Pulsar TeV Halos Explain the Diffuse TeV Excess Observed by Milagro

Milagro observations have found bright, diffuse TeV emission concentrated along the galactic plane of the Milky Way. The intensity and spectrum of this emission is difficult to explain with current models of hadronic $\gamma$-ray production, and has been named the “TeV excess.” We show that TeV emission from pulsars naturally explains this excess. Recent observations have detected “TeV halos” surrounding pulsars that are either nearby or particularly luminous. Extrapolating this emission to the full population of Milky Way pulsars indicates that the ensemble of “subthreshold” sources necessarily produces bright TeV emission diffusively along the Milky Way plane. Models indicate that the TeV halo $\gamma$-ray flux exceeds that from hadronic $\gamma$ rays above an energy of $\sim 500\mathrm{GeV}$. Moreover, the spectrum and intensity of TeV halo emission naturally matches the TeV excess. Finally, we show that upcoming HAWC observations will resolve a significant fraction of the TeV excess into individual TeV halos, conclusively confirming, or ruling out, this model.

Figure 1

The contribution of subthreshold TeV halos to the diffuse $\gamma$-ray emission along the galactic plane in the region $40°<\ell <100°$, and $|b|<5°$, compared to observations by the Fermi-LAT (described in the text), ARGO-YBJ [5], and Milagro [1]. The background (blue) corresponds to the predictions of 128 Galprop models of diffuse $\gamma$-ray emission [8]. The contribution from TeV halos (red) is described in the text. TeV halos naturally reproduce the TeV excess observed by Milagro, while remaining consistent with ARGO-YBJ observations. The dashed red region indicates our ignorance of low-energy $\gamma$-ray emission from TeV halos.

Figure 2

Same as Fig. 1 in the smaller region $65°<\ell <85°$, and $|b|<2°$ examined by Milagro [4] at a higher energy of 15 TeV, as well as ARGO-YBJ [5]. ARGO-YBJ observations are quoted in the latitude range $|b|<5°$. We renormalize the ARGO-YBJ points based on the ratio of the modeled Galprop diffuse emission flux in the $|b|<2°$ and $|b|<5°$ ROIs, which increases the flux of the three ARGO-YBJ points by 38%, 40%, and 42%, respectively.

Figure 3

The contribution of individual TeV halos to the TeV excess in the region $40°<\ell <100°$, and $|b|<5°$. We normalize our results at 7 TeV [19], assuming that individual TeV halos convert their spin-down luminosity into 7 TeV $\gamma$ rays with an identical efficiency as Geminga. Vertical lines correspond to the flux of Geminga, and the projected 10 yr HAWC sensitivity. Results are shown for the total $\gamma$-ray flux [$FdN/d{\log }_{10}(F)$, black, left $y$ axis], which indicates that most of the $\gamma$-ray intensity stems from the bright TeV halos, as well as for the source count [$dN/d{\log }_{10}(F)$, blue, right $y$ axis], which indicates that 10 yr HAWC data will observe $\sim 50\mathrm{TeV}$ halos in the ROI. For illustrative purposes, in this plot we show the contribution from TeV halos with individual fluxes exceeding Geminga, predicting the existence of only $\sim 1$ such system.