Chandra Observation of Luminous Sources in the Nearby Irregular Galaxy NGC 4449

Using archival Chandra ACIS-S data, 0.5–8.0keV X-ray spectra of two luminous X-ray sources in the nearby dwarf irregular galaxy NGC 4449 were studied. One, with an extremely high luminosity of 1.3 × 10 39 ergs −1 in the 0.5–8.0 keV band, shows a spectrum that is well described with a power-law model of photon index ∼ 2. Its properties are consistent with those of ultraluminous compact X-ray sources observed in nearby galaxies. The spectrum of the other, with a luminosity of 2.7 ×10 38 ergs −1 in the same band, is successfully represented with a so-called multi-color disk blackbody emission model with an inner-most disk temperature of ∼0.59keV. Its spectral parameters are typical of ordinary black hole binaries observed in our Galaxy and the Large Magellanic Cloud. These young population objects, together with a bright supernova remnant and diffuse hot gas already reported, suggest that the X-ray emission from irregular galaxies is generally enhanced by their recent star-forming activities.