Unbiased Spectroscopic Study of the Cygnus Loop with LAMOST. I. Optical
  Properties of Emission Lines and the Global Spectrum
  We present an unbiased spectroscopic study of the Galactic supernova remnant
(SNR) Cygnus Loop using the Large Sky Area Multi-object Fiber Spectroscopic
Telescope (LAMOST) DR5. LAMOST features both a large field of view and a large
aperture, which allow us to simultaneously obtain 4000 spectra at
$\sim$3700-9000 \AA with R$\approx$1800. The Cygnus Loop is a prototype of
middle-aged SNRs, which has the advantages of being bright, large in angular
size, and relatively unobscured by dust. Along the line of sight to the Cygnus
Loop, 2747 LAMOST DR5 spectra are found in total, which are spatially
distributed over the entire remnant. This spectral sample is free of the
selection bias of most previous studies, which often focus on bright filaments
or regions bright in [O III]. Visual inspection verifies that 368 spectra
(13$\%$ of the total) show clear spectral features to confirm their association
with the remnant. In addition, 176 spectra with line emission show ambiguity of
their origin but have a possible association to the SNR. In particular, the 154
spectra dominated by the SNR emission are further analyzed by identifying
emission lines and measuring their intensities. We examine distributions of
physical properties such as electron density and temperature, which vary
significantly inside the remnant, using theoretical models. By combining a
large number of the LAMOST spectra, a global spectrum representing the Cygnus
Loop is constructed, which presents characteristics of radiative shocks.
Finally, we discuss the effect of the unbiased spectral sample on the global
spectrum and its implication to understand a spatially unresolved SNR in a
distant galaxy.
