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

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 $T_{\rm in} = 1.08_{-0.03}^{+0.02}$ keV and $R_{\rm in} = 122.1_{-6.8}^{+7.7} \alpha^{1/2}$ 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 $L_{\rm bol} = 1.82 \times 10^{39} \alpha$ 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.Comment: Accepted for PASJ 3nd Suzaku special issu

In Orbit Timing Calibration of the Hard X-Ray Detector on Board Suzaku

The hard X-ray detector (HXD) on board the X-ray satellite Suzaku is designed to have a good timing capability with a 61 $\mu$s time resolution. In addition to detailed descriptions of the HXD timing system, results of in-orbit timing calibration and performance of the HXD are summarized. The relative accuracy of time measurements of the HXD event was confirmed to have an accuracy of $1.9\times 10^{-9}$ s s$^{-1}$ per day, and the absolute timing was confirmed to be accurate to 360 $\mu$s or better. The results were achieved mainly through observations of the Crab pulsar, including simultaneous ones with RXTE, INTEGRAL, and Swift.Comment: Accepted for publication on PASJ Vol.60, SP-1, 200