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Register a new userEnvironmental Impact on the Southeast Limb of the Cygnus Loop
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N. A. Levenson
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We analyze observations from the Chandra X-ray Observatory of the southeast knot of the Cygnus Loop supernova remnant. In this region, the blast wave propagates through an inhomogeneous environment. Extrinsic differences and subsequent multiple projections along the line of sight rather than intrinsic shock variations, such as fluid instabilities, account for the apparent complexity of the images. Interactions between the supernova blast wave and density enhancements of a large interstellar cloud can produce the morphological and spectral characteristics. Most of the X-ray flux arises in such interactions, not in the diffuse interior of the supernova remnant. Additional observations at optical and radio wavelengths support this account of the existing interstellar medium and its role in shaping the Cygnus Loop, and they demonstrate that the southeast knot is not a small cloud that the blast wave has engulfed. These data are consistent with rapid equilibration of electron and ion temperatures behind the shock front, and the current blast wave velocity v_{bw} approx 330 km/s. Most of this area does not show strong evidence for non-equilibrium ionization conditions, which may be a consequence of the high densities of the bright emission regions.
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We have observed the south-east (SE) limb of the Cygnus Loop with {it Suzaku}. Our spatially-resolved spectroscopic study shows that a one-$kT_mathrm{e}$ non-equilibrium ionization model represents our spectra fairly well. We find that the metal abundances obtained are all depleted relative to the solar values with a positional dependency along the radial direction of the Cygnus Loop. The abundances in the very edge of the limb shows about half the solar value, whereas other regions inside the Loop show about 0.2 times the solar value which has been believed as a typical value for the Cygnus Loop limb. The enhanced abundance in the very edge in the SE limb is quite similar to that found in the north-east (NE) limb of the Loop, and thus this is another evidence of abundance inhomogeneity in the limb regions of the Loop. The radio map shows a quite different feature: the NE limb is in the radio bright region while the SE limb shows almost no radio. Therefore, the metal abundance variation in the SE limb can not attribute to the non-thermal emission. The abundance inhomogeneity as well as the metal depletion down to 0.2 times the solar value still remain an open question.
Galaxy evolution reveals itself not only through the evolving properties of galaxies themselves but also through its impact on the surrounding environment. The intergalactic medium in particular holds a fossil record of past galaxy activity, imprinted on its thermodynamic and chemical properties. This is most easily discerned in small galaxy groups, where the gravitational heating of this gas renders it observable by X-ray telescopes while still leaving its properties highly susceptible to the effects of galactic feedback. X-ray observations of the hot gas in groups can therefore provide a view of galactic feedback history that can complement dedicated studies of AGN and star formation activity at low and high redshift. Based on high-quality X-ray data of a sample of nearby groups, we present initial results of such a study and discuss some implications for the AGN and star formation histories of the group members.
With Hubble Space Telescope Wide-Field Planetary Camera 2 observations of the Cygnus Loop supernova remnant, we examine the interaction of an interstellar cloud with the blast wave on physical scales of 10^15 cm. The shock front is distorted, revealing both edge-on and face-on views of filaments and diffuse emission, similar to those observed on larger scales at lower resolution. We identify individual shocks in the cloud of density n~15 cm^-3 having velocity v_s~170 km/s. We also find the morphologically unusual diffuse Balmer-dominated emission of faster shocks in a lower-density region. The obstacle diffracts these shocks, so they propagate at oblique angles with respect to the primary blast wave. The intricate network of diffuse and filamentary H alpha emission arises during the early stage of interaction between the cloud and blast wave, demonstrating that complex shock propagation and emission morphology occur before the onset of instabilities that destroy clouds completely.
We present the C III {lambda}977, O VI {lambda}{lambda}1032, 1038 and N IV] {lambda}1486 emission line maps of the Cygnus Loop, obtained with the newly processed data of Spectroscopy of Plasma Evolution from Astrophysical Radiation (SPEAR; also known as FIMS) mission. In addition, the Si IV+O IV] line complexes around 1400 {AA} are resolved into two separate emission lines, whose intensity demonstrates a relatively high Si IV region predicted in the previous study. The morphological similarity between the O VI and X-ray images, as well as a comparison of the O VI intensity with the value expected from the X-ray results, indicates that large portions of the observed O VI emissions could be produced from X-ray emitting gas. Comparisons of the far-ultraviolet (FUV) images with the optical and H I 21 cm images, reveal spatial variations of shock-velocity populations and high FUV extinction in the direction of a previously identified H I cloud. By calculating the FUV line ratios for several subregions of the Cygnus Loop, we investigate the spatial variation of the population of radiative shock velocities; and the effects of resonance scattering, X-ray emitting gas, and non-radiative shocks. The FUV and X-ray luminosity comparisons between the Cygnus Loop and the Vela supernova remnant suggest that the fraction of shocks in the early evolutionary stages is much larger in the Cygnus Loop.
We conducted a comprehensive study on the shell structure of the Cygnus Loop using 41 observation data obtained by the Suzaku and the XMM-Newton satellites. To investigate the detailed plasma structure of the Cygnus Loop, we divided our fields of view into 1042 box regions. From the spectral analysis, the spectra obtained from the limb of the Loop are well fitted by the single-component non-equilibrium ionization plasma model. On the other hand, the spectra obtained from the inner regions are well fitted by the two-component model. As a result, we confirmed that the low-temperature and the high-temperature components originated from the surrounding interstellar matter (ISM) and the ejecta of the Loop, respectively. From the best-fit results, we showed a flux distribution of the ISM component. The distribution clearly shows the limb-brightening structure, and we found out some low-flux regions. Among them, the south blowout region has the lowest flux. We also found other large low-flux regions at slightly west and the northeast from the center. We estimated the former thin shell region to be 1.3 degrees in diameter and concluded that there exists a blowout along the line of sight in addition to the south blowout. We also calculated the emission measure distribution of the ISM component and showed that the Cygnus Loop is far from the result obtained by a simple Sedov evolution model. From the results, we support that the Cygnus Loop originated from a cavity explosion. The emission measure distribution also suggests that the cavity-wall density is higher in the northeast than that in the southwest. These results suggest that the thickness of the cavity wall surrounding the Cygnus Loop is not uniform.
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