(Proposal ID) S16B-043

(PI) Ueda, Yoshihiro

(Proposal Title)
Deep Spectroscopy of ULXs Containing Intermediate Mass Black Hole

(Abstract)
Ultra-luminous X-ray sources (ULXs) are observed in external galaxies, whose 
apparent luminosities exceed those of the brightest black holes in our Galaxy 
by hundreds and thousands of times. There is a great interest in ULXs because 
they may be stellar-mass black holes with supercritical accretion disks, or 
they may represent a new class of black holes of intermediate masses (IMBHs). 
We found by using Subaru spectra (S11A-006) that the majority of bona-fide 
ULXs show spectra similar to those of WNLh stars (LBVs in their compact hot 
state), or SS 433 (the only known super-Eddington accretor in the Galaxy). 
Here we select a new type of ULXs, three most probable IMBH candidates, M82 
X-1, NGC 247 X-1, and IC 342 X-1, and an ultraluminous X-ray pulsar (M82 X-2). 
In our study of all previuosly studied ULXs, we have found a strong unambiguous
 criterion based on He~II and Halpha line widths to distinguish between very 
dense disk winds (LBVs/SS 433) and heated disks (IMBH). We aim to discover 
IMBHs at least in one of the targets, using these spectral criterion.