Discovery of An X-ray Photoionized Optical Nebula and A Radio Nebula Associated with the ULX NGC 4861 X-1

TL;DR

This study reports the discovery of both an X-ray photoionized nebula and a radio synchrotron nebula associated with ULX NGC 4861 X-1, revealing complex multi-band phenomena and challenging previous assumptions about ULX environments.

Contribution

First detection of a radio nebula emitting synchrotron radiation associated with an X-ray photoionized nebula around a ULX, combining multi-band observations in a single study.

Findings

NGC 4861 X-1 is the fourth ULX with an X-ray photoionized nebula.

Discovery of a radio nebula emitting optically thin synchrotron radiation.

Tentative detection of a faint optical continuum spectrum.

Abstract

We have conducted long-slit spectroscopic observations and analyzed archival radio data for the ultraluminous X-ray source (ULX) NGC 4861 X-1. Our spectral line analysis unveils that NGC 4861 X-1 is the fourth ULX situated within an X-ray photoionized nebula, following three previous findings made approximately two decades ago. Remarkably, we discover NGC 4861 X-1 also possesses a radio nebula emitting optically thin synchrotron radiation, which contradicts its X-ray photoionization and raises doubts about the four ULXs being a mere coincidence. Instead of gradually accumulating from different bands bit by bit, our multi-band discovery is made all at once. Moreover, we tentatively perceive a faint continuum spectrum of the optical nebula. Further observations are needed to ascertain its radio structures and verify the optical continuum.