# Using HAWC to Discover Invisible Pulsars

## TL;DR

This paper discusses the discovery of a new class of spatially extended TeV gamma-ray sources called TeV halos around pulsars, highlighting HAWC's role in detecting these and their implications for pulsar studies and cosmic-ray physics.

## Contribution

The paper introduces TeV halos as a new morphological class of gamma-ray sources and demonstrates HAWC's capability to detect them, including many pulsars not observable via traditional methods.

## Key findings

- HAWC can detect TeV halos around middle-aged pulsars.
- Predicted detection of approximately 37 TeV halos from misaligned pulsars within ten years.
- TeV halos could account for a significant fraction of HAWC sources, aiding pulsar and cosmic-ray research.

## Abstract

Observations by HAWC and Milagro have detected bright and spatially extended TeV gamma-ray sources surrounding the Geminga and Monogem pulsars. We argue that these observations, along with a substantial population of other extended TeV sources coincident with pulsar wind nebulae, constitute a new morphological class of spatially extended TeV halos. We show that HAWCs wide field-of-view unlocks an expansive parameter space of TeV halos not observable by atmospheric Cherenkov telescopes. Under the assumption that Geminga and Monogem are typical middle-aged pulsars, we show that ten-year HAWC observations should eventually observe 37$^{+17}_{-13}$ middle-aged TeV halos that correspond to pulsars whose radio emission is not beamed towards Earth. Depending on the extrapolation of the TeV halo efficiency to young pulsars, HAWC could detect more than 100 TeV halos from mis-aligned pulsars. These pulsars have historically been difficult to detect with existing multiwavelength observations. TeV halos will constitute a significant fraction of all HAWC sources, allowing follow-up observations to efficiently find pulsar wind nebulae and thermal pulsar emission. The observation and subsequent multi-wavelength follow-up of TeV halos will have significant implications for our understanding of pulsar beam geometries, the evolution of PWN, the diffusion of cosmic-rays near energetic pulsars, and the contribution of pulsars to the cosmic-ray positron excess.