Spectral transitions of an ultraluminous X-ray source, NGC 2403 Source 3

Abstract

Suzaku observation of an ultraluminous X-ray source, NGC 2403 Source 3, performed on 2006 March 16--17, is reported. The Suzaku XIS spectrum of Source 3 was described with a multi-color black-body-like emission from an optically thick accretion disk. The innermost temperature and radius of the accretion disk was measured to be $T_{\mathrm{in}}=1.08_{-0.03}^{+0.02}$ keV and $R_{\mathrm{in}}=122.1_{-6.8}^{+7.7}{\alpha }^{1/2}$ km, respectively, where $\alpha =(\cos 60^{\circ }/\cos i)$ with $i$ being the disk inclination. The bolometric luminosity of the source was estimated to be $L_{\mathrm{bol}}=1.82\times 10^{39}\alpha$ ergs s${}^{-1}$. Archival Chandra and XMM-Newton data of the source were analyzed for long-term spectral variations. In almost all observations, the source showed multi-color black-body-like X-ray spectra with parameters similar to those in the Suzaku observation. In only one Chandra observation, however, Source 3 was found to exhibit a power-law-like spectrum, with a photon index of $\Gamma =2.37\pm 0.08$, when it was fainter by about $\sim 15$ than in the Suzaku observation. The spectral behavior is naturally explained in terms of a transition between the slim disk state and the "very high" states, both found in Galactic black hole binaries when their luminosity approach the Eddington limit. These results are utilized to argue that ultraluminous X-ray sources generally have significantly higher black-hole masses than ordinary stellar-mass black holes.

Accepted for PASJ 3nd Suzaku special issue

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• The ultraluminous X-ray source population of NGC 4485/4490
• The Ages of High Mass X-ray Binaries in NGC 2403 and NGC 300
• The Chandra Local Volume Survey I: The X-ray Point Source Populations of NGC 55, NGC 2403, and NGC 4214
• Suzaku investigation into the nature of the nearest ultraluminous X-ray source, M33 X-8
• X-ray spectral and optical properties of a ULX in NGC 4258 (M106)
• In-depth studies of the NGC 253 ULXs with XMM-Newton: remarkable variability in ULX1, and evidence for extended coronae
• Two Power-Law States of the Ultraluminous X-ray Source IC342 X-1
• NGC 2403 XMM4: evidence for a super-Eddington neutron star with a possible transient pulsation