A change in the optical polarization associated with a γ-ray flare in the blazar 3C 279

Abstract

A small fraction of active galaxies are extreme phenomena, powered by the release of gravitational energy near the supermassive black hole at the galaxy's centre. Just what goes on in the emitting zone, where inflowing gases interact with the outflowing jets, is not clear. One such extreme object is the blazar 3C 279. Multi-band observations of 3C 279 using the Fermi space telescope have revealed a spectacular γ-ray flare coincident with a dramatic change of optical polarization angle. This points to co-spatiality of the optical and γ-ray emission regions and indicates a highly ordered jet magnetic field. Future observation of cosmic accelerators of this type should throw light on how the immense power required to accelerate matter to close to the speed of light is generated. It is widely accepted that strong and variable radiation detected over all accessible energy bands in a number of active galaxies arises from a relativistic, Doppler-boosted jet pointing close to our line of sight. However, the size of the emitting zone and the location of this region relative to the central supermassive black hole are poorly understood. Here, the coincidence of a γ-ray flare with a dramatic change of optical polarization angle is reported, providing evidence for co-spatiality of optical and γ-ray emission regions and indicating a highly ordered jet magnetic field. It is widely accepted that strong and variable radiation detected over all accessible energy bands in a number of active galaxies arises from a relativistic, Doppler-boosted jet pointing close to our line of sight1. The size of the emitting zone and the location of this region relative to the central supermassive black hole are, however, poorly known, with estimates ranging from light-hours to a light-year or more. Here we report the coincidence of a gamma (γ)-ray flare with a dramatic change of optical polarization angle. This provides evidence for co-spatiality of optical and γ-ray emission regions and indicates a highly ordered jet magnetic field. The results also require a non-axisymmetric structure of the emission zone, implying a curved trajectory for the emitting material within the jet, with the dissipation region located at a considerable distance from the black hole, at about 105 gravitational radii.