Decomposing the Suzaku spectrum of Ultra-Luminous X-ray source NGC 1313 X-1
Abstract
Ultra-Luminous X-ray sources (ULXs) are unusually luminous point sources located at off-nucleus regions of other galaxies. Their central objects are expected to be either intermediate mass black holes (BHs) accreting matters at sub-Eddington rates (e.g., Makishima et al. 2000) or stellar mass BHs/neutron stars with accretion rates above the Eddington (e.g., Mineshige 2007). Their X-ray spectra vary significantly as a function of luminosity, and some are known to exhibit a state called Multi-Color-Disk-like (MCD) state at their highest luminosities. Due to its convex continuum, the MCD state spectrum is often explained with a single accretion disk model, which is an accretion flow solution at near or above the Eddington (Watarai et al. 2000). However, some studies showed that an alternative multi-component modeling is favorable to explain variabilities of several MCD state spectra (e.g., Middleton et al. 2011). Since the state lacks characteristic spectral features, these modelings degenerate, and additional pieces of model-restricting information are required.In the present work, an X-ray data set of a representative ULX, NGC 1313 X-1, is analyzed. The observation was done with Suzaku on 2014 May 27th with an exposure of 100 ks. The source resided in the MCD state throughout the observation, yielding average X-ray luminosity of 2×1040 erg sec-1. Furthermore, the source showed strong variability in luminosity, and the change was prominent mainly in ≧ 1 keV. This suggests that a stable component is present in 0.5—1 keV band. To extract the spectrum of this possible extra component, a spectral-decomposing method introduced in an active galactic nucleus study (Noda et al. 2011) was employed in the ULX study for the first time. A spectrum of the “buried" extra component was successfully extracted from the data, and it was well fitted with a standard accretion disk model with inner-disk temperature and inner-disk radius of ∼ 0.3 keV and ∼ 1600 km, respectively. The results shows that the MCD state spectrum of NGC 1313 X-1 is actually composed of several components, and the central object is estimated to be as massive as ~80 M⊙. The mass estimation is consistent with a previous work (Bachetti et al. 2012).
- Publication:
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AAS/High Energy Astrophysics Division
- Pub Date:
- March 2019
- Bibcode:
- 2019HEAD...1711270K