The High-Altitude Water Cherenkov Gamma-Ray Observatory

Diffuse Emission from the Galactic Plane


Observations of the GeV and TeV sky have shown that the Galactic plane is the brightest feature in both wavelength bands. While some of the emission is likely due to unresolved point sources, a large fraction of the gamma rays likely originate when the cosmic rays interact with gas and dust in the Galaxy.

Gamma-ray observations are the most direct probe of the flux and spectrum of cosmic rays outside our solar neighborhood. Hadronic cosmic rays interacting with matter produce neutral pions that decay into gamma rays, whereas electrons create high-energy gamma rays through inverse Compton scattering with infrared photons and the cosmic microwave background. In addition, processes not directly related to cosmic-ray production may also contribute to the diffuse emission. For example, self-annihilating super-symmetric dark matter could play a significant role as an additional emission component with a distinct spectral signature.

Diffuse TeV emission
GALPROP simulations of diffuse emission from the Galactic plane at 1 TeV due to pion decay of cosmic rays (left) and inverse Compton scattering of low-energy photons (right). From A. Strong, MPI (2010).

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Observations of Diffuse Emission with HAWC

By observing nearly three orders of magnitude in energy — 100 GeV to 100 TeV — HAWC will map the diffuse emission in the Galaxy at multiple energies and be able to distinguish both the energy and spatial differences between the leptonic and hadronic emission mechanisms. The HAWC site is close to the equator and the field of view covers the inner Galaxy all the way to the Galactic center. Using observations from HAWC, we will be able to study nearby regions such as Cygnus at (1-2 kpc) as well as the more distant inner Galaxy (about 10 kpc). The Cygnus region could be dominated by a very few cosmic-ray accelerators whereas the cosmic rays from the inner Galaxy are from a large collection of sources and will reflect the cosmic-ray spectrum after propagation farther from their origins. These regions are hundreds of square degrees and require the large field of view of HAWC.

The diffuse GeV and TeV gamma-ray flux recorded with EGRET and Milagro respectively are above predictions based upon the assumption that local cosmic rays are representative of those elsewhere in the Galaxy. In order to match the EGRET data, the cosmic-ray density in the rest of the Galaxy must be two times higher than measured locally. Increasing the cosmic-ray density enough to match the Milagro data would violate the measured limits on the anti-proton flux. However, unresolved TeV sources may be contributing to the Milagro measurement of the flux from the Galactic plane.

For example, in the Cygnus region Milagro detects an excess, MGRO J2031+41, coincident with the largest matter density in the area. This Milagro source is also coincident with TeV J2032+41, observed by HEGRA, but the Milagro source is both brighter by a factor of 3 and more extended than the HEGRA source. Deep observations of the spatial and spectral morphology of this region with HAWC and IACTs will determine whether other sources exist in this region and whether the more localized TeV source could be the accelerator of protons which illuminate the entire region. The combination of the diffuse sensitivity of HAWC with the deeper, higher angular resolution IACT follow-up observations provides the most efficient way to map the entire Galactic plane over all angular scales.

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