Speaker
Chris Gordon
(University of Canterbury)
Description
The High-Energy Stereoscopic System (HESS) has detected diffuse TeV emission correlated
with the distribution of molecular gas along the Ridge at the Galactic Center. Diffuse, nonthermal
emission is also seen by the Fermi large area telescope (Fermi-LAT) in the GeV range
and by radio telescopes in the GHz range. Additionally, there is a distinct, spherically symmetric excess of gamma rays seen by Fermi-LAT in the GeV range. A cosmic ray flare, occurring in
the Galactic Center, 10 000 years ago has been proposed to explain the TeV Ridge (Aharonian et al.
2006). An alternative, steady-state model explaining all three data sets (TeV, GeV, and radio)
invokes purely leptonic processes (Yusef-Zadeh et al. 2013). We show that the flare model from
the Galactic Center also provides an acceptable fit to the GeV and radio data, provided the diffusion
coefficient is energy independent. However, if Kolmogorov-type turbulence is assumed for the diffusion coefficient, we find that two flares are needed, one for the TeV data (occurring
approximately 10 000 years ago) and an older one for the GeV data (approximately a hundred thousand years old). We find that the flare models we investigate do not fit the spherically symmetric GeV excess
as well as the usual generalized Navarro-Frenk-White spatial profile, but are better suited to
explaining the Ridge. We also show that a range of single-zone, steady-state models are able to
explain all three spectral data sets. A low gas density is needed for the steady-state model to be
wind driven. Higher gas densities can be accommodated by an energy independent diffusion or
streaming based steady-state model. Additionally, we investigate how the flare and steady-state
models may be distinguished with future gamma-ray data looking for a spatial dependence of
the gamma-ray spectral index.
| Registration number following "ICRC2015-I/" | 404 |
|---|---|
| Collaboration | -- not specified -- |
Primary author
Chris Gordon
(University of Canterbury)
