A Suzaku Study of Ejecta Structure and Origin of Hard X-Ray Emission in the Supernova Remnant G 156.2+5.7

Abstract

We report on an X-ray study of the evolved Galactic supernova remnant (SNR) G 156.2$+$5.7 based on six pointing observations with Suzaku. The remnant’s large extent (100$'$ in diameter) allows us to investigate its radial structure in the northwestern and eastern directions from the apparent center. The X-ray spectra were well fit with a two-component non-equilibrium ionization model representing the swept-up interstellar medium (ISM) and the metal-rich ejecta. We found prominent central concentrations of Si, S, and Fe from the ejecta component; the lighter elements of O, Ne, and Mg were distributed more uniformly. The temperature of the ISM component suggests a slow shock ($\sim $610–960 km s$^{-1}$); hence, the remnant’s age is estimated to be $\sim $7000–15000 yr, assuming its distance to be $\sim $1.1 kpc. G 156.2$+$5.7 has also been thought to emit hard, non-thermal X-rays, despite being considerably older than any other such remnant. In response to a recent discovery of a background cluster of galaxies (2XMM J045637.2$+$522411), we carefully excluded its contribution, and reexamined the origin of the hard X-ray emission. We found that the residual hard X-ray emission is consistent with the expected level of the cosmic X-ray background. Thus, no robust evidence for non-thermal emission was obtained from G 156.2$+$5.7. These results are consistent with the picture of an evolved SNR.

THE X-RAY PROPERTIES OF FIVE GALACTIC SUPERNOVA REMNANTS DETECTED BY THESPITZERGLIMPSE SURVEY

“…Observationally, overionization of the X-ray-emitting plasma associated with an SNR is manifested by significant residuals that appear when spectra of the plasmas are fitted with CIE and NEI models. For example, in the case of W44, Uchida et al (2012) found significant residuals at ∼1.5 keV and ∼ 2 keV, which are attributed to the Mg-Lyα line and the Si-Lyα line, respectively. In the cases of W44 (Uchida et al 2012), IC 443 (Yamaguchi et al 2009) and G359.1-0.5 (Ohnishi et al 2011), residuals that emerged after fitting with CIE models are seen at 2.7 and 3.5 keV: lines at these energies are associated with H-like Si (2.67 keV) and H-like S (3.48 keV), respectively.…”

“…For example, in the case of W44, Uchida et al (2012) found significant residuals at ∼1.5 keV and ∼ 2 keV, which are attributed to the Mg-Lyα line and the Si-Lyα line, respectively. In the cases of W44 (Uchida et al 2012), IC 443 (Yamaguchi et al 2009) and G359.1-0.5 (Ohnishi et al 2011), residuals that emerged after fitting with CIE models are seen at 2.7 and 3.5 keV: lines at these energies are associated with H-like Si (2.67 keV) and H-like S (3.48 keV), respectively. Finally, in the case of W49B, Ozawa et al (2009) identified residuals at the energies of 5.6, 6.2, 6.7, 6.9, 7.9, 8.5 keV: these lines are attributed to Cr Kα, Mnα and four Fe lines (of Heα rec , Lyα, Heβ rec and Heγ), respectively.…”

“…Overionization occurs when the electron temperature (T e ) is lower than the ionization temperature (T i ) of the plasma: in such cases, the recombination process is dominant. Examples of MM SNRs that feature overionized X-ray-emitting plasmas are IC 443 (based on observations made by ASCA, XMM-Newton and Suzaku -see Kawasaki et al (2002), Troja et al (2008 and Yamaguchi et al (2009)), W28, W44, W49B and G359.1−0.5 (all based on observations made by Suzaku -see Sawada & Koyama (2012), Uchida et al (2012), Ozawa et al (2009), Ohnishi et al (2011), respectively). Reviews of the phenomenon of overionized Xray-emitting plasmas associated with SNRs is provided by Vink (2012).…”