A 9 month long soft X-ray survey of the large magellanic cloud. I. X-ray map at ¼ keV

Abstract

We present a ¼ keV X-ray map of the Large Magellanic Cloud (LMC) obtained from 9 months of scanning observations with the HEAO 1 low energy detectors. This soft X-ray map is the most sensitive ¼ keV survey of this region yet reported. A detailed comparison of the ¼ keV X-ray observations with the neutral hydrogen column densities in the LMC obtained from a new 21 cm line survey shows no evidence for absorption effects is found, which in the ¼ keV X-ray flux from the LMC due to the neutral matter in the LMC. Instead, faint X-ray emission invalidates attempts to use the LMC to measure any hypothetical cosmic ¼ keV X-ray background, at least for instruments which are unable to spatially resolve the LMC emission. The extent of the observed emission is smaller than the size of the halo (24°) or the disk (14°) of the LMC. Assuming this ¼ keV emission to be diffuse, we identify it with two large regions in the LMC- a supergiant shell of optical nebulosity known as Shapely III and the bar of the LMC. The X-ray luminosities of these two regions are estimated to be 3.4×10³⁷ ergs s⁻¹ and 7.2×10³⁷ ergs s⁻¹ (0.1-1.0 keV), respectively. Shapely III could be the first X-ray superbubble observed in an external galaxy.