A ULX microquasar in NGC 5408?

TL;DR

This study investigates the radio emission from the ULX in NGC 5408, suggesting it is likely a jet-powered radio lobe rather than a transient jet, providing insights into black hole accretion and jet power balance.

Contribution

It presents evidence that the radio source is a jet-powered lobe, not a transient, and models its properties to estimate jet power and age, advancing understanding of ULX jet phenomena.

Findings

Radio source is steady and steep-spectrum, inconsistent with core emission.

Radio flux suggests a jet-powered lobe with an estimated power of 7 x 10^{38} erg/s.

Source likely has an age of about 10^5 years and expansion velocity of 80 km/s.

Abstract

We studied the radio source associated with the ultraluminous X-ray source in NGC 5408 (L_X ~ 10^{40} erg/s). The radio spectrum is steep (index ~ -1), consistent with optically-thin synchrotron emission, not with flat-spectrum core emission. Its flux density (~ 0.28 mJy at 4.8 GHz, at a distance of 4.8 Mpc) was the same in the March 2000 and December 2004 observations, suggesting steady emission rather than a transient outburst. However, it is orders of magnitude higher than expected from steady jets in stellar-mass microquasar. Based on its radio flux and spectral index, we suggest that the radio source is either an unusually bright supernova remnant, or, more likely, a radio lobe powered by a jet from the black hole. Moreover, there is speculative evidence that the source is marginally resolved with a radius ~ 30 pc. A faint HII region of similar size appears to coincide with the radio and X-ray sources, but its ionization mechanism remains unclear. Using a self-similar solution for the expansion of a jet-powered electron-positron plasma bubble, in the minimum-energy approximation, we show that the observed flux and (speculative) size are consistent with an average jet power ~ 7 x 10^{38} erg/s ~ 0.1 L_X ~ 0.1 L_{Edd}, an age ~ 10^5 yr, a current velocity of expansion ~ 80 km/s. We briefly discuss the importance of this source as a key to understand the balance between luminosity and jet power in accreting black holes.