ALMA CO Observations of Supernova Remnant N63A in the Large Magellanic Cloud: Discovery of Dense Molecular Clouds Embedded within Shock-Ionized and Photoionized Nebulae

We carried out new $^{12}$CO($J$ = 1-0, 3-2) observations of a N63A supernova remnant (SNR) from the LMC using ALMA and ASTE. We find three giant molecular clouds toward the northeast, east, and near the center of the SNR. Using the ALMA data, we spatially resolved clumpy molecular clouds embedded within the optical nebulae in both the shock-ionized and photoionized lobes discovered by previous H$\alpha$ and [S II] observations. The total mass of the molecular clouds is $\sim$$800$ $M_{\odot}$ for the shock-ionized region and $\sim$$1700$ $M_{\odot}$ for the photoionized region. Spatially resolved X-ray spectroscopy reveals that the absorbing column densities toward the molecular clouds are $\sim$$1.5$-$6.0\times10^{21}$ cm$^{-2}$, which are $\sim$$1.5$-$15$ times less than the averaged interstellar proton column densities for each region. This means that the X-rays are produced not only behind the molecular clouds, but also in front of them. We conclude that the dense molecular clouds have been completely engulfed by the shock waves, but have still survived erosion owing to their high-density and short interacting time. The X-ray spectrum toward the gas clumps is well explained by an absorbed power-law or high-temperature plasma models in addition to the thermal plasma components, implying that the shock-cloud interaction is efficiently working for both the cases through the shock ionization and magnetic field amplification. If the hadronic gamma-ray is dominant in the GeV band, the total energy of cosmic-ray protons is calculated to be $\sim$$0.3$-$1.4\times10^{49}$ erg with the estimated ISM proton density of $\sim$$190\pm90$ cm$^{-3}$, containing both the shock-ionized gas and neutral atomic hydrogen.Comment: 18 pages, 4 tables, 8 figures, accepted for publication in The Astrophysical Journal (ApJ

Simulating Cherenkov Telescope Array observation of RX J1713.7-3946

We perform simulations of Cherenkov Telescope Array (CTA) observations of a young supernova remnant RX J1713.7-3946. This target is not only one of the brightest sources ever discovered in very high-energy gamma rays but also well observed in other wavebands. In X-rays, the emission is dominated by synchrotron radiation, which links directly to the existence of high-energy electrons. Radio observations of CO and HI gas have revealed a highly inhomogeneous medium surrounding the SNR, such as clumpy molecular clouds. Therefore gamma rays from hadronic interactions are naturally expected. However, the spectrum in GeV energy range measured by Fermi/LAT indicates more typical of leptonic emission from accelerated electrons. Despite lots of multi-wavelength information, the competing interpretations have led to much uncertainty in the quest of unraveling the true origin of the gamma-ray emission from RX~J1713.7--3946. CTA will achieve highest performance ever in sensitivity, angular resolution, and energy resolution. We estimate CTA capability to examine the emission mechanisms of the gamma rays through simulated spatial distribution, spectra, and their time variation.Comment: 8 pages, 3 figures. In Proceedings of the 34th International Cosmic Ray Conference (ICRC2015), The Hague, The Netherlands. All CTA contributions at arXiv:1508.0589

Star Forming Dense Cloud Cores in the TeV {\gamma}-ray SNR RX J1713.7-3946

RX J1713.7-3946 is one of the TeV {\gamma}-ray supernova remnants (SNRs) emitting synchrotron X rays. The SNR is associated with molecular gas located at ~1 kpc. We made new molecular observations toward the dense cloud cores, peaks A, C and D, in the SNR in the 12CO(J=2-1) and 13CO(J=2-1) transitions at angular resolution of 90". The most intense core in 13CO, peak C, was also mapped in the 12CO(J=4-3) transition at angular resolution of 38". Peak C shows strong signs of active star formation including bipolar outflow and a far-infrared protostellar source and has a steep gradient with a r^{-2.2$\pm$0.4} variation in the average density within radius r. Peak C and the other dense cloud cores are rim-brightened in synchrotron X rays, suggesting that the dense cloud cores are embedded within or on the outer boundary of the SNR shell. This confirms the earlier suggestion that the X rays are physically associated with the molecular gas (Fukui et al. 2003). We present a scenario where the densest molecular core, peak C, survived against the blast wave and is now embedded within the SNR. Numerical simulations of the shock-cloud interaction indicate that a dense clump can indeed survive shock erosion, since shock propagation speed is stalled in the dense clump. Additionally, the shock-cloud interaction induces turbulence and magnetic field amplification around the dense clump that may facilitate particle acceleration in the lower-density inter-clump space leading to the enhanced synchrotron X rays around dense cores.Comment: 22 pages, 7 figures, to accepted in The Astrophysical Journal. A full color version with higher resolution figures is available at http://www.a.phys.nagoya-u.ac.jp/~sano/ApJ10/ms_sano.pd

