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

**Authors:** Jing Guo, Jianfeng Wu, Hua Feng, Zheng Cai, Ping Zhou, Changxing Zhou, Shiwu Zhang, Junfeng Wang, Mouyuan Sun, Wei-Min Gu, Shan-Shan Weng, Jifeng Liu  
**arXiv:** [2301.00022](https://arxiv.org/abs/2301.00022)  
**Date:** 2022-12-30  
**Summary page:** https://www.summarizepaper.com/en/paper/2301.00022v1/

## Key points

- Deep optical imaging campaign on starburst galaxy NGC 4631
- Goal: Identify optical counterparts of five brightest X-ray sources, four classified as ultraluminous X-ray sources (ULXs)
- Discovery of highly asymmetric bubble nebula surrounding X4, with different morphologies in Hα and [O III] images
- Bubble likely formed through shock ionization caused by one-sided jet/outflow
- Compact [O III] structure appears to be photoionized by ULX
- Expansion velocity and interstellar medium density of bubble estimated using MAPPINGS V code
- Mechanical power injected into bubble approximately $5\times10^{40}$ erg s$^{-1}$, suggesting a corresponding age of around $7\times 10^{5}$ years
- Relativistic jets with mass-loss rate of approximately $10^{-7}\ M_{\odot}\ \rm yr^{-1}$ provide high levels of mechanical power
- Black hole spin may serve as additional energy source for super-Eddington jet power
- Study sheds light on optical counterparts and extended nebulae associated with ULXs in NGC 4631
- Implications of relativistic jets provide insights into mechanisms driving energetic phenomena in starburst galaxies

## Summary

We present a deep optical imaging campaign on the starburst galaxy NGC 4631 with CFHT/MegaCam, supplemented by HST/ACS and Chandra/ACIS archival data. Our goal is to identify the optical counterparts of the five brightest X-ray sources in this galaxy, four of which are classified as ultraluminous X-ray sources (ULXs). To analyze the stellar environments of these X-ray sources, we use extinction-corrected color-magnitude diagrams and isochrone models. During our investigation, we make an intriguing discovery: a highly asymmetric bubble nebula surrounding X4. This bubble exhibits different morphologies in the Hα and [O III] images. By examining the [O III]/Hα ratio map, we find that the Hα-bright bubble is likely formed through shock ionization caused by a one-sided jet/outflow. In contrast, the more compact [O III] structure appears to be photoionized by the ULX. To further understand this phenomenon, we employ the MAPPINGS V code to constrain the expansion velocity of the bubble and estimate its interstellar medium density. Based on these calculations, we determine that the mechanical power injected into the bubble is approximately $5\times10^{40}$ erg s$^{-1}$, suggesting a corresponding bubble age of around $7\times 10^{5}$ years. Such high levels of mechanical power can only be provided by relativistic jets with a mass-loss rate of approximately $10^{-7}\ M_{\odot}\ \rm yr^{-1}$. We propose that besides accretion, black hole spin may serve as an additional energy source for this super-Eddington jet power. In summary, our study sheds light on the optical counterparts and extended nebulae associated with ultraluminous X-ray sources in NGC 4631. The presence of a highly asymmetric bubble nebula and implications of relativistic jets provide valuable insights into mechanisms driving these energetic phenomena. Our findings contribute to better understanding starburst galaxies and role of super-Eddington accretion in powering astrophysical jets.