1. Department of High Energy Astrophysics, Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency (ISAS/JAXA), 3-1-1 Yoshinodai, Sagamihara, Kanagawa, 229-8510, Japan
2. Dublin Institute for Advanced Studies, 5 Merrion Square, Dublin 2, Ireland
3. Max-Planck-Institut für Kernphysik, PO Box 103980, D69029 Heidelberg, Germany
4. Stanford Linear Accelerator Center, 2575 Sand Hill Road, M/S 29, Menlo Park, California 94025, USA

Correspondence to: Yasunobu Uchiyama¹ Correspondence and requests for materials should be addressed to Y.U. (Email: uchiyama@astro.isas.jaxa.jp).

Galactic cosmic rays (CRs) are widely believed to be accelerated by shock waves associated with the expansion of supernova ejecta into the interstellar medium¹. A key issue in this long-standing conjecture is a theoretical prediction that the interstellar magnetic field can be substantially amplified at the shock of a young supernova remnant (SNR) through magnetohydrodynamic waves generated by cosmic rays^{2, 3}. Here we report a discovery of the brightening and decay of X-ray hot spots in the shell of the SNR RX J1713.7-3946 on a one-year timescale. This rapid variability shows that the X-rays are produced by ultrarelativistic electrons through a synchrotron process and that electron acceleration does indeed take place in a strongly magnetized environment, indicating amplification of the magnetic field by a factor of more than 100. The X-ray variability also implies that we have witnessed the ongoing shock-acceleration of electrons in real time. Independently, broadband X-ray spectrometric measurements⁴ of RX J1713.7-3946 indicate that electron acceleration proceeds in the most effective ('Bohm-diffusion') regime. Taken together, these two results provide a strong argument for acceleration of protons and nuclei to energies of 1 PeV (10¹⁵ eV) and beyond in young supernova remnants.

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