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Durham University

Department of Physics

Staff profile

Publication details for Prof Tim Roberts

Vazquez, G.A., Hornschemeier, A.E., Colbert, E., Roberts, T.P., Ward, M.J. & Malhotra, S. (2007). Constraints on accretion in ultraluminous X-ray sources from Spitzer IRS observations of NGC 4485/4490: Infrared diagnostic diagrams. Astrophysical Journal 658(1): L21-L24.

Author(s) from Durham


Constraining the astrophysical nature of ultraluminous X-ray (ULX) sources, which have X-ray luminosities
exceeding 1039 ergs s1, has been elusive due to the optical faintness of any counterparts. With high spectral
resolution observations in the ∼10–30 mm 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.