Broad absorption features in wind-dominated ULXs?

TL;DR

This study analyzes X-ray spectra of ULXs and finds evidence of broad absorption features consistent with powerful winds, which could help distinguish between super-critical accretion and intermediate mass black holes.

Contribution

The paper provides the first evidence of wind-related absorption features in ULX spectra, supporting the super-critical accretion model.

Findings

Detection of broad, blue-shifted absorption features in ULX spectra

Evidence supporting the presence of radiatively-driven winds in ULXs

Initial estimates of wind physical parameters

Abstract

The luminosities of ultraluminous X-ray sources (ULXs) require an exotic solution with either super-critical accretion modes onto stellar mass black holes or sub-critical accretion onto intermediate mass black holes (IMBHs) being invoked. Discriminating between the two is non-trivial due to the present lack of a direct mass measurement. A key expectation of the super-critical mode of accretion is the presence of powerful radiatively-driven winds. Here we analyse XMM-Newton data from NGC 5408 X-1 and NGC 6946 X-1 and find that strong soft residuals present in the X-ray spectra can be reconciled with broadened, blue-shifted absorption by a partially ionised, optically thin phase of this wind. We derive initial values for the physical parameters of the wind; we also discuss other possible origins for the observed features.