Scientists have long believed there are “rogue black holes” wandering the universe, but we’ve never had good evidence of one until now. An international team of astronomers has released a study pointing to what appears to be the first roaming black hole, reports Space.com. It was no simple feat to spot an object that gives off no light as it drifts through the cold, empty space between stars. After years of observations, the preliminary study is now available.
Black holes were proposed in the early 20th century, but it wasn’t until decades later that we had proof of their existence. Once we knew these objects existed, it stood to reason that they might be ejected from their solar systems on occasion. It’s possible to study black holes even though they don’t emit light thanks to gravitational interactions with other objects. A black hole in a solar system might have a companion (binary) that orbits a certain way, or there might be enough material falling into the event horizon to form a glowing accretion disc. It’s harder to spot when the black hole is alone in the void.
Our first rogue black hole attracted attention in 2011 when a “gravitational microlensing” event was detected by two projects: Microlensing Observations in Astrophysics (MOA) in New Zealand and the Optical Gravitational Lensing Experiment (OGLE) in Chile. These events occur when a massive object passes in front of a star. Even though the star might be many light years away from the gravity source, its light is shifted before it reaches us, causing its apparent position and brightness to change. That’s what the teams saw when they looked at the MOA/OGLE event, but they didn’t see any light coming from the gravity source.
It took several observation campaigns with Hubble to gather enough data on the event, but the results are now freely available on the arXiv preprint server. Note, this study has not been peer-reviewed, so it may see revisions before official publication. With dozens of reputable scientists signed on, it’s unlikely the findings will be disproven, though. The data shows the lensing object is not another star because it emits no detectable light. It also can’t be a white dwarf or neutron star, both of which would be too small. That leaves a black hole, the first one we’ve detected that is on the move.
In the above series of images, you can see how the target star’s brightness and apparent position change over time. By 2017, the star’s appearance was close to normal, indicating the black hole had drifted out of alignment. The team calculated the black hole was a little over 5,000 light years away, and it was moving at 28 miles per second (45 kilometers per second). Mass estimates put it at 7.1 times the sun’s mass, which is smack in the middle of the accepted range for black holes. It was luck that this black hole passed through a region of space where we could detect it, but it’s probably just one representative of an uncountable horde floating around out there. Future projects like the Vera C. Rubin Observatory in Chile might help us spot more and improve our understanding of these weighty travelers.
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