Detection of Sub-TeV Gamma-Rays from the Galactic Center Direction by CANGAROO-II

TL;DR

This study reports the detection of sub-TeV gamma rays from the Galactic Center direction with the CANGAROO-II telescope, revealing a statistically significant excess above $E>250$ GeV and a very soft spectrum $dN/dE \propto E^{-4.6}$ compared to the Crab. The emission is positionally consistent with the GC and the EGRET GeV source 3EG J1746-2851, supporting a common origin and favoring a hadronic (π0-decay) scenario from protons accelerated in the GC region. The results imply a constrained hadronic acceleration environment and, simultaneously, provide upper limits on cold dark matter density in the GC via DM annihilation channels. Together with previous GeV data, these findings help delimit the GC gamma-ray production mechanisms and DM scenarios in the central Milky Way.

Abstract

We have detected sub-TeV gamma-ray emission from the direction of the Galactic Center (GC) using the CANGAROO-II Imaging Atmospheric Cherenkov Telescope (IACT). We detected a statistically signicant excess at energies greater than 250GeV. The flux was one order of magnitude lower than that of Crab at 1 TeV with a soft spectrum E^{-4.6+-0.5}. The signal centroid is consistent with the GC direction and the observed prole is consistent with a point-like source. Our data suggests that the GeV source 3EG J1746-2851 is identical with this TeV source and we study the combined spectra to determine the possible origin of the gamma-ray emission. We also obtain an upper limit on the cold dark-matter density in the Galactic halo.

Figures (3)

• Figure 2: Distributions of $\alpha$ (image orientation angle). From left to right, the a) 2001 data, b) 2002 data, and c) combined data, are shown. The points with error bars show the ON-source data and the hatched histograms are the normalized OFF-source data.
• Figure 3: The "significance map" obtained by the CANGAROO-II telescope is shown by the blue contours. The thin contours are a 12$\mu$ IRAS image. The position of Sgr A$^*$ (the telescope tracking center) is given by the cross. The inset is a 5 GHz VLA image showing Sgr A$^*$ and Sgr A East yusef. The uncertainty in the position for 3EG J1746$-$2851 analysed by mh98 is indicated by the orange dashed contour.
• Figure 4: Spectral energy distribution of the GC region. The cross-hatched area is the 1 $\sigma$ allowed region for the TeV observations in the energy range of Table \ref{['table1']}. Here the energy uncertainties in Table \ref{['table1']} were assumed to be correlated bin by bin. The arrow (W) is the Whipple 2 $\sigma$ upper limit at 2 TeV buckley97. The two analyses of the EGRET data are shown by the black hatched region mh98 and the crosses hartman99. The lines are estimations for $\pi^0$ gamma-rays, the details of which are given in the body of the figure and in the text.