Deep spectroscopy of stellar counterparts of ultraluminous X-ray sources
Abstract
We propose to carry out a deep spectral study of faint stars in X-ray boxes of four ultraluminous X-ray sources (ULXs) in nearby galaxies. They are the most luminous bone-fide ULXs, whose stellar counterparts have unambiguous identification. They were well-studied in the HST and ground-based imaging. Our analysis of archival Subaru spectra (2 out of the 4 targets) has shown that the spectra were taken with inadmissible short exposures resulting in too low S/N, and the spectral range did not include main diagnostical spectral features. All previous studies of the ULXs did not answer the question on their nature. It is astonishing that there was no spectroscopy of their optical counterparts, though it is possible to take good spectra of them in a long-exposure spectroscopy with 8-m class of telescopes. The predictions for the ULX optical counterpart spectrum are clear enough to distinguish between two the most popular models (supercritical accretion disks like that in SS 433 and intermediate-mass black holes with a "normal disk"). The main goal is to distinguish an accretion regime in ULXs, because in both cases the ULXs must be close binaries with a massive donor. This spectroscopy is the necessary first step for subsequent phase-resolved spectroscopic studies to determine the black hole mass.
- Publication:
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Subaru Proposal
- Pub Date:
- January 2011
- Bibcode:
- 2011suba.prop....6U