Discovery of Very-High-Energy Gamma-Rays from the Galactic Centre Ridge

Abstract

The origin of Galactic cosmic rays (with energies up to 10^15 eV) remains unclear, though it is widely believed that they originate in the shock waves of expanding supernova remnants. Currently the best way to investigate their acceleration and propagation is by observing the gamma-rays produced when cosmic rays interact with interstellar gas. Here we report observations of an extended region of very high energy (VHE, >100 GeV) gamma-ray emission correlated spatially with a complex of giant molecular clouds in the central 200 pc of the Milky Way. The hardness of the gamma-ray spectrum and the conditions in those molecular clouds indicate that the cosmic rays giving rise to the gamma-rays are likely to be protons and nuclei rather than electrons. The energy associated with the cosmic rays could have come from a single supernova explosion around 10,000 years ago.

Figures (3)

• Figure 1: VHE $\gamma$-ray images of the GC region. Top: $\gamma$-ray count map, bottom: the same map after subtraction of the two dominant point sources, showing an extended band of gamma-ray emission. White contour lines indicate the density of molecular gas, traced by its CS emission. The position and size of the composite SNR G 0.9+0.1 is shown with a yellow circle. The position of Sgr A$^{\star}$ is marked with a black star. The 95% confidence region for the positions of the two unidentified EGRET sources in the region are shown as dashed green ellipses EGRETEllipses. These smoothed and acceptance corrected images are derived from 55 hours of data consisting of dedicated observations of Sgr A$^{\star}$, G 0.9+0.1 and a part of the data of the H.E.S.S. Galactic plane survey HESSSCAN. The excess observed along the Galactic plane consists of $\approx$3500 $\gamma$-ray photons and has a statistical significance of 14.6 standard deviations. The absence of any residual emission at the position of the point-like $\gamma$-ray source G 0.9+0.1 demonstrates the validity of the subtraction technique. The energy threshold of the maps is 380 GeV due to the tight $\gamma$-ray selection cuts applied here to improve signal/noise and angular resolution. We note that the ability of H.E.S.S. to map extended $\gamma$-ray emission has been demonstrated for the shell-type SNRs RX J1713.7 $-$3946 HESS_RXJ1713 and RX J0852.0$-$4622 HESS_VelaJnr. The white contours are evenly spaced and show velocity integrated CS line emission from Tsuboi et al. Tsuboi, and have been smoothed to match the angular resolution of H.E.S.S..
• Figure 2: Distribution of $\gamma$-ray emission in Galactic latitude (for individual slices in longitude, top) and in Galactic longitude (bottom). The red curves show the density of molecular gas, traced by CS emission. The upper curves show acceptance corrected (and cosmic-ray background subtracted) $\gamma$-ray counts for 0.3$^{\circ}$ wide bands in longitude. The point-source subtracted counts are shown in black. The dashed blue histogram shows the unsubtracted values (the $y$-scale is truncated). The red curves correspond to the smoothed CS map of Fig. \ref{['fig_map']} and are drawn only in the regions where CS measurements are available. The dashed red lines show nominal zero CS density in regions away from the Galactic plane. The lower plot shows $\gamma$-ray counts versus $l$ for $-0.2^{\circ}<b<0.2^{\circ}$. The CS line flux may be underestimated close to $l=-1^{\circ}$ due to a narrower coverage in $b$ at this longitude. The dashed line shows the $\gamma$-ray flux expected if the CR density distribution can be described by a Gaussian centred at $l=0^{\circ}$ and with rms $0.8^{\circ}$, as expected in a simple model for diffusion away from a central source of age $\sim 10^{4}$ years. In all plots the background level is estimated using events from the regions $0.8^{\circ}<|b|<1.5^{\circ}$. Error bars show $\pm1$ standard deviation.
• Figure 3: $\gamma$-ray flux per unit solid angle in the GC region (data points), in comparison with the expected flux assuming a cosmic-ray spectrum as measured in the solar neighbourhood (shaded band). The spectrum of the region $-0.8^{\circ}<l<0.8^{\circ}$, $|b|<0.3^{\circ}$ is shown using full circles. These data can be described by a power law: $dN/dE = k(E/\mathrm{TeV})^{-\Gamma}$, with $k = (1.73 \pm0.13_{stat}\pm0.35_{sys}) \times 10^{-8}$ TeV$^{-1}$ cm$^{-2}$ s$^{-1}$ sr$^{-1}$ and a photon index $\Gamma=2.29\pm0.07_{stat}\pm0.20_{sys}$. The shaded box shows the range of expected $\pi^{0}$-decay fluxes from this region assuming a CR spectrum identical to that found in the solar neighbourhood and a total mass of $1.7-4.4\times10^{7}$ solar masses in the region $-0.8^{\circ}<l<0.8^{\circ}$, $|b|<0.3^{\circ}$, estimated from CS measurements. Above 1 TeV an enhancement by a factor $3-9$ relative to this prediction is observed. Using independent mass estimates derived from sub-millimeter measurements SCUBA, $5.3\pm1.0\times10^{7}$ solar masses, and from C$^{18}$O measurements C18O, $3^{+2}_{-1}\times10^{7}$ solar masses, results in enhancement factors of $4-6$ and $5-13$, respectively. The strongest emission away from the bright central source HESS J1745$-$290 occurs close to the Sagittarius B complex of giant molecular clouds Lis. In a box covering this region ($0.3^{\circ}<l<0.8^{\circ}$, $-0.3^{\circ}<b<0.2^{\circ}$), integrated CS emission suggests a molecular target mass of $6-15\times10^{6}$ solar masses. The energy spectrum of this region is shown using open circles. The measured $\gamma$-ray flux ($>$ 1 TeV) implies a high-energy cosmic-ray density which is $4-10$ times higher than the local value. Standard $\gamma$-ray selection cuts are applied here, yielding a spectral analysis threshold of 170 GeV. The spectrum of the central source HESS J1745-290 is shown for comparison (using an integration radius of $0.14^{\circ}$). All error bars show $\pm1$ standard deviation.