A small anomaly in the measurements from the Gaia satellite can help weigh invisible objects around stars. Astronomers have shown that this “astrometric noise” contains information about the orbit of a hidden companion.
Gaia measures the position of stars very precisely: when a star has a companion, the two objects orbit a common centre of gravity. From Earth, the visible star then appears to make a slight back-and-forth movement in the sky.
The Gaia space observatory.
Image Wikimedia
This displacement is sometimes too small or too incomplete for Gaia to reconstruct the entire orbit. The Gaia DR3 catalogue then reports an “excess astrometric noise”. This value indicates that the observed positions deviate more than expected from the path of an isolated star.
The researchers wanted to know whether this deviation could reveal the orbital inclination. This angle indicates how the system is oriented with respect to our line of sight. It is important for calculating the mass of the companion, because the same variation in velocity can correspond to very different masses depending on the orientation.
For each system, the team simulated many possible orbits. They varied their inclination, then calculated the level of astrometric noise that Gaia should have measured in each case. The researchers then compared these simulated values with the noise actually recorded in Gaia DR3.
The method was first tested on 221 binary systems whose orbits were already known from other analyses. For 83.7% of them, the range of inclinations calculated with the new procedure contained the value published by Gaia. The test therefore verifies that the method generally recovers an already known orientation.
The results are better when the star’s motion is more visible. For the systems producing the clearest astrometric signal, the correct inclination was found within the calculated range in about 92.1% of cases.
The astronomers then applied the method to Gaia BH1, Gaia BH2 and LB-1, three systems suspected of harbouring a dark companion. It narrowed the possible values for inclination and mass. However, it does not by itself prove that the invisible object is a black hole.
A non-visible companion can also be a very faint star, a white dwarf, or another stellar remnant. Gaia DR4 should provide more individual positions taken at different dates. These data will make it possible to reconstruct orbital motions more directly and further improve mass estimates.