First Detection of Ar-K Line Emission from the Cygnus Loop

We observed the Cygnus Loop with XMM-Newton (9 pointings) and Suzaku (32 pointings) between 2002 and 2008. The total effective exposure time is 670.2 ks. By using all of the available data, we intended to improve a signal-to-noise ratio of the spectrum. Accordingly, the accumulated spectra obtained by the XIS and the EPIC show some line features around 3 keV that are attributed to the S Heβ and Ar Heα lines, respectively. Since the Cygnus Loop is an evolved (∼10000yr) supernova remnant whose temperature is relatively low (<1keV) compared with other young remnants, its spectrum is generally faint above 3.0 keV, no emission lines, such as the Ar-K line, have ever been detected. The detection of the Ar-K line is the first time, and we found that its abundance is significantly higher than that of the solar value: 9.0^+4.0_-3.8 and 8.4^+2.5_-2.7 (in units of solar), estimated from the XIS and the EPIC spectra, respectively. We conclude that the Ar-K line originated from the ejecta of the Cygnus Loop. Follow-up X-ray observations to tightly constrain the abundances of Ar-rich ejecta will be useful to accurately estimate the progenitor's mass.