A Radio Nebula Surrounding the Ultraluminous X-Ray Source in NGC 5408

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"-2.0". 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 α=-0.8+/-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 3.8×10³⁴ erg s^-1 and the minimum energy required to power the nebula is ~1×10⁴⁹ erg. These values are 2 orders of magnitude larger than in any Galactic nebula powered by an accreting compact object.