Swift observations of the ultraluminous X-ray source Holmberg IX X-1

TL;DR

This study analyzes Swift X-ray observations of Holmberg IX X-1, revealing its spectral behavior during a prolonged low luminosity state and suggesting it hosts a black hole accreting at super-Eddington or near-Eddington rates.

Contribution

It provides the first detailed spectral analysis of Holmberg IX X-1 during an extended low state using co-added Swift spectra, proposing a dual thermal model for its emission.

Findings

Spectral fits favor a dual thermal model over simple power-law or disk models.

Holmberg IX X-1 may contain a 10 solar-mass black hole accreting above Eddington or a 100 solar-mass black hole at Eddington.

The observations suggest emission from both the inner accretion disk and outflows.

Abstract

Holmberg IX X-1 is a well-known ultraluminous X-ray source with an X-ray luminosity of ~1e40 erg/s. The source has been monitored by the X-ray Telescope of Swift regularly. Since 2009 April, the source has been in an extended low luminosity state. We utilize the co-added spectra taken at different luminosity states to study the spectral behavior of the source. Simple power-law and multi-color disk blackbody models can be ruled out. The best overall fits, however, are provided by a dual thermal model with a cool blackbody and a warm disk blackbody. This suggests that Holmberg IX X-1 may be a 10 solar-mass black hole accreting at 7 times above the Eddington limit or a 100 solar-mass maximally rotating black hole accreting at the Eddington limit, and we are observing both the inner regions of the accretion disk and outflows from the compact object.