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

TL;DR

This study uses ALMA CO observations to reveal small-scale shocked molecular cloudlets and filaments in the supernova remnant RX J1713.7-3946, linking these structures to gamma-ray and X-ray emission features.

Contribution

First detailed ALMA CO mapping of shocked molecular cloudlets at 0.01 pc scales in this supernova remnant, highlighting their role in shock interactions and magnetic field amplification.

Findings

Discovery of dozens of shocked cloudlets and filaments with radii of 0.03-0.05 pc.

Cloudlets located along X-ray filaments and hotspots, indicating shock-cloud interactions.

Magnetic field amplification up to mG due to shock-cloudlet interactions.

Abstract

RX J1713.7-3946 is a unique core-collapse 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 ALMA $^{12}$CO($J$ = 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 $\sim$0.03-0.05 pc and densities of $\sim$$10^4$ cm$^{-3}$. These cloudlets and filaments are located not only along synchrotron X-ray filaments, but also in the vicinity of X-ray hotspots with month or year-scale time variations. We argue that X-ray hotspots were generated by shock-cloudlet interactions through magnetic-field amplification up to mG. The ISM density contrast of $\sim$$10^5$, coexistence of molecular cloudlets and low-density diffuse medium of $\sim$0.1 cm$^{-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.