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We present an unbiased spectroscopic study of the Galactic supernova remnant(SNR) Cygnus Loop using the Large Sky Area Multi-object Fiber SpectroscopicTelescope (LAMOST) DR5. LAMOST features both a large field of view and a largeaperture, which allow us to simultaneously obtain 4000 spectra at$\sim$3700-9000 \AA with R$\approx$1800. The Cygnus Loop is a prototype ofmiddle-aged SNRs, which has the advantages of being bright, large in angularsize, and relatively unobscured by dust. Along the line of sight to the CygnusLoop, 2747 LAMOST DR5 spectra are found in total, which are spatiallydistributed over the entire remnant. This spectral sample is free of theselection bias of most previous studies, which often focus on bright filamentsor regions bright in [O III]. Visual inspection verifies that 368 spectra(13$\%$ of the total) show clear spectral features to confirm their associationwith the remnant. In addition, 176 spectra with line emission show ambiguity oftheir origin but have a possible association to the SNR. In particular, the 154spectra dominated by the SNR emission are further analyzed by identifyingemission lines and measuring their intensities. We examine distributions ofphysical properties such as electron density and temperature, which varysignificantly inside the remnant, using theoretical models. By combining alarge number of the LAMOST spectra, a global spectrum representing the CygnusLoop is constructed, which presents characteristics of radiative shocks.Finally, we discuss the effect of the unbiased spectral sample on the globalspectrum and its implication to understand a spatially unresolved SNR in adistant galaxy.

Unbiased Spectroscopic Study of the Cygnus Loop with LAMOST. I. Optical Properties of Emission Lines and the Global Spectrum