Searching for Massive Outflows in Holmberg IX X-1 and NGC 1313 X-1: The Iron K Band

TL;DR

This study analyzed XMM-Newton data of two ultraluminous X-ray sources to search for iron K band features indicative of massive outflows, but found no significant lines, challenging models predicting strong super-Eddington winds.

Contribution

The paper provides the first stringent observational limits on iron K band features in Holmberg IX X-1 and NGC 1313 X-1, constraining super-Eddington outflow models.

Findings

No statistically significant iron K emission or absorption lines detected.

Equivalent width limits are ~30 eV for Holmberg IX X-1 and ~50 eV for NGC 1313 X-1.

Results suggest either absence or unfavorable viewing angles of outflows.

Abstract

We have analysed all the good quality XMM-Newton data publicly available for the bright ULXs Holmberg IX X-1 and NGC 1313 X-1, with the aim of searching for discrete emission or absorption features in the Fe K band that could provide observational evidence for the massive outflows predicted if these sources are accreting at substantially super-Eddington rates. We do not find statistically compelling evidence for any atomic lines, and the limits that are obtained have interesting consequences. Any features in the immediate Fe K energy band (6-7 keV) must have equivalent widths weaker than ~30 eV for Holmberg IX X-1, and weaker than ~50 eV for NGC 1313 X-1 (at 99 per cent confidence). In comparison to the sub-Eddington outflows observed in GRS 1915+105, which imprint iron absorption features with equivalent widths of ~30 eV, the limits obtained here appear quite stringent, particularly when Holmberg IX X-1 and NGC 1313 X-1 must be expelling at least 5-10 times as much material if they host black holes of similar masses. The difficulty in reconciling these observational limits with the presence of strong line-of-sight outflows suggests that either these sources are not launching such outflows, or that they must be directed away from our viewing angle.