The High-Energy, Arcminute-Scale Galactic Center Gamma-Ray Source

Abstract

Employing data collected during the first 25 months of observations by the Fermi-LAT, we describe and subsequently seek to model the very high energy (>300 MeV) emission from the central few parsecs of our Galaxy. We analyze the morphological, spectral, and temporal characteristics of the central source, 1FGL J1745.6−2900. The data show a clear, statistically significant signal at energies above 10 GeV, where the Fermi-LAT has angular resolution comparable to that of HESS at TeV energies. This makes a meaningful joint analysis of the data possible. Our analysis of the Fermi data (alone) does not uncover any statistically significant variability of 1FGL J1745.6−2900 at GeV energies on the month timescale. Using the combination of Fermi data on 1FGL J1745.6−2900 and HESS data on the coincident, TeV source HESS J1745−290, we show that the spectrum of the central gamma-ray source is inflected with a relatively steep spectral region matching between the flatter spectrum found at both low and high energies. We model the gamma-ray production in the inner 10 pc of the Galaxy and examine cosmic ray (CR) proton propagation scenarios that reproduce the observed spectrum of the central source. We show that a model that instantiates a transition from diffusive propagation of the CR protons at low energy to almost rectilinear propagation at high energies can explain well the spectral phenomenology. We find considerable degeneracy between different parameter choices which will only be broken with the addition of morphological information that gamma-ray telescopes cannot deliver given current angular resolution limits. We argue that a future analysis performed in combination with higher-resolution radio continuum data holds out the promise of breaking this degeneracy.

Leptonic origin of the 100 MeVγ-ray emission from the Galactic centre

“…3 is consistent with the previously reported spectrum of Chernyakova et al (2011), but extends to lower energies (down to 60 MeV). This potentially opens a possibility of distinguishing between the leptonic and hadronic models because the hadronic spectral models are generically expected to possess a spectral feature in the 100 MeV energy range (energy comparable to the mass of the pions).…”

“…Aharonian & Neronov (2005a), Chernyakova et al (2011), Kusunose & Takahara (2012). The proposed models use different types of radiationproducing particles, and the majority of them can be divided into two broad classes, leptonic and hadronic.…”

“…4 one could see that the sensitivity of Fermi/LAT is marginally sufficient for the detection of this decrease. Aharonian & Neronov (2005b) and Chernyakova et al (2011) developed a model in which the GC Fermi/HESS spectrum is explained by hadronic radiation from relativistic protons, which diffuse by ∼5 pc away from the central source. Data above 100 MeV are well described by the model for different sets of parameters (density profile of surrounding medium, characteristics of the diffusion coefficient, injection rate history).…”

“…The source of the GeV and TeV γ-rays is not variable and it is not clear if the emission comes from the vicinity of the black hole, if it is produced in an extended region around the black hole, or, finally, if it comes from an unrelated source positionally coincident with the supermassive black hole in Sgr A*. The best positional uncertainty of 6 on TeV source HESS J1745-303 (Acero et al 2010) still allows several sources to be responsible for the observed very high energy(VHE) emission: the central black hole itself (Aharonian & Neronov 2005a), a plerion within several arcseconds from the GC (Wang et al 2006;Hinton & Aharonian 2007), a putative black hole plerion around Sgr A* (Atoyan & Dermer 2004), and the central diffuse region around the GC (Aharonian & Neronov 2005b;Ballantyne et al 2007;Chernyakova et al 2011).…”

“…The analysis of 25 months of Fermi/LAT observations by Chernyakova et al (2011) has revealed a GeV "bump" in the source spectrum. Assuming the identification of the Fermi source with the GC and HESS J1745-303, Chernyakova et al (2011) proposed a model that allowed the authors to explain the combined Fermi/LAT -HESS spectrum with a model of π 0 decay γ-ray emission from protons diffusing away from the supermassive black hole.…”