We report on results from Suzaku broadband X-ray observations of the southwest part of the galactic supernova remnant (SNR) RXJ1713.7-3946 with an energy coverage of 0.4-40keV. The X-ray spectrum, presumably of synchrotron origin, is known to be completely lineless, making this SNR ideally suited for a detailed study of the X-ray spectral shape formed through efficient particle acceleration at high-speed shocks. With a sensitive hard X-ray measurement from the HXD PIN aboard Suzaku, we determined the hard X-ray spectrum in the 12-40keV range to be described by a power law with photon index of Γ = 3.2 ± 0.2, significantly steeper than the soft X-ray index of Γ = 2.4 ± 0.05 measured previously with ASCA and other missions. We find that a simple power law fails to describe the full spectral range of 0.4-40 keV, and instead a power-law with an exponential cutoff with a hard index of Γ = 1.50 ± 0.09 and a high-energy cutoff of εc = 1.2 ± 0.3 keV formally provides an excellent fit over the full bandpass. If we use the so-called SRCUT model, as an alternative model, it gives a best-fit rolloff energy of εroll = 0.95 ± 0.04 keV. Together with the TeV γ-ray spectrum, ranging from 0.3 to 100TeV, recently obtained by HESS observations, our Suzaku observations of RX J1713.7-3946 provide stringent constraints on the highest-energy particles accelerated in a supernova shock.
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