Resolving the Crab pulsar wind nebula at teraelectronvolt energies

TL;DR

This study uses gamma-ray observations to pinpoint the emission region of the highest energy gamma rays in the Crab nebula, revealing a larger angular extension than at X-ray energies, thus advancing understanding of particle acceleration.

Contribution

First measurement of the Crab nebula's gamma-ray emission region at very high energies, showing a larger extension than at X-ray energies, and testing emission models.

Findings

Gamma-ray emission region is 52 arcseconds in size.

The gamma-ray emission region is larger than the X-ray region.

Simulation results align with the new measurement.

Abstract

The Crab nebula is one of the most studied cosmic particle accelerators, shining brightly across the entire electromagnetic spectrum up to very high-energy gamma rays. It is known from radio to gamma-ray observations that the nebula is powered by a pulsar, which converts most of its rotational energy losses into a highly relativistic outflow. This outflow powers a pulsar wind nebula (PWN), a region of up to 10~light-years across, filled with relativistic electrons and positrons. These particles emit synchrotron photons in the ambient magnetic field and produce very high-energy gamma rays by Compton up-scattering of ambient low-energy photons. While the synchrotron morphology of the nebula is well established, it was up to now not known in which region the very high-energy gamma rays are emitted. Here we report that the Crab nebula has an angular extension at gamma-ray energies of 52 arcseconds (assuming a Gaussian source width), significantly larger than at X-ray energies. This result closes a gap in the multi-wavelength coverage of the nebula, revealing the emission region of the highest energy gamma rays. These gamma rays are a new probe of a previously inaccessible electron and positron energy range. We find that simulations of the electromagnetic emission reproduce our new measurement, providing a non-trivial test of our understanding of particle acceleration in the Crab nebula.