Exploring the case for hard-X-ray beaming in NGC 6946 X-1

TL;DR

This study investigates whether optical emission from nebulae around ULXs can reveal the beaming and collimation of X-ray and UV radiation, focusing on NGC 6946 X-1, and finds nebulae are more sensitive to UV than hard X-rays.

Contribution

It demonstrates the potential of nebular optical line emission as a probe for ULX radiation beaming and highlights the limitations in constraining hard X-ray beaming through nebula feedback.

Findings

Nebulae are highly sensitive to UV emission from ULXs.

Hard X-ray emission has little impact on nebular optical lines.

Cannot definitively rule out hard X-ray beaming based on nebula feedback.

Abstract

In order to understand the nature of super-Eddington accretion we must explore both the emission emerging directly from the inflow and its impact on the surroundings. In this paper we test whether we can use the optical line emission of spatially resolved, ionized nebulae around ultraluminous X-ray sources (ULXs) as a proxy for their X-ray luminosity. We choose the ULX NGC 6946 X-1 and its nebula, MF16, as a test case. By studying how the nebular optical line emission responds to assumed irradiation, we can infer the degree to which we require the UV or X-ray emission from the inflow to be collimated by optically thick winds seemingly ubiquitously associated with ULXs. We find that the nebula is highly sensitive to compact UV emission but mostly insensitive to hard X-rays. Our attempts to quantify the beaming of the soft and hard X-rays therefore strongly depends on the UV luminosity of the ULX in the center of the nebula. We find that it is not possible to conclude a lack of geometrical beaming of hard X-rays from such sources via nebula feedback.