Bubble in the Whale: Identifying the Optical Counterparts and Extended Nebula for the Ultraluminous X-ray Sources in NGC 4631

TL;DR

This study identifies optical counterparts and a nebula around ULXs in NGC 4631, revealing shock-ionized bubbles and jet activity, and estimates the energy and age of the nebula using multi-wavelength data and modeling.

Contribution

It provides the first detailed analysis of the optical nebulae associated with ULXs in NGC 4631, linking shock ionization and jet activity to the ULX environment.

Findings

Discovery of a highly asymmetric bubble nebula around X4

The nebula shows shock ionization and photoionization features

Estimated bubble age of approximately 700,000 years

Abstract

We present a deep optical imaging campaign on the starburst galaxy NGC 4631 with CFHT/MegaCam. By supplementing the HST/ACS and Chandra/ACIS archival data, we search for the optical counterpart candidates of the five brightest X-ray sources in this galaxy, four of which are identified as ultraluminous X-ray sources (ULXs). The stellar environments of the X-ray sources are analyzed using the extinction-corrected color-magnitude diagrams and the isochrone models. We discover a highly asymmetric bubble nebula around X4 which exhibits different morphology in the H$\alpha$ and [O III] images. The [O III]/H$\alpha$ ratio map shows that the H$\alpha$-bright bubble may be formed mainly via the shock ionization by the one-sided jet/outflow, while the more compact [O III] structure is photoionized by the ULX. We constrain the bubble expansion velocity and interstellar medium density with the MAPPINGS V code, and hence estimate the mechanical power injected to the bubble as $P_w \sim 5\times10^{40}$ erg s$^{-1}$ and the corresponding bubble age of $\sim7\times 10^{5}$ yr. Relativistic jets are needed to provide such level of mechanical power with a mass-loss rate of $\sim10^{-7}\ M_{\odot}\ \rm yr^{-1}$. Besides the accretion, the black hole spin is likely an additional energy source for the super-Eddington jet power.