Open AccessAn ultraluminous X‐ray microquasar in NGC 5408?
Author(s) -
Soria R.,
Fender R. P.,
Hannikainen D. C.,
Read A. M.,
Stevens I. R.
Publication year - 2006
Publication title -
monthly notices of the royal astronomical society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.058
H-Index - 383
eISSN - 1365-2966
pISSN - 0035-8711
DOI - 10.1111/j.1365-2966.2006.10250.x
Subject(s) - physics , astrophysics , radius , luminosity , flux (metallurgy) , jet (fluid) , spectral index , supernova , supernova remnant , black hole (networking) , astronomy , synchrotron , galaxy , spectral line , computer network , routing protocol , materials science , computer security , routing (electronic design automation) , computer science , nuclear physics , metallurgy , thermodynamics , link state routing protocol
We studied the radio source associated with the ultraluminous X‐ray source in NGC 5408 ( L X ≈ 10 40 erg s −1 ) . 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 (BH). Moreover, there is speculative evidence that the source is marginally resolved with a radius ∼30 pc. A faint H ii 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 × 10 38 erg s −1 ∼ 0.1 L X ∼ 0.1 L Edd , an age ≈10 5 yr, a current velocity of expansion ≈80 km s −1 . We briefly discuss the importance of this source as a key to understand the balance between luminosity and jet power in accreting BHs.