Multiwavelength Observations of the ULX in NGC 6946
Abstract
We present optical observations of the brightest ultraluminous X-ray source (ULX), ULX4, in NGC 6946 (associated with optical source MF16). Using Keck long-slit optical spectroscopy, we obtain a 3100-9360 Angstrom spectrum. The spectrum is dominated by emission lines, suggesting that we are observing a circum-source nebula or the accretion disk rather than the companion star. The emission lines include not only low-excitation lines, but the extended wavelength coverage of this object allows for the identification of previously unobserved high-ionization lines such as the emission-line pair [Ne V] 3346, 3426 and the emission line [Ne III] 3869. Other high-ionization lines detected include N V 4604 and [Ar IV] 7171. In all, some 50 lines are identified, almost double the number of any previous study. Of particular interest, He II 4686 emission is detected; the photoionizing luminosity necessary to create this line is > 5 × 1039 erg/s. Thus, NGC 6946 ULX4 is truly ultraluminous (ruling out the beaming scenario). It can either be an intermediate-mass BH or a stellar-mass BH with super-Eddington flows. Furthermore, we report on our HST ACS UV observations (PI: Kaaret) of NGC 6946 ULX4. The predicted point-source is detected, making this the first directly detected ultraluminous UV source (ULUV). We discuss the origin of the UV continuum.
- Publication:
-
38th COSPAR Scientific Assembly
- Pub Date:
- 2010
- Bibcode:
- 2010cosp...38.2461W