ALMA CO Observations of the Gamma-Ray Supernova Remnant RX J1713.7-3946: Discovery of Shocked Molecular Cloudlets and Filaments at 0.01 pc Scales

RX J1713.7-3946 is a unique core-collapse supernova remnant (SNR) that emits bright TeV gamma-rays and synchrotron X-rays caused by cosmic rays, in addition to interactions with interstellar gas clouds. We report here on results of Atacama Large Millimeter/submillimeter Array ¹²CO(J = 1-0) observations toward the northwestern shell of the SNR. We newly found three molecular complexes consisting of dozens of shocked molecular cloudlets and filaments with typical radii of ∼0.03-0.05 pc and densities of ∼10⁴ cm^-3. These cloudlets and filaments are located not only along synchrotron X-ray filaments, but also in the vicinity of X-ray hotspots with month- or year-scale time variations. We argue that X-ray hotspots and filaments were generated by shock-cloudlet interactions through magnetic-field amplification up to mG. The interstellar medium density contrast of ∼10⁵, coexistence of molecular cloudlets and low-density diffuse medium of ∼0.1 cm^-3, is consistent with such a magnetic field amplification as well as a wind-bubble scenario. The small-scale cloud structures also affect hadronic gamma-ray spectra considering the magnetic field amplification on surface and inside clouds.