![]() ![]() 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. Similarly, light in stellar spectra tell us about a star’s motion directly towards 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. The discovery also shows up gaps in current astronomical knowledge, namely about the formation of binary star systems. Now, a group of astronomers led by Kareem El-Badry (Max Planck Institute for Astronomy and Harvard-Smithsonian Center for Astrophysics) has used a novel method to discover the closest known black hole. So-called stellar black holes, in particular, with a few solar masses, are the end state of very massive stars. Still, these objects have long found their place in astrophysics. 2016), ESA/Gaia/DPAC (CC BY-SA 3.0 IGO)īlack holes are difficult to observe, by their definition: mass concentrated in a region with a diameter so small that the resulting extremely strong gravity allows nothing to escape, not even light.
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