| Description |
Ultraluminous X-ray sources are non-nuclear sources in normal disk galaxies that are second only to AGNs in point-source luminosity. Their X-ray luminosities exceed the Eddington limit for stellar mass black holes (15 Msun), suggesting the need for intermediate mass black holes. This inference depends on the X-ray emission being isotropic, an assumption that we test here. X-ray spectral studies show that much of the soft X-ray emission is absorbed by gas and dust, which will be reemitted isotropically in the far-infrared. The ratio of the absorbed X-ray luminosity to the FIR luminosity is a direct measure of the anisotropy of the X-ray emission. Our previous study with Spitzer, which focused on PAH emission, suggests that the X-ray emission is highly anisotropic. However, if the X-rays destroyed the PAHs, we should focus on the longer wavelength emission, where MIPS observations indicate weak detections. The limitations of the MIPS observations were the poor point spread function and short exposure time, which will be improved upon by the proposed PACS 70 um observations of three highly luminous and nearby ULXs. These data will determine whether ULXs are sub-Eddington or super-Eddington emitters. |
