Absorption of very high energy gamma rays in the Milky Way

Abstract

Galactic gamma ray astronomy at very high energy (Eγ 30 TeV) is a vital tool in the study of the nonthermal universe. The interpretation of the observations in this energy region requires the precise modeling of the attenuation of photons due to pair production interactions (γγ → e + e − ) where the targets are the radiation fields present in interstellar space. For gamma rays with energy Eγ 300 TeV the attenuation is mostly due to the photons of the cosmic microwave background radiation. At lower energy the most important targets are infrared photons with wavelengths in the range λ ≃ 50-500 µm emitted by dust. The evaluation of the attenuation requires a good knowledge of the density, and energy and angular distributions of the target photons for all positions in the Galaxy. In this work we discuss a simple model for the infrared radiation that depends on only few parameters associated to the space and temperature distributions of the emitting dust. The model allows to compute with good accuracy the effects of absorption for any space and energy distribution of the diffuse Galactic gamma ray emission. The absorption probability due to the Galactic infrared radiation is maximum for Eγ ≃ 150 TeV, and can be as large as P abs ≃ 0.45 for distant sources on lines of sight that pass close to the Galactic center. The systematic uncertainties on the absorption probability are estimated as ∆P abs 0.08.

Detection of GeV Gamma-Rays from HESS J1534−571 and Multiwavelength Implications for the Origin of the Nonthermal Emission

“…Different contributions to the gamma-ray flux from IC scattering of far infrared (FIR) photon fields from heated dust and near-infrared star light (NIR), both described by modified blackbody spectra, as well as from the Cosmic Microwave Background (CMB), are included. The contribution from NIR is negligible for typical values in the Galaxy and the temperature and density found near the Solar System are used here, 3000 K and 0.4 eV/cm 3 (e.g., Shibata et al 2011;Vernetto & Lipari 2016). With the adopted model parameters, a FIR temperature and density of 20 K and 0.8 eV/cm 3 predict GeV-TeV fluxes that are reasonably consistent with the data.…”

“…With the adopted model parameters, a FIR temperature and density of 20 K and 0.8 eV/cm 3 predict GeV-TeV fluxes that are reasonably consistent with the data. This value of the FIR density is found at a distance of 5 kpc from the Galactic center to the source using standard Galactic models (Shibata et al 2011;Vernetto & Lipari 2016). An estimation of the dust temperature using GALPROP (Grondin et al 2011) for a Galactocentric distance of 5 kpc gives ∼ 30 K, but we note that changing the dust temperature from 10 K to 50 K has little effect on the resulting IC fluxes.…”

GeV–TeV Counterparts of SS 433/W50 from Fermi-LAT and HAWC Observations

“…u B = B 2 /(8π) and u CMB = 0.26 eV cm −3 are the energy density of magnetic field and the CMB. We also adopt a farinfrared (FIR) background at 20 K with u FIR = 0.3 eV cm −3 motivated by the dust emission in the solar neighborhood (Vernetto & Lipari 2016). Background photons with higher energies are not important due to the Klein-Nishina 5 https://data.hawc-observatory.org/datasets/ss433 2018/index.php 6 https://github.com/threeML/threeML 7 https://fermi.gsfc.nasa.gov/ssc/data/analysis/software/ 8 https://github.com/threeML/hawc hal Figure C1.…”

Detection of a γ -ray halo around Geminga with the Fermi -LAT data and implications for the positron flux

“…These ISRF models are based on the estimate for the interactions between SL and the interstellar matter, which take into account an accurate knowledge of the stars, gas, and dust in the Galaxy, and the IR emissivities per dust grain. Our results are obtained for the ISRF energy density in the local Galaxy reported in [35]. We do not consider any spatial variation in the model.…”

“…We also test the effect on the e + and ICS γ-ray flux for different choices of the ISRF, propagation model, and Galactic magnetic field strength. A change from the ISRF model in [35] to the one in [36] has a negligible effect for e + flux and of about 10% in the case of the ICS γ-ray flux. The difference in the e + flux as propagated using the K15 model and the one in G15 is a normalization factor of about 1.5.…”