A Detailed Study of the Molecular and Atomic Gas Toward the {\gamma}-ray SNR RX J1713.7-3946: Spatial TeV {\gamma}-ray and ISM Gas Correspondence

RX J1713.7$-$3946 is the most remarkable TeV $\gamma$-ray SNR which emits $\gamma$-rays in the highest energy range. We made a new combined analysis of CO and \ion{H}{1} in the SNR and derived the total protons in the interstellar medium (ISM). We have found that the inclusion of the \ion{H}{1} gas provides a significantly better spatial match between the TeV $\gamma$-rays and ISM protons than the H$_2$ gas alone. In particular, the southeastern rim of the $\gamma$-ray shell has a counterpart only in the \ion{H}{1}. The finding shows that the ISM proton distribution is consistent with the hadronic scenario that comic ray (CR) protons react with ISM protons to produce the $\gamma$-rays. This provides another step forward for the hadronic origin of the $\gamma$-rays by offering one of the necessary conditions missing in the previous hadronic interpretations. We argue that the highly inhomogeneous distribution of the ISM protons is crucial in the origin of the $\gamma$-rays. Most of the neutral gas was likely swept up by the stellar wind of an OB star prior to the SNe to form a low-density cavity and a swept-up dense wall. The cavity explains the low-density site where the diffusive shock acceleration of charged particles takes place with suppressed thermal X-rays, whereas the CR protons can reach the target protons in the wall to produce the $\gamma$-rays. The present finding allows us to estimate the total CR proton energy to be $\sim 10^{48}$ ergs, 0.1% of the total energy of a SNe.Comment: 21 pages, 20 figures, 1 table, accepted for publication in the Astrophysical Journa

Associated molecular and atomic clouds with X-ray shell of superbubble 30 Doradus C in the LMC

30 Doradus C is a superbubble which emits the brightest nonthermal X- and TeV gamma-rays in the Local Group. In order to explore detailed connection between the high energy radiation and the interstellar medium, we have carried out new CO and HI observations using the Atacama Large Millimeter$/$Submillimeter Array (ALMA), Atacama Submillimeter Telescope Experiment, and the Australia Telescope Compact Array with resolutions of up to 3 pc. The ALMA data of $^{12}$CO($J$ = 1-0) emission revealed 23 molecular clouds with the typical diameters of $\sim$6-12 pc and masses of $\sim$600-10000 $M_{\odot}$. The comparison with the X-rays of $XMM$-$Newton$ at $\sim$3 pc resolution shows that X-rays are enhanced toward these clouds. The CO data were combined with the HI to estimate the total interstellar protons. Comparison of the interstellar proton column density and the X-rays revealed that the X-rays are enhanced with the total proton. These are most likely due to the shock-cloud interaction modeled by the magnetohydrodynamical simulations (Inoue et al. 2012, ApJ, 744, 71). Further, we note a trend that the X-ray photon index varies with distance from the center of the high-mass star cluster, suggesting that the cosmic-ray electrons are accelerated by one or multiple supernovae in the cluster. Based on these results we discuss the role of the interstellar medium in cosmic-ray particle acceleration.Comment: 20 pages, 14 figures, 3 tables, accepted for publication in The Astrophysical Journa

ALMA Observations of Supernova Remnant N49 in the LMC. I. Discovery of CO Clumps Associated with X-Ray and Radio Continuum Shells

N49 (LHA 120-N49) is a bright X-ray supernova remnant (SNR) in the Large Magellanic Cloud. We present new 12CO (J = 1–0, 3–2), H i, and 1.4 GHz radio continuum observations of the SNR N49 using Mopra, ASTE, ALMA, and ATCA. We have newly identified three H i clouds using ATCA with an angular resolution of ~20'': one associated with the SNR and the others located in front of the SNR. Both the CO and H i clouds in the velocity range from 281 to 291 km s−1 are spatially correlated with both the soft X-rays (0.2–1.2 keV) and the hard X-rays (2.0–7.0 keV) of N49 on a ~10 pc scale. CO 3–2/1–0 intensity ratios indicate higher values of the CO cloud toward the SNR shell with an angular resolution of ~45'', and thus a strong interaction was suggested. Using the ALMA, we have spatially resolved CO clumps embedded within or along the southeastern rim of N49 with an angular resolution of ~3''. Three of the CO clumps are rim brightened on a 0.7–2 pc scale in both hard X-rays and the radio continuum: this provides further evidence for dynamical interactions between the CO clumps and the SNR shock wave. The enhancement of the radio synchrotron radiation can be understood in terms of magnetic field amplification around the CO clumps via a shock–cloud interaction. We also present a possible scenario in which the recombining plasma that dominates the hard X-rays from N49 was formed via thermal conduction between the SNR shock waves and the cold/dense molecular clumps