We present the results of a systematic study of the Large Magellanic Cloud (LMC) with the X-ray satellite, Advanced Satellite for Cosmology and Astrophysics (ASCA). We analyzed all the available data in the vicinity of the LMC, obtained during the ASCA mission, the period from 1993 until 2000, including the data from the ASCA LMC survey and all archival pointing observations. The X-ray mosaic images in the soft (0.7-2.0 keV) as well as hard (2.0-7.0 keV) bands were constructed. In particular, the hard band image of the LMC is first revealed, thanks to the wide energy band of ASCA. We extracted 71 sources from these images with a criterion of S/N > 5 and made systematic spectral analyses of them. From the information of spatial correlations as well as the spectra, the comparison with ROSAT PSPC and HRI source catalogues (Haberl et al. 1999 ; Sasaki et al. 2000) are carried out. As a results, the first ASCA LMC source catalogue is revealed.
We made intensive spectral analyses for 9 SNRs, for which no detailed spectral analyses have been carried out (DEM L316A, DEM L316B, 0532-71.0, 0534-69.9, 0548-70.4, 0520-69.4, DEM L214, 0532-67.5, and N86; sample-1), applying the non-equilibrium ionization (NEI) model coded by Masai 1994. The NEI model well describes the ASCA spectra of sample-1 SNRs, allowing us to derive the plasma parameters within them, such as ages, and densities.
Among the sample-1 SNRs is included a pair of SNRs of considerable interest, a possible site for the collision between two remnants. The ASCA spectra of these two remnants exhibit totally different characteristics; one shows clear iron overabundance, while the other does not. These facts lead us to the conclusion that they were not due to a single supernova explosion into a pre-existing bubbles made by a stellar wind or SNe. We propose one of these have resulted from Type Ia explosion. We also examined the relation between the Balmer-dominated SNRs in the LMC and those which exhibit iron over abundances in the X-ray spectra. Based on the spatial correlations with OB associations or H II regions and those remnants, we found that it is highly possible that characteristic to show the dominant Balmer-lines in optical spectra does not always become the evidence for the Type Ia SNRs.
Our sample-1 SNRs together with 10 SNRs in Hughes et al. (1998) cover all the remnants for which the ASCA spectra with enough statistics are available. They exhibit various ages of ~ 103-105 yr. We found that the densities within them decrease as their ages increase, surely tracing the course of the evolution of the plasma within SNRs. The density of the oldest SNR in our sample should restrict the upper limit of the ambient plasma density, namely that of interstellar matter (ISM).
From all the available pointing observations, we accumulated a spectrum of diffuse X-ray emission which is widely spreading in the LMC body, whose origin is still uncertain, but probably arising from hot ISM in the LMC. The spectral analysis of the diffuse X-ray emission with such a good energy resolution and enough statistics is first attempted. We find that the spectrum cannot be reproduced with a non-thermal model, instead, is well described with a NEI model (kTe ~ 0.9 keV and neti ~1010.5 s cm-3), showing that it is surely thermal in origin, which is a strong support for the diffuse X-ray emission to be arising from the hot ISM. are available. They exhibit various ages of ~ 103--105 yr. We found that the densities within them decrease as their ages increase, surely tracing the course of the evolution of the plasma within SNRs. The density of the oldest SNR in our sample should restrict the upper limit of the ambient plasma density, namely that of interstellar matter (ISM).
We then attempted to interpret the diffuse X-ray emission in terms of sum of SNRs based on the derived plasma parameters. The diffuse X-ray emission cannot be described as an ensemble of either young, middle-aged, nor old SNRs; the expected temperature in young as well as old remnants differ from those of the diffuse X-ray emission. The potential candidates, middle-aged SNRs, though they have plasma parameters well resemble to those of the diffuse X-ray emission, also fail to be the major contributor of the diffuse X-ray emission, as long as we assumed that they follow the simple Sedov solution. The negative results to explain the diffuse X-ray emission with an ensemble of `usual' SNRs are not surprising because most of them are considered to interact with other physical systems in the course of their evolution, thus the Sedov solution is not applicable. We suppose that most of the SNRs in the LMC do not end their lives normally to be silently merged into the cold or warm ISM, instead, they have merged into the hot tenuous medium in the course of their evolution, realized within the bubbles, giant shells, or super giant shells which themselves are formed by the repetitious merging of SNRs, and do not show the distinct remnants at all, thus have become a part of truly diffuse hot plasma. Indeed, only ~ 10 % of the expected number of remnants to exist from the energy sustainment of the diffuse X-ray emission at this moment, are resolved in the LMC. The rest of them should be surely 'missing' within the truly diffuse hot plasma.