The First Detection of [O IV] from an Ultraluminous X-ray Source with Spitzer. II. Evidence for High Luminosity in Holmberg II ULX

TL;DR

This study presents the first detection of the [O IV] infrared emission line from an ultraluminous X-ray source, providing evidence for high luminosity and photoionization by the accretion disk, with minimal shock contribution.

Contribution

It offers detailed photoionization modeling of the [O IV] emission in Holmberg II ULX, demonstrating high luminosity and the influence of the companion star's spectral type on emission strength.

Findings

[O IV] emission consistent with photoionization by the accretion disk's radiation.

Luminosity exceeds 10^40 erg s^-1, indicating a highly luminous source.

Shocks contribute little to the high-ionization line fluxes.

Abstract

This is the second of two papers examining Spitzer Infrared Spectrograph (IRS) observations of the ultraluminous X-ray source (ULX) in Holmberg II. Here we perform detailed photoionization modeling of the infrared lines. Our analysis suggests that the luminosity and morphology of the [O IV] 25.89 $\mu$m emission line is consistent with photoionization by the soft X-ray and far ultraviolet (FUV) radiation from the accretion disk of the binary system and inconsistent with narrow beaming. We show that the emission nebula is matter-bounded both in the line of sight direction and to the east, and probably radiation-bounded to the west. A bolometric luminosity in excess of 10$^{40}$ erg s$^{-1}$ would be needed to produce the measured [O IV] flux. We use modeling and previously published studies to conclude that shocks likely contribute very little, if at all, to the high-ionization line fluxes observed in the Holmberg II ULX. Additionally, we find that the spectral type of the companion star has a surprisingly strong effect on the predicted strength of the [O IV] emission. This finding could explain the origin of [O IV] in some starburst systems containing black hole binaries.