Chandra observations of the luminous infrared galaxy NGC 3256

Abstract

We present a detailed analysis of high-resolution Chandra observations of the merger system NGC 3256, the most infrared-luminous galaxy in the nearby universe. The X-ray data show that several discrete sources embedded in complex diffuse emission contribute 20 per cent of the total emission in the 0.5–10 keV energy range). The compact sources are hard and extremely bright and their emission is probably dominated by accretion-driven processes. Both galaxy nuclei are detected with LX3–10×1040 erg s1. No evidence is found for the presence of an active nucleus in the southern nucleus, contrary to previous speculation. Once the discrete sources are removed, the diffuse component has a soft spectrum that can be modelled by the superposition of three thermal plasma components with temperatures kT=0.6, 0.9 and 3.9 keV. Alternatively, the latter component can be described as a power law with index Γ3. Some evidence is found for a radial gradient of the amount of absorption and temperature of the diffuse component. We compare the X-ray emission with optical, Hα and NICMOS images of NGC 3256 and find a good correlation between the inferred optical/near-infrared and X-ray extinctions. Although inverse Compton scattering could be important in explaining the hard X-rays seen in the compact sources associated with the nuclei, the observed diffuse emission is probably of thermal origin. The observed X-ray characteristics support a scenario in which the powerful X-ray emission is driven solely by the current episode of star formation.