Space Telescope has played a crucial role in the study of supermassive black holes. Shortly after the telescope launched into space in 1990, it “discovered that throughout the Universe can contain supermassive black holes at their centers weighing millions or billions of times the mass of our Sun,” NASA explains. However, Hubble also found that galaxies can have millions of much smaller black holes that weigh less than 100 times the mass of the Sun. These relatively tiny black holes are born when especially massive stars die.
However, while supermassive black holes can be found throughout the cosmos, and scientists can observe small black holes, there is a third class of black holes that has proven much more elusive: intermediate-mass black holes (IMBHs).
In true Goldilocks fashion — not too big and not too small — these IMBHs are often invisible to astronomers because they do not “gobble as much gas and stars” as supermassive black holes, which creates powerful, detectable radiation.
Instead, much like other transient cosmic events, IMBHs can only be observed when they happen to be detected while they are actively consuming a star.
“When [IMBHs] occasionally devour a hapless bypassing star — in what astronomers call a tidal disruption event — they pour out a gusher of radiation,” NASA writes.
The latest “probable” IMBH, NGC 6099 HLX-1, was “caught snacking” in telescope data about 40,000 light-years from NGC 6099’s galactic center. In the associated research, newly published in The Astrophysical Journal, scientists Yi-Chi Chang, Roberto Soria, Albert K.H. Kong, Alister W. Graham, Kirill A. Grishin, and Igor V. Chilingarian explain how they initially discovered unusual X-rays in Chandra observations from 2009. The team then followed up using the European Space Agency’s (ESA) XMM-Newton space observatory.
“X-ray sources with such extreme luminosity are rare outside galaxy nuclei and can serve as a key probe for identifying elusive IMBHs. They represent a crucial missing link in black hole evolution between stellar mass and supermassive black holes,” says the lead author of the research, Yi-Chi Chang of the National Tsing Hua University, Hsinchu, Taiwan.
The X-ray emissions from NGC 6099 HLX-1 were three million degrees, which is consistent with expected observations of a tidal disruption event. Hubble data revealed evidence that a small cluster of stars was located around the suspected IMBH, which would provide ample sustenance for the medium-sized black hole. The stars are incredibly close together, separated by only a few light-months, or about 500 billion miles.
The suspected IMBH hit its maximum observed brightness in 2012 and declined in intensity until 2023.
“If the IMBH is eating a star, how long does it take to swallow the star’s gas? In 2009, HLX-1 was fairly bright. Then in 2012, it was about 100 times brighter. And then it went down again,” explains study co-author Roberto Soria of the Italian National Institute for Astrophysics (INAF). “So now we need to wait and see if it’s flaring multiple times, or there was a beginning, there was peak, and now it’s just going to go down all the way until it disappears.”
NGC 6099 HLX-1 remains a fascinating area of study because scientists believe intermediate-mass black holes may continue growing and someday become supermassive black holes through coalescence with other IMBHs that may be orbiting a galaxy’s center, like satellite black holes.
An alternative theory is that supermassive black holes in galaxies formed from dark-matter halos in the early Universe, rather than being born from multiple black holes combining or through ongoing mass accumulation.
“So if we are lucky, we’re going to find more free-floating black holes suddenly becoming X-ray bright because of a tidal disruption event. If we can do a statistical study, this will tell us how many of these IMBHs there are, how often they disrupt a star, how bigger galaxies have grown by assembling smaller galaxies.” Soria adds.
However, the studies are complicated by the fact that space telescopes, especially ones like Chandra and XMM-Newton that only look at a very small portion of the sky at a time, very rarely discover tidal disruption events. The Vera C. Rubin Observatory, newly opened in Chile, looks at a huge portion of the sky each night and could help locate tidal disruption events for further study, albeit in optical light.
Image credits: Science: NASA, ESA, CXC, Yi-Chi Chang (National Tsing Hua University); Image Processing: Joseph DePasquale (STScI)