The archetypical very-high-energy gamma-ray blazar Mrk 421 was monitored for more than 3 years with the Gas Slit Camera onboard Monitor of All Sky X-ray Image (MAXI), and its longterm X-ray variability was investigated. The MAXI lightcurve in the 3 -
- 10 keV range was transformed to the periodogram in the frequency range $f = 1 times 10^{-8}$ -- $2 times 10^{-6}$ Hz. The artifacts on the periodogram, resulting from data gaps in the observed lightcurve, were extensively simulated for variations with a power-law like Power Spectrum Density (PSD). By comparing the observed and simulated periodograms, the PSD index was evaluated as $alpha = 1.60 pm 0.25$. This index is smaller than that obtained in the higher frequency range ($f > 1 times 10^{-5}$ Hz), namely, $alpha = 2.14 pm 0.06$ in the 1998 ASCA observation of the object. The MAXI data impose a lower limit on the PSD break at $f_{rm b} = 5 times 10^{-6}$ Hz, consistent with the break of $f_{rm b} = 9.5 times 10^{-6}$ Hz, suggested from the ASCA data. The low frequency PSD index of Mrk 421 derived with MAXI falls well within the range of the typical value among nearby Seyfert galaxies ($alpha = 1$ -- $2$). The physical implications from these results are briefly discussed.
The X-ray spectrum of the nearest ultraluminous X-ray source, M33 X-8, obtained by Suzaku during 2010 January 11 -- 13, was closely analyzed to examine its nature. It is, by far, the only data with the highest signal statistic in 0.4 -- 10 keV range.
Despite being able to reproduce the X-ray spectrum, Comptonization of the disk photons failed to give a physically meaningful solution. A modified version of the multi-color disk model, in which the dependence of the disk temperature on the radius is described as r^(-p) with p being a free parameter, can also approximate the spectrum. From this model, the innermost disk temperature and bolometric luminosity were obtained as T_in = 2.00-0.05+0.06 keV and L_disk = 1.36 x 10^39 (cos i)^(-1) ergs/s, respectively, where i is the disk inclination. A small temperature gradient of p = 0.535-0.005+0.004, together with the high disk temperature, is regarded as the signatures of the slim accretion disk model, suggesting that M33 X-8 was accreting at high mass accretion rate. With a correction factor for the slim disk taken into account, the innermost disk radius, R_in =81.9-6.5+5.9 (cos i)^(-0.5) km, corresponds to the black hole mass of M sim 10 M_sun (cos i)^(-0.5). Accordingly, the bolometric disk luminosity is estimated to be about 80 (cos i)^(-0.5)% of the Eddington limit. A numerically calculated slim disk spectrum was found to reach a similar result. Thus, the extremely super-Eddington luminosity is not required to explain the nature of M33 X-8. This conclusion is utilized to argue for the existence of intermediate mass black holes with M > 100 M_sun radiating at the sub/trans-Eddington luminosity, among ultraluminous X-ray sources with L_disk > 10^(40) ergs/s.
This paper reports the discovery of a bright X-ray transient source, Suzaku J1305-4913, in the south-west arm of the nearby Seyfert II galaxy NGC 4945. It was detected at a 0.5 -- 10 keV flux of $2.2 times 10^{-12}$ erg cm$^{-2}$ s$^{-1}$ during the
Suzaku observation conducted on 2006 January 15 -- 17, but was undetectable in a shorter observation on 2005 August 22 --23, with an upper limit of $1.7 times 10^{-14}$ erg cm$^{-2}$ s$^{-1}$ (90% confidence level). At a distance of 3.7 Mpc, the bolometric luminosity of the source becomes $L_{rm bol} = 4.4 times 10^{39} alpha$ erg s$^{-1}$, where $alpha = (cos 60^circ / cos i)$ and $i$ is the disk inclination. Therefore, the source is classified into so-called ultraluminous X-ray sources (ULXs). The time-averaged X-ray spectrum of the source is described by a multi-color disk model, with the innermost accretion disk temperature of $T_{rm in} = 1.69_{-0.05}^{+0.06}$ keV. During the 2006 January observation, it varied by a factor of 2 in intensity, following a clear correlation of $L_{rm bol} propto T_{rm in}^4$. It is inferred that the innermost disk radius $R_{rm in}$ stayed constant at $R_{rm in} = 79_{-3.9}^{+4.0} alpha^{1/2}$ km, suggesting the presence of a standard accretion disk. Relating $R_{rm in}$ with the last stable orbit around a non-rotating black hole yields a rather low black hole mass, $sim 9 alpha^{1/2}$ solar masses, which would imply that the source is shining at a luminosity of $sim3 alpha^{1/2} $ times the Eddington limit. These results can be better interpreted by invoking sub-Eddington emission from a rapidly spinning black hole with a mass of 20 -- 130 solar masses.
