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

TL;DR

This study uses Spitzer infrared observations to analyze the nebula and X-ray source NGC 6946 X-1, constraining its spectral energy distribution and testing accretion models, favoring a combined stellar and disk emission scenario.

Contribution

First detailed infrared spectral analysis of the ULX NGC 6946 X-1 and its nebula, constraining its SED and testing accretion models against IR line ratios.

Findings

The nebula shows prominent [O IV] emission similar to AGN.

A combined stellar and accretion disk model best fits the data.

The accretion disk dominates the extreme-UV and X-ray emission.

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.