Molecular and Atomic Gas toward HESS J1745-303 in the Galactic Center: Further Support for the Hadronic Scenario

We have compared the TeV gamma-rays with the new 12CO J=2-1 data toward HESS J1745-303 in the Galactic center and confirmed that the molecular gas MG358.9-0.5 toward (l, b)=(358.9, -0.5 at VLSR=-100-0 km s-1 shows a reasonable positional agreement with the primary peak (northern part) of the gamma-ray source. For the southern part of HESS J1745-303, we see no CO counterpart, whereas the HI gas in the Parkes 21 cm HI dataset shows a possible counterpart to the gamma-ray source. This HI gas may be optically thick as supported by the HI line shape similar to the optically thick 12CO. We estimate the total mass of interstellar protons including both the molecular and atomic gas to be 2x10^6 Mo and the cosmic-ray proton energy to be 6x10^{48} ergs in the hadronic scenario. We discuss possible origins of the cosmic-ray protons including the nearby SNR G359.1-0.5. The SNR may be able to explain the northern gamma-ray source but the southern source seems to be too far to be energized by the SNR. As an alternative, we argue that the second-order Fermi acceleration in the inter-clump space surrounded by randomly moving high-velocity clumps may offer a possible mechanism to accelerate protons. The large turbulent motion with velocity dispersion of ~15 km s-1 has energy density two orders of magnitude higher than in the solar vicinity and is viable as the energy source.Comment: To be published in PASJ Vol. 64 No.1 (February 2012

Discovery of a molecular cloud possibly associated with the youngest Galactic SNR G1.9+0.3

The youngest known Galactic supernova remnant (SNR) G1.9+0.3 has high-velocity supernova shock beyond 10000 km s-1, and it is considered to be one of the major candidates of a PeVatron. Despite these outstanding properties, the surrounding interstellar matter of this object is poorly understood. We investigated the interstellar gas toward G1.9+0.3 using the 12CO(J=3-2) data with the angular resolution of 15" obtained by the CHIMPS2 survey by the James Clerk Maxwell Telescope, and discovered three individual clouds at -1, 7, and 45 km s-1. From its morphological and velocity structures, the -1 km s-1 cloud, having the largest velocity width >20 km s-1 and located at the distance of the Galactic Center, is possibly associated with the SNR. The associated cloud shows a cavity structure both in space and velocity and coincides well with the SNR. We found that the associated cloud has higher column densities toward three bright, radio synchrotron-emitted rims where the radial expansion velocity of the supernova shock is decelerated, and the cloud is faint in the other parts of the SNR. This is the first direct evidence indicating that the highly anisotropic expansion of G1.9+0.3 observed by previous studies results from the deceleration by the interaction between the supernova shock and surrounding dense interstellar medium.Comment: 17 pages, 10 figures, accepted by PAS