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Title:
Constraints on Accretion in Ultraluminous X-Ray Sources from Spitzer IRS Observations of NGC 4485/4490: Infrared Diagnostic Diagrams
Authors:
Vázquez, Gerardo A.; Hornschemeier, Ann E.; Colbert, Edward; Roberts, Timothy P.; Ward, Martin J.; Malhotra, Sangeeta
Affiliation:
AA(Physics and Astronomy Department, Johns Hopkins University, Baltimore, MD 21218; ), AB(NASA/GSFC, Code 662, Laboratory for X-ray Astrophysics, Greenbelt, MD 20771; ), AC(Physics and Astronomy Department, Johns Hopkins University, Baltimore, MD 21218; ), AD(Department of Physics, Durham University, South Road, Durham DH1 3LE, UK; , ), AE(Department of Physics, Durham University, South Road, Durham DH1 3LE, UK; , ), AF(Department of Physics and Astronomy, Arizona State University, Tempe, AZ 85287-1504 )
Publication:
The Astrophysical Journal, Volume 658, Issue 1, pp. L21-L24. (ApJL Homepage)
Publication Date:
03/2007
Origin:
UCP
ApJ Keywords:
Galaxies: General, Galaxies: Individual: NGC Number: NGC 4485, Galaxies: Individual: NGC Number: NGC 4490
DOI:
10.1086/513423
Bibliographic Code:
2007ApJ...658L..21V

Abstract

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.
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