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

TL;DR

This study reports the first detection of Ar-K line emission from the Cygnus Loop supernova remnant, revealing unexpectedly high argon abundance in its ejecta, which provides new insights into its composition and progenitor characteristics.

Contribution

First detection of Ar-K line emission in the Cygnus Loop, indicating high argon abundance in its ejecta, based on combined XMM-Newton and Suzaku observations.

Findings

Detected Ar-K line emission at ~3 keV for the first time.

Argon abundance significantly exceeds solar levels, around 8-9 times solar.

Ejecta origin of the Ar-K line confirmed.

Abstract

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 He$\beta$ and Ar He$\alpha$ lines, respectively. Since the Cygnus Loop is an evolved ($\sim$10,000 yr) supernova remnant whose temperature is relatively low ($<$1 keV) compared with other young remnants, its spectrum is generally faint above 3.0 keV, no emission lines, such as the Ar-K line have ever been detected. The detection of the Ar-K line is the first time and we found that its abundance is significantly higher than that of the solar value; 9.0$^{+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.