Simulations of Supernova Remnants in Diffuse Media. II. Three Remnants and Their X‐Ray Emission

Abstract

This paper provides detailed descriptions of the X-ray emission from supernova remnants (SNRs) evolving in warm, low-density, nonthermal pressureÈdominated regions K, n \ 0.001 cm~3, (T 0 \ 104 or 7200 K cm~3). Nonequilibrium ionization hydrocode simulations are used to predict the P nt \ 1800 high-resolution spectra, and keV ROSAT PSPC count rates, spatial appearance, color temperatures, 1 4 3 4 and ratios of O VII to O VIII emission-line Ñuxes as a function of time. If undisturbed, the remnants are quite long lived, surviving for D1.2 ] 107 to D1.6 ] 107 yr. During their brief energy-conserving phases, the hot, highly pressurized gas behind their shock fronts copiously emits X-rays. Thus, their keV 1 4 surface brightnesses are thousands of ROSAT PSPC counts s~1 arcmin~2 and the remnants appear strongly edge brightened. The onset of the radiative phase heralds the end of the extreme X-ray luminosities but not the end of the X-ray emission. After the cool shell forms behind the shock front, the hot SNR bubble slowly radiates away the remaining energy, with a diminutive fraction released in the form of X-rays. Thus, the keV surface brightnesses are tens to hundreds of ROSAT PSPC counts s~1 1 4 arcmin~2 and the remnants appear "" centrally Ðlled.ÏÏ The hot plasma within the remnant bubbles is always out of collisional ionizational equilibrium. Early on, the ionization states are much lower than expected for such hot gas. Later on, the ionization states are higher. This paper also applies the standard observational analyses for determining the color temperature, electron density, and thermal pressure to ROSAT "" observations ÏÏ of one of the simulated remnants, thus providing a map between observational results and physical conditions. The paper reports the O`5, N`4, and C`3 column densities for the simulated remnants. The simulations may be of interest and are applied to studies of the Galactic halo and Local Bubble. They may also be of interest to studies of external galaxies and interarm regions of the Milky Way.

Supernova Remnants in the Magellanic Clouds. IV. X‐Ray Emission from the Largest Supernova Remnant in the Large Magellanic Cloud

“…The optical and X-ray properties described above are consistent with an SNR in this age range. Slavin & Cox (1992) and Shelton (1999) predict that as a remnant ages, the expansion velocity slows to the point where X-rays are no longer generated by the shock front. The remaining X-ray emission is from the hot interior of the SNR, where material had been shocked to high temperatures by the previous phases of more rapid SNR expansion.…”

“…A detailed description of the SNR model is given by Shelton (1999). The particular simulation used here assumed an explosion energy of 10 51 ergs, an ambient density of 0.1 cm À3 , and an effective ambient magnetic field of 5 G. We followed the SNR development up to an age of 3 ; 10 6 yr, showing the evolution of the soft (0.1-2.2 keV ) X-ray spectrum over this period.…”

“…To estimate the effects of the ambient density on the model results, we also compared our data with modeled spectra for a remnant in an ambient ISM of 0.01 cm À3 (SNR C from Shelton 1999). Unsurprisingly, the best fits occurred at the high end of the age range (95,000 yr) and also required higher absorption column densities of $5 ; 10 21 cm À2 .…”

“…Theoretical models of hot gas in SNRs of ages 10 5 -10 6 yr (e.g., Slavin & Cox 1992, 1993Shelton 1999) cannot be compared easily with observations because few evolved SNRs are known and they are expected to be faint and difficult to observe. SNR 0450À70.9 ( Mathewson et al 1985) is the largest known remnant in the Large Magellanic Cloud ( LMC).…”

Diffuse X‐Ray Emission from M101

“…Absence of evidence is not, however, evidence of absence, and the presence of such a component may be obscured by other emission components. A further caution concerning the supernova attribution can be derived from the hydrodynamic supernova models of Shelton (1999). In those models of individual supernovae in low-density ambient media, typical time-averaged temperatures were d0.35 keV, substantially cooler than the observed component.…”