Gamma-Ray Halos May Be Common Around Pulsars
In 2017, researchers with the High-Altitude Water Cherenkov (HAWC) Observatory discovered that two nearby pulsars are surrounded by halos of tera-electron-volt (TeV) gamma rays [1]. Now the HAWC Collaboration has found evidence that TeV halos may be ubiquitous around middle-aged pulsars [2]. The result suggests that TeV observations could be used to find pulsars whose orientations render them invisible in other wave bands.
Pulsars are natural particle accelerators that convert the rotational energy of a spinning neutron star into the kinetic energy of relativistic particles. In a young pulsar, these particles (primarily electrons and positrons) are confined by the star’s strong magnetic field. But by the time the pulsar has reached middle age—at about 10,000 years―this field has declined enough for the particles to escape into interstellar space. Astrophysicists believe that TeV halos are produced around such pulsars because these escaped particles repeatedly scatter off ambient photons from stars and the cosmic microwave background, boosting the photons to TeV energies via inverse Compton scattering.
This result could help to settle a debate regarding the diffusion of pulsar-emitted particles. The sizes of the TeV halos suggest that the accelerated particles diffuse more slowly than they would in a typical region of the interstellar medium. When only a handful of TeV halos were known, this difference could be explained by regions of anomalous relativistic electron transport. But the possible ubiquity and uniformity of the halos indicate that the unexpectedly slow diffusion is due to a yet-unknown particle-transport phenomenon inherent to all middle-aged pulsars.
–Marric Stephens
Marric Stephens is a Corresponding Editor for Physics Magazine based in Bristol, UK.
References
- A. U. Abeysekara et al., “Extended gamma-ray sources around pulsars constrain the origin of the positron flux at Earth,” Science 358 (2017).
- A. Albert et al., “Extended TeV halos may commonly exist around middle-aged pulsars,” Phys. Rev. Lett. 134, 171005 (2025).