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 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 progenitors mass.

Abundance Inhomogeneity in the Northeastern Rim of the Cygnus Loop Revealed by Suzaku Observatory

We present the results of a spatially resolved spectral analysis from four Suzaku observations covering the northeastern rim of the Cygnus Loop. A two-kT_e non-ionization equilibrium (NEI) model fairly well represents our data, which confirms the NEI condition of the plasma there. The metal abundances are depleted relative to the solar values almost everywhere in our field of view. We find abundance inhomogeneities across the field: the northernmost region (Region A) has enhanced absolute abundances compared with other regions. In addition, the relative abundances of Mg/O and Fe/O in Region A are lower than the solar values, while those in the other regions are twice higher than the solar values. As far as we are concerned, neither a circumstellar medium, fragments of ejecta, nor abundance inhomogeneities of the local interstellar medium around the Cygnus Loop can explain the relatively enhanced abundance in Region A. This point is left as an open question for future work.

Asymmetric Ejecta Distribution of the Cygnus Loop revealed with Suzaku

We observed a linearly sliced area of the Cygnus Loop from the north-east to the south-west with Suzaku in seven pointings. After dividing the entire fields of view (FOV) into 119 cells, we extracted spectra from all of the cells and performed spectral analysis for them. We then applied both one- and two-component non-equilibrium ionization (NEI) models for all of the spectra, finding that almost all were significantly better fitted by the two-component NEI model rather than the one-component NEI model. Judging from the abundances, the high-kT_e component must be the ejecta component, while the low-kT_e component comes from the swept-up matter. Therefore, the ejecta turn out to be distributed inside a large area (at least our FOV) of the Cygnus Loop. We divided the entire FOV into northern and southern parts, and found that the ejecta distributions were asymmetric to the geometric center: the ejecta of Si, S, and Fe seem to be distributed more in the south than in the north of the Cygnus Loop by a factor of about 2. The degree of ejecta-asymmetry is consistent with that expected by recent supernova explosion models.