Fermi-LAT Observations of the Diffuse Gamma-Ray Emission: Implications for Cosmic Rays and the Interstellar Medium

By The Fermi-LAT Collaboration. 2012

194 citations recorded by the source corpus.

The γ-ray sky > 100 MeV is dominated by the diffuse emissions from interactions of cosmic rays with the interstellar gas and radiation fields of the Milky Way. Observations of these diffuse emissions provide a tool to study cosmic-ray origin and propagation, and the interstellar medium. We present measurements from the first 21 months of the Fermi–LAT mission and compare with models of the diffuse γ-ray emission generated using the GALPROP code. The models are fitted to cosmic-ray data and incorporate astrophysical input for the distribution of cosmic-ray sources, interstellar gas and radiation fields. To assess uncertainties associated with the astrophysical input, a grid of models is created by varying within observational limits the distribution of cosmic-ray sources, the size of the cosmic-ray confinement volume (halo), and the distribution of interstellar gas. An all-sky maximum-likelihood fit is used to determine the XCO-factor, the ratio between integrated CO-line intensity and H2 column density, the fluxes and spectra of the γ-ray point sources from the first Fermi–LAT catalogue, and the intensity and spectrum of the isotropic background including residual cosmic rays that were misclassified as γ-rays, all of which have some dependency on the assumed diffuse emission model. The models are compared on the basis of their maximum likelihood ratios as well as spectra, longitude, and latitude profiles. We also provide residual maps for the data following subtraction of the diffuse emission models. The models are consistent with the data at high and intermediate latitudes but under-predict the data in the inner Galaxy for energies above a few GeV. Possible explanations for this discrepancy are discussed, including the contribution by undetected point source populations and spectral variations of cosmic rays throughout the Galaxy. In the outer Galaxy, we find that the data prefer models with a flatter distribution of cosmic-ray sources, a larger cosmic-ray halo, or greater gas density than is usually assumed. Our results in the outer Galaxy are consistent with other Fermi–LAT studies of this region that used different analysis methods than employed in this paper. Subject headings: ISM: general — (ISM:) cosmic rays — (ISM:) dust extinction — Gamma rays: general — Gamma rays: ISM — radiation mechanisms: nonthermal Institució Catalana de Recerca i Estudis Avançats (ICREA), Barcelona, Spain NASA Postdoctoral Program Fellow, USA Consorzio Interuniversitario per la Fisica Spaziale (CIFS), I-10133 Torino, Italy Dipartimento di Fisica, Università di Roma “Tor Vergata”, I-00133 Roma, Italy

Subfields: Astronomy and Astrophysics, Nuclear and High Energy Physics

View Paper: https://www.semanticscholar.org/paper/eb9290df75477361ff8be32e8b307c60c7d467b9

How researchers see this paper

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In Astronomy and Astrophysics

1. Christoph Weniger
2. A. Strong
3. I. Cholis
4. E. Orlando
5. F. Calore
6. Anna Franckowiak
7. C. Gordon
8. O. Macias

In Nuclear and High Energy Physics

1. Christoph Weniger
2. I. Cholis
3. A. Strong
4. Anna Franckowiak
5. E. Orlando
6. F. Calore
7. C. McCabe
8. C. Gordon

Citations in the corpus

• A tale of tails: Dark matter interpretations of the Fermi GeV excess in light of background model systematics (2014, background): “The emission associated with the inner Galaxy was found to be larger than expected from standard Galactic diffuse emission models [where the distribution of the cosmic-ray (CR) sources peaks at kpc distances from the GC [27]].”
• Limits on dark matter annihilation signals from the Fermi LAT 4-year measurement of the isotropic gamma-ray background (2015, methodology): “Templates of the Galactic diffuse emission are produced with the GALPROP code(13) [73], based on three distinct diffuse model setups, dubbed models A, B and C (see Appendix A and [8] for further details).”
• Dark matter and pulsar model constraints from Galactic Center Fermi-LAT gamma-ray observations (2013, methodology): “By varying the parameters used in GALPROP for the distribution of cosmic-rays, interstellar gas and radiation fields, the Fermi-LAT team reported a systematic error of order 10% for the inner Galaxy and for energies less than 10 GeV with unresolved point sources being cited as a likely cause [43].”
• Cosmic-ray propagation with DRAGON2: I. numerical solver and astrophysical ingredients (2016, methodology): “The second anomaly is about the γ-ray spectrum: the comprehensive collection of GALPROP-based models provided by the Fermi-LAT collaboration in [65] underpredicts the data above 10 GeV in the inner Galactic plane; this discrepancy is present in all the considered setups, with different prescriptions”