Transition of an X-ray binary to the hard ultraluminous state in the blue compact dwarf galaxy VII Zw 403

TL;DR

This study observes a transition in an X-ray binary within a low-metallicity galaxy to a high luminosity ultraluminous state, revealing spectral characteristics consistent with super-Eddington stellar black holes, which impacts models of early universe heating.

Contribution

It provides the first detailed spectral analysis of an HMXB in a BCD galaxy during a luminosity transition, highlighting its ultraluminous state and implications for cosmic reionization models.

Findings

The galaxy's X-ray source reached ultraluminous levels of 1.7x10^40 erg s^-1.

Spectra during high flux are hard and fit a disk plus Comptonization model.

Contrasts with other BCD HMXBs suggest diverse spectral states in low-metallicity environments.

Abstract

We examine the X-ray spectra of VII Zw 403, a nearby low-metallicity blue compact dwarf (BCD) galaxy. The galaxy has been observed to contain an X-ray source, likely a high mass X-ray binary (HMXB), with a luminosity of 1.3-23x10^38 erg s^-1 in the 0.3-8 keV energy range. A new Suzaku observation shows a transition to a luminosity of 1.7x10^40 erg s^-1 [0.3-8 keV], higher by a factor of 7-130. The spectra from the high flux state are hard, best described by a disk plus Comptonization model, and exhibit curvature at energies above 5 keV. This is consistent with many high-quality ultraluminous X-ray source spectra which have been interpreted as stellar mass black holes (StMBH) accreting at super-Eddington rates. However, this lies in contrast to another HMXB in a low-metallicity BCD, I Zw 18, that exhibits a soft spectrum at high flux, similar to Galactic black hole binaries and has been interpreted as a possible intermediate mass black hole. Determining the spectral properties of HMXBs in BCDs has important implications for models of the Epoch of Reionization. It is thought that the main component of X-ray heating in the early universe was dominated by HMXBs within the first galaxies. Early galaxies were small, metal-deficient, star forming galaxies with large H I mass fractions --- properties shared by local BCDs we see today. Understanding the spectral evolution of HMXBs in early universe analogue galaxies, such as BCDs, is an important step in estimating their contribution to the heating of the intergalactic medium during the Epoch of Reionization. The strong contrast between the properties of the only two spectroscopically studied HMXBs within BCDs motivates further study on larger samples of HMXBs in low metallicity environments in order to properly estimate the X-ray heating in the early universe.