Diffuse ultrahigh-energy gamma-ray emission from TeV halos

The LHAASO Collaboration has recently reported a measurement of the diffuse gamma-ray emission from the Galactic Plane at energies between 10 TeV and 1 PeV. While this emission is brighter than that expected from cosmic-ray interactions in the interstellar medium alone, we show that the intensity, spectrum, and morphology of this excess are in good agreement with that predicted from the “TeV halos” which surround the Milky Way’s pulsar population. These results support the conclusion that TeV halos dominate the ultrahigh-energy sky, and that these objects convert $\sim 5\%$ of their total spindown power into very high and ultrahigh-energy photons.

Figure 1

A realization of our Monte Carlo simulation of the ultrahigh-energy gamma-ray emission from TeV halos, before (a) and after (b) convolving the map with LHAASO-KM2A’s point spread function and applying a mask around the brightest sources. Both (a) and (b) are subdivided into the inner galaxy (top) and outer galaxy (bottom) regions.

Figure 2

The spectrum of the diffuse ultrahigh-energy gamma-ray emission averaged over the unmasked portions of the inner ($15°<l<125°$, $-5°<b<5°$) and outer ($125°<l<235°$, $-5°<b<5°$) Galaxy. The spectrum reported by the LHAASO Collaboration [1] is compared to that predicted from cosmic-ray interactions with gas in the interstellar medium, from TeV halos, and from the sum of these contributions. The combined emission from TeV halos and cosmic-ray interactions is in good agreement with the observed spectrum.

Figure 3

Longitude and latitude profiles of the diffuse ultrahigh-energy gamma-ray emission measured by the LHAASO Collaboration [1], compared to the predictions from cosmic-ray interactions with gas, from TeV halos, and from the sum of these contributions. In the left, center, and right frames, the flux has been integrated over $-5°<b<5°$, $15°<l<125°$, and $125°<l<235°$, respectively. The top (bottom) frames show the fluxes as integrated between 10–63 TeV (63–1000 TeV). The combined emission from TeV halos and cosmic-ray interactions is in good agreement with the observed spectrum.

Figure 4

Same as Fig. 2 but for three different values of $\alpha$.