NGC 5408 X-2: ULX of Superlatives
Abstract
Close to the well-studied ULX NGC 5408 X-1 and its associated X-ray ionised nebula we discovered an even more powerful cousin: X-2. Apparently much less X-ray active (Lx = 1E37 erg/s) than X-1, it nevertheless also excites an extended HeIII region/ synchroton nebula displaying high-ionisation HeII4686 and [NeV]3426 emission lines. This suggests an intrinsic (or previous) X-ray power of several 1E40 erg/s. Even more outstanding is the very luminous Mv = -10 optical counterpart that underwent drastic spectral evolution with strong Halpha emission that increased its equivalent width of some 200A twenty-five years ago to an unprecedented 2000A. The system appears to be in a very rapid stage of binary evolution with currently ongoing huge mass loss and mass transfer to the compact binary component. We will discuss our findings in the framework of a common envelope phase and point out its relevance to evolutionary scenarios of stellar-mass BH-BH mergers.
- Publication:
-
The X-ray Universe 2017
- Pub Date:
- October 2017
- Bibcode:
- 2017xru..conf..167P