An ultraluminous X-ray microquasar in NGC 5408? |
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| Authors: | R Soria R P Fender D C Hannikainen A M Read I R Stevens |
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| Institution: | Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA;School of Physics &Astronomy, University of Southampton, Southampton SO17 1BJ;Observatory PO Box 14, 00014 University of Helsinki, Finland;Department of Physics &Astronomy, University of Leicester, Leicester LE1 7RH;School of Physics and Astronomy, University of Birmingham, Birmingham B15 2TT |
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| Abstract: | We studied the radio source associated with the ultraluminous X-ray source in NGC 5408 ( L X≈ 1040 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 × 1038 erg s?1～ 0.1 L X～ 0.1 L Edd , an age ≈105 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. |
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| Keywords: | black hole physics supernova remnants radio continuum: ISM X-rays: binaries X-rays: individual: NGC 5408 X-1 |
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