Using HAWC to discover invisible pulsars

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 ${37}_{-13}^{+17}$ 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 misaligned 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 pulsar wind nebulae, the diffusion of cosmic rays near energetic pulsars, and the contribution of pulsars to the cosmic-ray positron excess.

Figure 1

The new discovery space provided by HAWC observations of TeV halos in the Milky Way (shaded orange region). Orange points represent 2HWC sources associated with ATNF pulsars, as listed in Tables 1 and 2. Blue (black) data points represent H.E.S.S. TeV halos associated (potentially associated) with ATNF pulsars, as provided by [13]. Gray data points represent ATNF radio pulsars with known distances and spin-down energies [34]. The gray circles include all ATNF pulsars, regardless of whether they lie within the field of view of HAWC. Thus, we stress that isolated gray points above the HAWC sensitivity threshold do not indicate failed detections. The green squares include only middle-aged ATNF pulsars that lie within the HAWC field of view. The orange dashed (solid) line represents the sensitivity of HAWC in the 2HWC catalog (after 10 years of observation), assuming all pulsars produce TeV halos with luminosities calculated using our Geminga-like model. The blue H.E.S.S. angular sensitivity lower limit excludes regions of parameter space where the TeV halo would be expected to be extended by more than 0.6° [13]. H.E.S.S. observations also include a flux sensitivity limit (not shown), which falls within a factor of $\sim 2$ of the HAWC 10 year sensitivity limit, depending on the H.E.S.S. observation time. The large number of gray data points in the HAWC sensitivity region (orange shaded) demonstrate the potential for HAWC to observe a large number of new TeV halos.