# Searching for Synchrotron Emission from the Geminga TeV Halo using the Planck Satellite

**Authors:** Dan Hooper, Elena Pinetti, Anastasia Sokolenko  
**arXiv:** [2405.06739](https://arxiv.org/abs/2405.06739)  
**Date:** 2024-05-10  
**Summary page:** https://www.summarizepaper.com/en/paper/2405.06739v1/

## Key points

- Pulsars convert a significant portion of their spin-down power into high-energy electrons, forming TeV halos.
- TeV halos have been observed at multi-TeV energies by collaborations like HAWC, LHAASO, and HESS.
- Pulsars generate spatially extended emission and efficiently accelerate electron-positron pairs at high energies.
- Gamma rays are produced through inverse Compton scattering by these particles.
- TeV halos extend up to about 25 parsecs in radius, indicating inefficient cosmic ray propagation near pulsars compared to the interstellar medium.
- Questions remain regarding the physics of TeV halos, particularly why diffusion is slower near pulsars compared to other areas in the interstellar medium.
- Recent study on Geminga pulsar's TeV halo using Planck satellite data did not find conclusive evidence of synchrotron emission within Planck's frequency range.
- Inability to detect significant synchrotron emission from Geminga suggests current measurements are insufficient to constrain important parameters related to low-energy diffusion and electron acceleration near pulsars.
- Further research is needed to understand particle acceleration and propagation processes near pulsars and elucidate the physics of TeV halos associated with them.

## Summary

Pulsars are known to convert a significant portion of their total spin-down power into very high-energy electrons, resulting in the formation of TeV halos. These halos have been observed at multi-TeV energies by various collaborations such as HAWC, LHAASO, and HESS. This indicates that pulsars generate spatially extended emission and efficiently accelerate electron-positron pairs at very high energies. These particles then produce gamma rays through inverse Compton scattering. The TeV halos extend up to approximately 25 parsecs in radius, highlighting the inefficient propagation of cosmic rays in the vicinity of these sources compared to the interstellar medium. Despite these observations, several questions remain unanswered regarding the physics of TeV halos. One key question is why diffusion is significantly slower in the regions surrounding these pulsars compared to other areas within the interstellar medium. Understanding the mechanisms responsible for this slow diffusion could provide valuable insights into the acceleration and propagation of particles near pulsars. In a recent study focusing on the Geminga pulsar's TeV halo, researchers attempted to use data from the Planck satellite to constrain its nature. However, no conclusive evidence of synchrotron emission from Geminga was found within Planck's frequency range. Even models with favorable diffusion parameters or soft injection spectra predicted that the synchrotron flux from Geminga would be well below the total flux measured by Planck. The inability to detect significant synchrotron emission from Geminga using Planck data suggests that current measurements are not sufficient to constrain important parameters related to low-energy diffusion and electron acceleration near pulsars. Further research is needed to better understand the processes governing particle acceleration and propagation in these environments and shed light on the physics of TeV halos associated with pulsars.