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X-ray Hotspots in the Northwest Shell of the Supernova Remnant RX J1713.7$-$3946

The supernova remnant (SNR) RX J1713.7$-$3946 is one of the best-studied accelerators of cosmic rays because of its strong nonthermal X-ray and gamma-ray radiation. We have analyzed accumulated \chandra\ observations with a total exposure time of $\sim$266 ks in the northwest rim of RX J1713.7$-$3946. We detect a substantially large number of point-like sources, referred to as ``hotspots'', which are likely associated with the remnant. The spectra of the hotspots are well described by an absorbed power-law model. The spectral properties ($10^{21}\ \mathrm{cm^{-2}}\lesssim N_H \lesssim 10^{23}\ \mathrm{cm^{-2}}$ and $ 0.5\lesssim Γ\lesssim 6$) are different from diffuse X-ray emission in RX J1713.7$-$3946, and the harder hotspot tends to have the larger $N_H$. We also confirm yearly and monthly variabilities of flux for some hotspots. We propose that RX J1713.7$-$3946 is embedded in a complex surroundings where some dense molecular clumps and cores exist inside a wind-blown cavity, and that the hotspot traces synchrotron emission caused by an interaction of shock waves of the SNR and dense molecular cores with a number density of $10^{5}-10^{7}~ \mathrm{cm}^{-3}$. The X-ray radiation of the hotspot might be emitted from both primary electrons accelerated at the shocks and secondary electrons produced by the interaction of accelerated protons with the cores.