TeV halos are everywhere: Prospects for new discoveries

Milagro and the High Altitude Water Cherenkov (HAWC) observatory have detected extended TeV gamma-ray emission around nearby pulsar wind nebulae. Building on these discoveries, T. Linden et al., Phys. Rev. D 96, 103016 (2017). identified a new source class—TeV halos—powered by the interactions of high-energy electrons and positrons that have escaped from the PWN, but which remain trapped in a larger region where diffusion is inhibited compared to the interstellar medium. Many theoretical properties of TeV halos remain mysterious, but empirical arguments suggest that they are ubiquitous. The key to progress is finding more halos. We outline prospects for new discoveries and calculate their expectations and uncertainties. We predict, using models normalized to current data, that future HAWC and Cherenkov Telescope Array observations will detect in total $\sim 50–240\mathrm{TeV}$ halos, though we note that multiple systematic uncertainties still exist. Further, the existing High Energy Stereoscopic System source catalog could contain $\sim 10–50\mathrm{TeV}$ halos that are presently classified as unidentified sources or PWN candidates. We quantify the importance of these detections for new probes of the evolution of TeV halos, pulsar properties, and the sources of high-energy gamma rays and cosmic rays.

Figure 1

Schematic illustration of a TeV halo in relation to the more familiar PWN and supernova remnant (SNR). A TeV halo may not form early, and the SNR may be fading when the halo appears.

Figure 2

Evolution of the pulsar spin-down power for six representative cases, as labeled.

Figure 3

TeV $\gamma$-ray luminosity functions of TeV halos for two choices of $P_0$ distribution. The upper panel shows source counts and the lower panel shows the contributions to the total luminosity.

Figure 4

Predicted numbers of TeV halos in the 2HWC catalog for all sources (top) and sources with radio beams aligned towards Earth (bottom), for different choices of $⟨P_0⟩$ and $T_{\min }$. Unmarked curves have $T_{\min }=0$. In the bottom panel, the cumulative histogram for TeV halo candidates is also shown, separated into young and middle-aged candidates following Ref. [7].

Figure 5

Cumulative number distribution of the differential $\gamma$-ray flux at an energy of 7 TeV for all TeV halos within the HAWC field of view, as labeled. The 2HWC sensitivity is approximately equal to 10% of the Geminga flux.

Figure 6

Cumulative contribution to the diffuse galactic $\gamma$-ray flux from unresolved TeV halos with ages above $T_{\mathrm{age}}$, compared to the measurement by Milagro above 3.5 TeV. Solid unmarked curves have $T_{\min }=0$. Line colors have the same meaning as Fig. 4.

Figure 7

Same as Fig. 4, but using 10 years of HAWC observations. Solid unmarked curves have $T_{\min }=0$.

Figure 8

The prediction for the galactic longitude distribution of the TeV halo number (top) and the diffuse $\gamma$-ray flux from unresolved TeV halos (bottom) from $|b|<5°$ in bins of $\mathrm{\Delta }l=5°$. All results utilize a model with $⟨P_0⟩=120\mathrm{ms}$ and $T_{\min }=1\mathrm{kyr}$. A “HAWC-like” sensitivity is modeled as a theoretical instrument with a sensitivity of 3% of the Geminga flux across the entire sky.

Figure 9

Same as the upper panel of Fig. 7, but for the LMC survey with CTA. Curves have $T_{\min }=0$ unless marked.

Figure 10

Same as Fig. 4, but for the HGPS. In the bottom panel, the histogram of HGPS sources associated with ATNF pulsars is also shown, divided into three classes following Refs. [5, 6]. Note that from all unidentified sources in HGPS we only plot those associated with radio pulsars.

Figure 11

Same as Fig. 4, but for different models for the $P_0$ distribution with Gaussian (dotted) and uniform (solid). The cyan curve shows the case of a uniform distribution from 0 to 500 ms.

Figure 12

Same as the upper panel of Fig. 4, but for alternative models in the case of $⟨P_0⟩=120\mathrm{ms}$.