A Radio Nebula Surrounding the Ultra-luminous X-ray Source in NGC 5408

TL;DR

This study presents high-resolution radio observations of an ultraluminous X-ray source in NGC 5408, revealing a resolved nebula with synchrotron emission, indicating a powerful, extended radio nebula unlike typical Galactic counterparts.

Contribution

First resolved radio imaging of an ULX nebula, demonstrating its size, spectral properties, and energetic scale, challenging previous beamed jet interpretations.

Findings

Radio emission is resolved with a size of 35-46 pc.

Spectral index of the radio emission is -0.8 pm 0.2.

The nebula's energy is about 10^49 erg, much larger than Galactic nebulae.

Abstract

New radio observations of the counterpart of the ultraluminous X-ray source in NGC 5408 show for the first time that the radio emission is resolved with an angular size of 1.5 to 2.0 arcseconds. This corresponds to a physical size of 35-46 pc, and rules out interpretation of the radio emission as beamed emission from a relativistic jet. In addition, the radio spectral index of the counterpart is well determined from three frequencies and found to be alpha=-0.8 pm 0.2. The radio emission is likely to be optically-thin synchrotron emission from a nebula surrounding the X-ray source. The radio luminosity of the counterpart is 3.8 x 10^34 erg/s and the minimum energy required to power the nebula is ~1 x 10^49 erg. These values are two orders of magnitude larger than in any Galactic nebula powered by an accreting compact object.