Just when you’ve started to put your Halloween decorations away, astronomers found a new spook just 1,560 light-years from Earth. In cosmic terms, that’s pretty darn close.
The new spook is Gaia BH1, which is now the closest known black hole to Earth. For a black hole, it’s fairly small at 10 stellar masses. For reference, the black hole at the center of the Milky Way Galaxy, known as Sagittarius A*, is a huge one at 4 million stellar masses.
Unlike Sagittarius A*, Gaia BH1 was hard to spot because it isn’t currently consuming matter. Most known black holes can be detected through their effect on surrounding material that passed a little too close to them and were captured by their gravity. Their gravity can tear apart even larger objects like stars if they get too close. Then the material from these objects swirl around the black holes as it starts falling past the event horizon as if it was about to go down a three-dimensional drain.
What makes this weird is that Gaia BH1 is part of a binary system that also includes a sun-like star. The black hole and the star are only as far apart as Earth and our sun are. Others have claimed to find a system like this, but they could have gotten faked out by their readings.
“While there have been many claimed detections of systems like this, almost all these discoveries have subsequently been refuted. … This is the first unambiguous detection of a sunlike star in a wide orbit around a stellar-mass black hole in our galaxy,” study lead author Kareem El-Badry said.
The researchers involved in the study used data from the European Space Agency’s Gaia spacecraft. Gaia was launched in 2014 as part of an ambitious effort produce a three-dimensional map of the galaxy. Data from Gaia produced a series of surprising discoveries, including a small galaxy that is currently colliding with ours.
The star that formed Gaia BH1 must have been even bigger.
Gaia BH1 is small for a black hole but could have come from a star that was twice as big. The research team estimates that the star that formed it could have been 20 stellar masses. These giant stars typically live only a few million years, and then puff up and blow off a lot of their outer material before their cores collapse.
According to current models, the collapse could have destroyed its companion star or forced it into a closer orbit if it formed nearby at around the same time.
“It is interesting that this system is not easily accommodated by standard binary evolution models,” El-Badry said. “It poses many questions about how this binary system was formed, as well as how many of these dormant black holes there are out there.”
El-Badry’s team adapted a technique for detecting big exoplanets.
The data can also assist with finding black holes that might be part of a binary system with visible stars. Gaia BH1 might not be eating its companion star right now, but it is certainly tugging on the star. This causes it to wobble in a way that could only be explained if the sunlike star had an invisible companion.
Exoplanet hunters use a similar method to find especially big exoplanets that have just enough gravity to cause a noticeable “wobble” in their host stars. They call this method radial velocity – essentially, the star and exoplanet are both orbiting a common center of gravity as their gravitational pull influence one another. Earth and the Moon actually do the same thing with their center of gravity being a point under Earth’s surface that isn’t quite at Earth’s actual center.
This method helped the research team led by El-Badry confirm Gaia BH1’s presence: “We could find no plausible astrophysical scenario that can explain the observed orbit of the system that doesn’t involve at least one black hole.”
The research team confirmed Gaia BH1’s presence using data from ground-based telescopes like the Gemini North and Keck 1 telescopes in Hawaii and the Magellan Clay and MPG/ESO telescopes in Chile. By pointing these telescopes at the visible star long enough for follow-up observations, they could get a better picture of exactly what was going on with Gaia BH1’s binary system.
A paper on Gaia BH1 was published in Monthly Notices of the Royal Astronomical Society.