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arXiv · 2301.00022

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

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
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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$α$ and [O III] images. The [O III]/H$α$ ratio map shows that the H$α$-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.

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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. 2022-12-30. Bubble in the Whale: Identifying the Optical Counterparts and Extended Nebula for the Ultraluminous X-ray Sources in NGC 4631. https://doi.org/10.3847/1538-4357%2Facaddd

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