Astrometric Lensing - What Gaia can and cannot do
Abstract: Context: Data from the recently launched astrometric satellite Gaia will be coming in soon with the final data release expected in 2022. This will provide a very precise map of the Galaxy. The Solar system and Galaxy is thought to be filled with invisible bodies (planetesimals, planets, brown dwarfs, neutron stars, black holes, etc.) and they will affect the observations via gravitational lensing. Aim: I investigate to what extent it is possible to use gravitational lensing to study the otherwise invisible bodies. Also investigated is the possibility of determining masses for visible bodies for which it otherwise is hard to directly determine the mass of. Method : I utilize the existing Gaia simulation and testing environment to simulate observations which are perturbed by placing a massive body in the line of sight. Three tests are conducted: In the first test the observation finishes without trying to adjust for the body, this leaves traces of the lensing in the observational residuals. This simulation is repeated whilst the body is moved further out in order to determine at what distance the traces of a body of a given mass no longer appears in the residuals. In the second test an attempt fit a 3D lensing model to a lens that is detected in the residuals is made. In the third test the position of the lens is assumed to be known and only the mass is attempted to be recovered. Results: I find that the detection scaling for the mass-distance relation is highly dependent on the chosen line of sight and assumed stellar density. With the assumptions calculations show that there will be on the order of ∼ 0.004 detections. Doing the fit with both position and mass was found to be very difficult due to the data reduction utilizing a linear algorithm whereas the problem is highly non-linear. The mass determination works better but becomes unstable at large distances leaving few interesting bodies of which mass can be determined. Conclusions: The detection criterion was strict. Relaxing it and redoing the calculation gives ∼ 4 detections. The predictions is however very sensitive to the assumed density of sources close to the lens and can thus vary by orders of magnitude. I also argue that one should differentiate between the detection of a lensing event and the detection of a lens in these discussions as many more lensing events will be detected than can be confirmed as lenses. For this reason alternative methods of detection are discussed.
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