arXiv subjects

Yasuo Tanaka

Publications and source records attributed to Yasuo Tanaka.

7 recordsLinked to original sources

arXiv2008

Iron Emission Lines on the Galactic Ridge Observed with Suzaku

In order to elucidate origin of the Galactic Ridge X-ray Emission, we analyzed Suzaku data taken at various regions along the Galactic plane and studied their Fe-K emission line features. Suzaku resolved the Fe line complex into three narrow lines at ~6.4 keV,~6.7 keV and ~6.97 keV, which are K-lines from neutral (or low-ionized), He-like, and H-like iron ions, respectively. The 6.7 keV line is clearly seen in all the observed regions and its longitudinal distribution is consistent with that determined from previous observations. The 6.4 keV emission line was also found in various Galactic plane regions (b~0). Differences in flux ratios of the 6.4 keV/6.7 keV and 6.97 keV/6.7 keV lines between the Galactic plane and the Galactic center regions are studied and its implication is discussed.

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arXiv2007

Spectral Study of the Galactic Ridge X-ray Emission with Suzaku

We have observed a typical Galactic plane field at (l,b) = (28.46d, -0.20d) with Suzaku for 100 ksec to carry out a precise spectral study of the Galactic Ridge X-ray Emission (GRXE). The field is known to be devoid of X-ray point sources brighter than ~2 x 10^{-13} ergs s^{-1} cm^{-2} (2--10 keV), and already deeply observed with Chandra. Thanks to the low and stable background and high spectral resolution of Suzaku, we were able to resolve, for the first time, three narrow iron K-emission lines from low-ionized (6.41 keV), helium-like (6.67 keV), and hydrogenic ions (7.00 keV) in the GRXE spectrum. These line features constrain the GRXE emission mechanisms: The cosmic-ray ion charge exchange model or the non-equilibrium ionization plasma model are unlikely, since they require either broad emission lines or lines at intermediate ionization states. Collisional ionization equilibrium plasma is the likely origin for the 6.67 keV and 7.00 keV lines, while origin of the 6.41 keV line, which is due to fluorescence from cold material, is not elucidated. Low non-X-ray background and little stray-light contamination of Suzaku allowed us to precisely measure the absolute X-ray surface brightness in the direction of the Galactic plane. Excluding the point sources brighter than ~2 x 10^{-13} ergs s^{-1} cm^{-2} (2--10 keV), the total surface brightness on the Galactic plane is ~6.1 x 10^{-11} ergs s^{-1} cm^{-2} deg^{-2} (2--10 keV), including the contribution of the cosmic X-ray background that is estimated to be ~1.3x 10^{-11} ergs s^{-1} cm^{-2} deg^{-2}.

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arXiv2006

Suzaku observations of the hard X-ray variability of MCG-6-30-15: the effects of strong gravity around a Kerr black hole

Suzaku has, for the first time, enabled the hard X-ray variability of the Seyfert 1 galaxy MCG-6-30-15 to be measured. The variability in the 14-45 keV band, which is dominated by a strong reflection hump, is quenched relative to that at a few keV. This directly demonstrates that the whole reflection spectrum is much less variable than the power-law continuum. The broadband spectral variability can be decomposed into two components - a highly variable power-law and constant reflection - as previously inferred from other observations in the 2-10 keV band. The strong reflection and high iron abundance give rise to a strong broad iron line, which requires the inner disc radius to be at about 2 gravitational radii. Our results are consistent with the predictions of the light bending model which invokes the very strong gravitational effects expected very close to a rapidly spinning black hole.

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arXiv2006

Erratum: ASCA Observations of the Absorption-Line Features from the Super-Luminal Jet Source GRS 1915+105

We have discovered errors in the calculation of some of the Einstein coefficients in Table 5 and some plots in Figure 3. Due to the errors, the square-root section of the curves of growth of Fe xxvi Ka and Kb, and Ni xxviii Ka in Figure 3 were a few times underestimated. We correct Table 5 and re-plot Figure 3 together with the unaffected curves. The program to calculate the corrected curves of growth is available at http://www.hp.phys.titech.ac.jp/kotani/cog/index.html

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arXiv2004

Partial Covering Interpretation of the X-Ray Spectrum of the NLS1 1H 0707-495

The X-ray spectrum of 1H 0707-495 obtained with XMM-Newton showing a deep flux drop at ~7 keV (Boller et al. 2002) is studied based on the partial covering concept. The previously inferred extreme iron overabundance can be reduced down to ~3x solar if the hard component gradually steepens at high energies. The spectral shape supports that 1H 0707-495 is an AGN analogue of the galactic black-hole binaries in the soft state. Interpreting the soft excess as the emission from an optically-thick disk, the minimum black hole mass M is estimated to be 2 x 10^6 solar masses from the intrinsic luminosity corrected for partial covering. Based on the slim disk model, the observed disk temperature implies that the luminosity is close to the Eddington limit. The rapid and large flux variations with little change in the spectral shape can also be explained, if not all, as due to changes in the partial covering fraction. Partial covering may account for the large variability characteristics of NLS1.

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arXiv2001

A New Measurement of the X-ray Temperature Function of Clusters of Galaxies

We present a newly measured X-ray temperature function of galaxy clusters using a complete flux-limited sample of 61 clusters. The sample is constructed with the total survey area of 8.14 steradians and the flux limit of 1.99e-11 ergs/s/cm^2 in the 0.1-2.4keV band. X-ray temperatures and fluxes of the sample clusters were accurately measured with ASCA and ROSAT data. The derived temperature function covers an unprecedentedly wide temperature range of 1.4-11keV. By fitting these data with theoretically predicted temperature functions given by the Press-Schechter formalism together with a recent formation approximation and the CDM power spectrum, we obtained tight and individual constraints on Omega_m,0 and sigma_8. We also employed the Formation-Epoch model in which the distribution in the formation epoch of clusters as well as the temperature evolution are taken into account, showing significantly different results. Systematics caused by the uncertainty in the mass-temperature relation are studied and found to be as large as the statistical errors.

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arXiv2000

RXTE observation of NGC 6240: a search for the obscured active nucleus

The wide-band energy spectrum of NGC6240 over the range 0.5-200keV is investigated using the RXTE and ASCA data. The RXTE data provide the spectrum beyond the ASCA range (0.5-10 keV) with significant detection of signals up to 20keV and the upper limits above 20keV. The spectrum above 10keV is found to be very flat. A strong iron-K emission line discovered in the previous ASCA observation is also confirmed with the RXTE PCA. These results provide further evidence for the dominance of a reflection component, i.e. emission from cool material illuminated by an AGN. By fitting the spectra obtained with RXTE and ASCA simultaneously, we satisfactorily modeled the AGN spectrum with a Compton reflection component and probably a transmitted AGN component penetrating through a thick absorber. The X-ray luminosity of the AGN is estimated to be in the range 4x10^43 - 6x10^44 ergs/s in the range 2-10keV, which categorizes NGC6240 among the most luminous Seyfert nuclei. The ratio of the 2-10keV X-ray luminosity to the infrared luminosity, L_X(2-10keV)/L_IR, is 0.01 - 0.1, which implies a quite substantial, if not dominant, contribution of AGN to the infrared luminosity.

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