The Inverse Compton and Extragalactic Components of Diffuse Gamma-Ray Emission

Abstract

We present spectra of the inverse-Compton and extragalactic components of the high-energy gamma-radiation based on an analysis of eight high-latitude (29°.5 < |b| < 74°) regions. The spectrum of gamma-radiation that is correlated with atomic hydrogen (H I) column density indicates that this component originates in cosmic-ray/matter interactions. The gamma-ray emission uncorrelated with H I varies as a function of galactic longitude and is correlated with radio continuum emission (408 MHz), which is dominated by synchrotron radiation from cosmic-ray electrons. We interpret the longitude-dependent variation in the intensity of gamma-ray emission uncorrelated with H I as a model-independent, direct measure of the spatially varying part of the inverse-Compton (IC) emission from the galactic halo. We find an intensity change (center versus anticenter) of (3.3 ± 0.7) × 10 -6 photons cm -2 s -1 ster -1 for E > 100 MeV, a statistically significant (> 4σ) result. This value is a lower limit to the intensity of inverse-Compton gamma-ray emission towards the galactic center at high latitudes.We correlate the gamma-ray intensity with a model consisting of an isotropic component, a component proportional to the 408 MHz synchrotron radiation, and an HI component with different emissivities in eight galactic octants. Using this fit, we measure the spectrum and absolute intensity of the IC emission. We obtain a spectral index of -1.85 ± 0.17, in agreement with measurements of the near-earth electron cosmic-ray spectrum, and an average intensity of (5.0 ± 0.8) × 10 -6 photons cm -2 s -1 ster -1 for E > 100 MeV. Extrapolating the IC emission to zero galactic radio continuum intensity, we find that the extragalactic gamma-radiation has a spectral index of -2.15 ± 0.06 and an intensity of (1.24 ± 0.06) × 10 -5 photons cm -2 s -1 ster -1 for E > 100 MeV. The spectrum is consistent with those measured for gamma-ray loud AGN and lends support to interpretation of the extragalactic radiation as due to unresolved gamma-ray emitting AGN.

Evidence for a Galactic gamma-ray halo

“…A definitive answer on this topic will perhaps require a "smoking gun" in this or another energy band, or may have to wait for the GLAST mission. Further, as noted by previous authors (Smialkowski, Wolfendale, and Zhang 1997;Chen, Dwyer, and Kaaret 1996;Strong et al 1983), the existence of such an extended excess may impact estimates of the extragalactic γ-ray background.…”

“…Previous authors (Strong et al 1983;Strong 1984;Chen, Dwyer, and Kaaret 1996;Smialkowski, Wolfendale, and Zhang 1997;Willis 1996) have noted the following characteristics:…”

“…The possibility of high latitude inverse Compton (IC) emission has been discussed previously (see, e.g., Smialkowski, Wolfendale, and Zhang 1997, Chen, Dwyer, and Kaaret 1996, Strong 1984, Strong and Youssefi 1995, Willis 1996. The distribution of Galactic IC emission is not well-understood, and it is unlikely that one can make a definitive statement as to whether or not IC is the emission mechanism without extensive modeling and further analysis.…”

“…The alternate approach (see e.g. Chen, Dwyer, andKaaret 1996 andWillis 1996) is non-parametric, where one attempts to find the flux distribution as a function of position without reference to a particular model. The two classes complement each other in the sense that the strengths of one are often the weaknesses of the other.…”

The case for a low extragalactic gamma-ray background

“…A sequential subtraction of tracer-correlated components further requires the tracers to be uncorrelated, and the errors introduced by each two-component model fit are cumulative. However, since this method has been previously used (Chen et al 1996), we examine it quantitatively here.…”