Spitzer Observations of MF 16 Nebula and the associated Ultraluminous X-ray Source

Abstract

We present Spitzer Infrared Spectrograph (IRS) observations of the ultra-luminous X-ray source (ULX) NGC 6946 X-1 and its associated nebula MF 16. This ULX has very similar properties to the famous Holmberg II ULX, the first ULX to show a prominent infrared [O IV] emission line comparable to those found in AGN. This paper attempts to constrain the ULX Spectral Energy Distribution (SED) given the optical/UV photometric fluxes and high-resolution X-ray observations. Specifically, Chandra X-ray data and published Hubble optical/UV data are extrapolated to produce a model for the full optical to X-ray SED. The photoionization modeling of the IR lines and ratios is then used to test different accretion spectral models. While either an irradiated disk model or an O-supergiant plus accretion disk model fit the data very well, we prefer the latter because it fits the nebular parameters slightly better. In this second case the accretion disk alone dominates the extreme-UV and X-ray emission, while an O-supergiant is responsible for most of the far-UV emission.

26 pages, 7 figures, accepted by ApJ

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• A New, Larger Sample of Supernova Remnants in NGC 6946
• Elevated Hot Gas and High-Mass X-ray Binary Emission in Low Metallicity Galaxies: Implications for Nebular Ionization and Intergalactic Medium Heating in the Early Universe
• Ultra-luminous X-ray Sources as Supercritical Accretion Disks: Spectral Energy Distributions
• MUSE spectroscopy of the ULX NGC 1313 X-1: a shock-ionised bubble, an X-ray photoionised nebula, and two supernova remnants
• On the contribution of the X-ray source to the extended nebular HeII emission in IZw18
• A broadband look at the old and new ULXs of NGC 6946
• The Local Environment of Ultraluminous X-ray Sources Viewed by XMM Newton's Optical Monitor
• Spitzer IRAC observations of IR excess in Holmberg IX X-1: A circumbinary disk or a variable jet?
• Hot gas and magnetic arms of NGC 6946: indications for reconnection heating?
• Constraining soft and hard X-ray irradiation in ultraluminous X-ray sources