The mystery of negative parallaxes in radiosource positions

par Dr Oleg Titov (Geoscience Australia)

mercredi 19 nov. 2025, 11:00 → 13:00 Europe/Paris

Denisse (Paris)

The effect of negative parallax was predicted due to the microlensing effect (Sazhin, Zharov and Kalinina, 2002). Nowadays, the Gaia optical astrometric mission has measured accurate positions of many million objects on the sky, including extragalactic objects also observed by Very Long baseline Interferometry (VLBI). The recent Gaia EDR3 release reported an effect of negative parallax for extragalactic radio sources with magnitude of -17 µas (parallax zero point), presumably due to the technical reason. To test the theory, we analysed more than 15 million individual group delays since 1993 for more than 5000 objects and detected the negative parallax with an amplitude of -15.8 +/- 0.5 µas. Cross-spectrum analysis of the individual position time series reveals strong annual signal for the radio sources with a good observational history. However, there is not a plausible theoretical explanation of the observed effect by now. Possible reasons comprise the microlensing effect of the metric perturbation on the global scale.

Zoom link : https://cnrs.zoom.us/j/91693764755?pwd=bKm8uCgmHzEqCf9Pbrxw1KBketFabQ.1