Constraints on accretion in ultraluminous X-ray sources from Spitzer IRS observations of NGC 4485/4490: Infrared diagnostic diagrams

Constraining the astrophysical nature of ultraluminous X-ray (ULX) sources, which have X-ray luminosities exceeding 10³⁹ ergs s^-1, has been elusive due to the optical faintness of any counterparts. With high spectral resolution observations in the ∼10-30 μm wavelength range we have conducted an experiment to study six ULX sources in the NGC 4485/4490 galaxy pair. We have found that five of the six ULXs, based on mid-infrared spectral diagnostics, show the characteristic higher ionization features that are found in AGNs. The sixth source, ULX-1, is consistent with being a supernova remnant. The chief infrared spectral diagnostics used are the ratios of [S III]/[Si II] versus [Ne III]/[Ne II]. In two instances fits to the continuum and polycyclic aromatic hydrocarbon (PAH) features also indicate higher dust temperatures, which are characteristic of accreting sources. Overall, however, we find that the continuum is dominated by stellar processes, and the best diagnostic features are the emission lines. High spectral resolution studies in the mid-infrared thus appear to show great promise for determining the astrophysical nature of ULXs.