A few thousand light-years away from Earth, a lone celestial object that is more massive than the sun but much smaller is roving the galaxy. The solitary stellar-mass black hole may be the first to be found in the Milky Way. Or it might be one of the known neutron stars with the highest mass.
When its gravity momentarily amplified the light from a more distant star in 2011, the intergalactic wanderer first made itself known. Its real nature, however, escaped experts at the time. Now that the traveler’s identity has been revealed, two teams of astronomers have examined Hubble Space Telescope photos and reached substantially different conclusions.
According to one team’s work that is now under review by the Astrophysical Journal, the enigmatic rogue black hole is around seven times as large as the sun. Another team says in a paper under review in the Astrophysical Journal Letters that it is either a remarkably massive neutron star or a little bit lighter — just two to four times the mass of our closest star.
When huge stars that are at least several times as heavy as the sun collapse under their own gravity towards the conclusion of their lifetimes, neutron stars and stellar-mass black holes are created. About 100 million stellar-mass black holes and one billion neutron stars are thought to be hidden within our galaxy (SN: 8/18/17). These things, however, are difficult to see. Because neutron stars are so small, around the size of a city, they don’t emit a lot of light. Black holes don’t even produce any light.
Scientists often investigate these things’ effects on their environment in order to find them. According to Kailash Sahu, an astronomer at the Space Telescope Science Institute in Baltimore, “the only way that we can identify them is if they impact something else.”
Nearly 20 stellar-mass black holes have been found so far, according to astronomers. The supermassive behemoths that reside at the centers of most galaxies, including our own (SN: 1/18/21) are dwarfed by these comparatively little black holes. To achieve this, scientists have observed interactions between these particles and surrounding celestial neighbors. A black hole snatches stuff from its partner star while they are engaged in a gravitational dance. Telescopes in orbit around the Earth may see the X-rays that are released when the material falls upon the black hole.
The discovery of black holes in binary systems, however, does not represent the entire black hole kingdom. It is difficult to pinpoint the mass at which these things formed since they are constantly absorbing materials.
According to Sahu, looking at binary systems has a huge disadvantage since birthweight is a crucial component of a black hole. Finding isolated black holes is preferable if we wish to study their characteristics.
The quest for lone black holes has been going on for more than ten years. Einstein’s theory of general relativity, which asserts that any large object, even one that is invisible, bends space in its vicinity, has been the basis for the searches .
The result of such bending is a phenomena called gravitational microlensing, which magnifies and distorts light from background stars. Scientists can determine the mass of the object acting as a lens by analyzing variations in the brightness and apparent location of stars; this method has also helped identify a few extrasolar planets.
Researchers discovered a star that suddenly became more than 200 times brighter in 2011, and they reported their discovery. The star’s apparent location may have also been moving, but those original measurements, taken using telescopes in Chile and New Zealand, were unable to reveal this. And determining the mass of the intervening item requires this knowledge.
If it weighs a lot, its gravitational force would cause space to warp such that the star would appear to be moving. Even a “large” movement in the star’s location, however, would have been incredibly minute and difficult to notice. Additionally, the tumultuous atmosphere of our planet tends to distort fine features in astronomical photographs taken by ground-based telescopes.
Two separate teams of astronomers used the Hubble Space Telescope to get beyond this Earthly restriction. Since this observatory orbits above the majority of Earth’s atmosphere, it can take incredibly detailed pictures.
Both teams discovered that the star’s position changed over numerous years. One of the teams, lead by Sahu, came to the conclusion that something nearly seven times as massive as the sun was responsible for the star’s apparent dance. In the Hubble photos, a star of such mass should have been dazzlingly brilliant, yet the astronomers saw nothing. The crew concludes that something that heavy and dark must be a black hole.
However, a different team of researchers, under the direction of astronomer Casey Lam at the University of California, Berkeley, came to a different conclusion. According to Lam and her coworkers’ calculations, the lensing object’s mass was less, just two to four times that of the sun. The group came to the conclusion that it may be either a neutron star or a black hole.
UC Berkeley astronomer Jessica Lu, a member of Lam’s team, believes that whatever it is, it is a fascinating item. That is as a result of the mass being somewhat peculiar. According to Lu, it’s either one of the most massive neutron stars ever seen or one of the least massive black holes ever found. It is located in the weird area known as the massgap.
Will M. Farr, an astronomer at Stony Brook University in New York who was not involved in either research, believes that despite the controversy, these are exciting discoveries. It is tremendously fascinating to be working at the instrumental limit, at the cutting edge of what is quantifiable.