![]() ![]() A fifth candidate is still under consideration, awaiting additional spectral measurements. In those cases, the available radial velocity data flatly contradicted the Gaia reconstruction of the binary orbit.Īnother candidate could be ruled out by the bad fit of the Gaia data to the reconstructed orbit, with an orbital period so long that Gaia should not have been able to measure it in the first place. Using existing spectral data available in astronomical archives, the astronomers were able to rule out three of the candidates right away. ![]() The fact that radial velocities and Gaia positions are two sides of the same coin, so to speak, enables crucial consistency checks. All six candidates warranted a closer look: with the help of the complementary information of radial velocity measurements derived from the star’s spectrum, giving information about motion directly towards us or away from us. Gaia DR3 contains data for 168,065 such tiny ellipses, or parts thereof.Īpplying selection criteria that were particularly likely to pick out systems in which a luminous star is dragged around by an invisible companion with considerable mass, the researchers narrowed their set down to six possible candidates. Generally, as two objects in a binary system orbit each other, they each trace out a tiny ellipse in the sky. When Gaia’s data release 3 (Gaia DR3), the first to contain the orbital data for binary systems detected with Gaia, was published in mid-June 2022, Kareem El-Badry, together with MPIA director Hans-Walter Rix and their colleagues directly set about sifting the data for likely candidates. That is why, in this case, Gaia’s scope is just as important as the survey’s accuracy: high-quality data for more than a hundred thousand binary systems makes for a fair chance to find the needle in the haystack, the black hole binary among the many ordinary binaries. This kind of binary containing a black hole would still be very rare, compared to the overall number of binaries. This includes the ability to detect a visible star’s motion on the sky, and from that to deduce the presence of an unseen companion. Gaia is designed for ultra-precise measurements of stellar position. ![]() The missing information is a fundamental source of uncertainty – and it’s also where ESA’s Gaia mission promises help!įor a few years now, there has been hope that ESA’s astrometry mission Gaia would open up a new way of detecting and characterizing black holes in binary star systems by providing information that is complementary to what stellar spectra deliver. The key problem: Spectra give only part of the information about stellar motion, and hence about the orbit and about the companion’s mass. However, all but one of them (the June 2022 discovery of the binary system VFTS 243, with El-Badry as co-author) have since been challenged or downright refuted by follow-up studies. Over the past few years, there have been several claims of quiescent black hole discoveries that tried to deduce a binary’s orbit and the mass of an unseen companion exclusively from stellar spectra. After arriving at the location, we see the orbit of a Sun-like star around Gaia BH1. This video zooms into the Milky Way to the position of the stellar black hole “Gaia BH1,” currently the black hole closest to Earth. Similarly, light in stellar spectra tell us about a star’s motion directly toward us or away from us. We know this, from everyday life, from the “Doppler effect” for sound: an ambulance with a blaring siren will sound higher-pitched when it is coming towards us, and lower-pitched once it has passed us. The tool of choice: stellar spectra, the rainbow-like decomposition of star light, which contain information about a star’s motion. There have been several attempts to also find “quiescent” black holes in binary systems – black holes without an X-ray-emitting disk. There are 20 known “X-ray binaries” of this kind, with an additional 50 candidate objects. The gas then becomes hot enough in the process to emit considerable amounts of X-rays. Of those few dozen stellar black holes that have been detected using telescope observations, most orbit a companion star closely enough for the black hole’s gravity to pull hydrogen gas from the companion star into a so-called accretion disk that surrounds the black hole. Some have been detected by gravitational wave detectors, which have measured almost a hundred mergers of stellar black holes, yielding additional data about black hole masses. There are an estimated hundred million stellar black holes in our home galaxy, the Milky Way, but only a small fraction has been detected so far. Background: region of the Milky Way galaxy Panel 1: an image of the star orbiting the black hole Panel 2: reconstructed orbit of the star Panel 3: relativistic light-bending effects that would be visible if we could see star and black hole up close. ![]()
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