Quasi-isotropic UV Emission in the ULX NGC~1313~X--1

TL;DR

This study investigates the anisotropy of UV/EUV emission in super-Eddington accretion flows around ULX NGC 1313 X-1 by analyzing nebular emission lines and spectral energy distributions, revealing nearly isotropic UV emission likely from wind photospheres.

Contribution

It provides observational constraints on the anisotropy of UV/EUV emission in ULXs, using multi-band spectroscopy and nebular line ratios to compare observed SEDs with nebular predictions, highlighting the role of wind photospheres.

Findings

UV emission is approximately four times dimmer in the nebula than along the line of sight.

The nebula 'sees' a nearly isotropic UV emission, suggesting wind photosphere origin.

Differences in ULX nebulae are likely due to mass-transfer rates, not inclination.

Abstract

A major prediction of most super-Eddington accretion theories is the presence of anisotropic emission from supercritical disks, but the degree of anisotropy and its dependency with energy remain poorly constrained observationally. A key breakthrough allowing to test such predictions was the discovery of high-excitation photoionized nebulae around Ultraluminous X-ray sources (ULXs). We present efforts to tackle the degree of anisotropy of the UV/EUV emission in super-Eddington accretion flows by studying the emission-line nebula around the archetypical ULX NGC~1313~X--1. We first take advantage of the extensive wealth of optical/near-UV and X-ray data from \textit{Hubble Space Telescope}, \textit{XMM-Newton}, \textit{Swift}-XRT and \textit{NuSTAR} observatories to perform multi-band, state-resolved spectroscopy of the source to constrain the spectral energy distribution (SED) along the line of sight. We then compare spatially-resolved \texttt{Cloudy} predictions using the observed line-of-sight SED with the nebular line ratios to assess whether the nebula `sees' the same SED as observed along the line of sight. We show that to reproduce the line ratios in the surrounding nebula, the photo-ionizing SED must be a factor $\approx 4$ dimmer in ultraviolet emission than along the line-of-sight. Such nearly-iosotropic UV emission may be attributed to the quasi-spherical emission from the wind photosphere. We also discuss the apparent dichotomy in the observational properties of emission-line nebulae around soft and hard ULXs, and suggest only differences in mass-transfer rates can account for the EUV/X-ray spectral differences, as opposed to inclination effects. Finally, our multi-band spectroscopy suggest the optical/near-UV emission is not dominated by the companion star.