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A Measurement of the Spatial Distribution of Diffuse TeV Gamma Ray Emission from the Galactic Plane with Milagro

A. A. Abdo, B. Allen, T. Aune, D. Berley, E. Blaufuss, S. Casanova, C. Chen, B. L. Dingus, R. W. Ellsworth, L. Fleysher, R. Fleysher, M. M. Gonzalez, J. A. Goodman, C. M. Hoffman, P. H. H"untemeyer, B. E. Kolterman, C. P. Lansdell, J. T. Linnemann, J. E. McEnery, A. I. Mincer, I. V. Moskalenko, P. Nemethy, D. Noyes, T. A. Porter, J. Pretz, J. M. Ryan, P. M. Saz Parkinson, A. Shoup, G. Sinnis, A. J. Smith, A. W. Strong, G. W. Sullivan, V. Vasileiou, G. P. Walker, D. A. Williams, G. B. Yodh

TL;DR

This study uses the Milagro observatory to map diffuse TeV gamma-ray emission from the Galactic plane, testing predictions from GALPROP. By subtracting known sources and modeling the diffuse component, the authors find general agreement with GALPROP in the inner Galaxy but a pronounced excess in the Cygnus region, suggesting local hadronic cosmic-ray accelerators interacting with dense interstellar material. The results highlight a regional variation in the gamma-ray emission mechanisms, with implications for high-energy CR propagation and potential neutrino production. The findings motivate next-generation wide-field detectors to resolve the diffuse and source components across the Milky Way at TeV energies.

Abstract

Diffuse $γ$-ray emission produced by the interaction of cosmic-ray particles with matter and radiation in the Galaxy can be used to probe the distribution of cosmic rays and their sources in different regions of the Galaxy. With its large field of view and long observation time, the Milagro Gamma Ray Observatory is an ideal instrument for surveying large regions of the Northern Hemisphere sky and for detecting diffuse $γ$-ray emission at very high energies. Here, the spatial distribution and the flux of the diffuse $γ$-ray emission in the TeV energy range with a median energy of 15 TeV for Galactic longitudes between 30$^\circ$ and 110$^\circ$ and between 136$^\circ$ and 216$^\circ$ and for Galactic latitudes between -10$^\circ$ and 10$^\circ$ are determined. The measured fluxes are consistent with predictions of the GALPROP model everywhere except for the Cygnus region ($l\in[65^\circ,85^\circ]$). For the Cygnus region, the flux is twice the predicted value. This excess can be explained by the presence of active cosmic ray sources accelerating hadrons which interact with the local dense interstellar medium and produce gamma rays through pion decay.

A Measurement of the Spatial Distribution of Diffuse TeV Gamma Ray Emission from the Galactic Plane with Milagro

TL;DR

This study uses the Milagro observatory to map diffuse TeV gamma-ray emission from the Galactic plane, testing predictions from GALPROP. By subtracting known sources and modeling the diffuse component, the authors find general agreement with GALPROP in the inner Galaxy but a pronounced excess in the Cygnus region, suggesting local hadronic cosmic-ray accelerators interacting with dense interstellar material. The results highlight a regional variation in the gamma-ray emission mechanisms, with implications for high-energy CR propagation and potential neutrino production. The findings motivate next-generation wide-field detectors to resolve the diffuse and source components across the Milky Way at TeV energies.

Abstract

Diffuse -ray emission produced by the interaction of cosmic-ray particles with matter and radiation in the Galaxy can be used to probe the distribution of cosmic rays and their sources in different regions of the Galaxy. With its large field of view and long observation time, the Milagro Gamma Ray Observatory is an ideal instrument for surveying large regions of the Northern Hemisphere sky and for detecting diffuse -ray emission at very high energies. Here, the spatial distribution and the flux of the diffuse -ray emission in the TeV energy range with a median energy of 15 TeV for Galactic longitudes between 30 and 110 and between 136 and 216 and for Galactic latitudes between -10 and 10 are determined. The measured fluxes are consistent with predictions of the GALPROP model everywhere except for the Cygnus region (). For the Cygnus region, the flux is twice the predicted value. This excess can be explained by the presence of active cosmic ray sources accelerating hadrons which interact with the local dense interstellar medium and produce gamma rays through pion decay.

Paper Structure

This paper contains 4 sections, 3 figures.

Table of Contents

  1. Introduction
  2. Analysis
  3. Results
  4. Discussion

Figures (3)

  • Figure 1: Galactic longitude profile of the $\gamma$-ray emission around 15 TeV in the Galactic plane as measured by Milagro. Upper plot: Red data points with dashed error bars -- no subtraction of source contributions, black data points -- after subtraction of source contributions. Lower plot: Source-subtracted profile overlaid with prediction of the optimized GALPROP model -- the red line is the pion contribution, the green line the IC contribution, and the blue line represents the total flux prediction between Galactic latitudes $\pm$ 2 degrees. There are no data points in the region of longitude $l\in[-144^\circ,29^\circ]$ because it is below the Milagro horizon. The region $l\in[111^\circ,135^\circ]$ is excluded because the analysis method is insensitive here (see text for details).
  • Figure 2: Gamma-ray spectra of the diffuse emission as predicted by the optimized GALPROP model for the Galactic plane -- left plot: inner Galaxy ($l\in[30^\circ,65^\circ]$), right plot: Cygnus region ($l\in[65^\circ,85^\circ]$). The red bars represent EGRET data, the black bar the Milagro measurement, where the length of the bar represents the statistical uncertainty only. The dark blue line represents the total diffuse flux predicted by the optimized GALPROP model, the dark gray line the extragalactic background, and the light blue line the bremsstrahlung component. The two contributions at Milagro energies are shown as red line, the pion contribution, and green line, the total IC contribution. The green dashed line shows the dominant IC contribution from scattering of electrons off the cosmic microwave background, which amounts to about 60 to 70% of the IC component at Milagro energies. Other IC contributions which are less important, such as infrared and optical, are not shown separately.
  • Figure 3: Source-subtracted Galactic latitude profile of the $\gamma$-ray emission around 15 TeV in the inner Galaxy (left plot), in the Cygnus region (middle plot), and in the region above Cygnus (right plot) as measured by Milagro (points with errors) and predicted by the optimized GALPROP model. The blue curve is the total $\gamma$-ray flux, the red curve the pion and the green curve the IC contribution.