arXiv subjects

Hiroaki Sameshima

Publications and source records attributed to Hiroaki Sameshima.

42 records · Page 3Linked to original sources

arXiv2016

Origin of the Broad Iron Line Feature and the Soft X-ray Variation in Seyfert Galaxies

Many Seyfert galaxies are known to exhibit significant X-ray spectral variations and seemingly broad iron K-emission line features. In this paper, we show that the "variable partial covering model", which has been successfully proposed for MCG-6.30-15 (Miyakawa, Ebisawa & Inoue 2012) and 1H0707.495 (Mizumoto, Ebisawa & Sameshima 2014), can also explain the spectral variations in 2-10 keV as well as the broad iron line features in 20 other Seyfert galaxies observed with Suzaku. In this model, the absorbed spectral component through the optically-thick absorbing clouds has a significant iron K-edge, which primarily accounts for the observed seemingly broad iron line feature. Fluctuation of the absorbing clouds in the line of sight of the extended X-ray source results in variation of the partial covering fraction, which causes an anti-correlation between the direct (not covered) spectral component and the absorbed (covered) spectral component below ~10 keV. Observed spectral variation in 2-10 keV in a timescale of less than ~day is primarily explained by such variations of the partial covering fraction, while the intrinsic soft X-ray luminosity is hardly variable.

astro-ph.HE↗

arXiv2015

Near-Infrared Spectroscopy of Quasars at z~3 and Estimates of Their Supermassive Black Hole Masses

We present the results of new infrared spectroscopic observations of 37 quasars at z~3, selected based on the optical r'-band magnitude and the availability of nearby bright stars for future imaging follow-up with Adaptive Optics system. The supermassive black hole (SMBH) masses (M_BH) were successfully estimated in 28 out of 37 observed objects from the combination of the H_beta emission linewidth and continuum luminosity at rest-frame 5100A. Comparing these results with those from previous studies of quasars with similar redshift, our sample exhibited slightly lower (~ -0.11 dex in median) Eddington ratios; and, the SMBH masses are slightly (~ 0.38 dex in median) higher. The SMBH growth time, t_grow, was calculated by dividing the estimated SMBH mass by the mass accretion rate measured using optical luminosity. We found, given reasonable assumptions, that t_grow was smaller than the age of the universe at the redshift of individual quasars for a large fraction of observed sources, suggesting that the SMBHs in many of our observed quasars are in growing phase with high accretion rates. A comparison of the SMBH masses derived from our H_beta data and archived CIV data indicated considerable scattering, as indicated in previous studies. All quasars with measured SMBH masses have at least one nearby bright star, such that they are suitable targets for adaptive optics observations to study the mass relationship between SMBHs and host galaxies' stellar component at high redshift.

astro-ph.GA↗

arXiv2014

Interpretation of the X-ray Spectral Variation of 1H0707-495 with a Variable Double Partial Covering Model

The Narrow-line Seyfert 1 galaxy 1H0707-495 is known to exhibit significant X-ray spectral variations. Its X-ray energy spectrum is characterized by a strong soft excess emission, an extremely deep iron K-edge structure at ~7 keV, and a putative iron L-line/edge feature at ~1 keV. We have found that the energy spectrum of 1H0707-495 in 0.5-10 keV is successfully explained by a "variable double partial covering model" where the original continuum spectrum, which is composed of the soft multi-color disk blackbody component and the hard power-law component, is partially covered by two ionized absorption layers with different ionization states and the same partial covering fraction. The lower-ionized and thicker absorption layer primarily explains the iron K-edge feature, and the higher-ionized and thinner absorption layer explains the L-edge feature. We have discovered that the observed significant intensity/spectral variation within a ~ day is mostly explained by only variation of the partial covering fraction. In our model, the intrinsic luminosity and spectral shape are hardly variable within a ~ day, while some intrinsic variability above 3 keV is recognized. This is consistent with the picture that the multi-color disk blackbody spectrum is almost invariable in this timescale, and the hard power-law component is more variable. We propose that the observed spectral variation of 1H0707-495 is caused by three physically independent variations with different timescales; (1) intrinsic luminosity variation over days, (2) variation of partial covering fraction at a timescale of hours, and (3) small intrinsic hard component variation above 3 keV in a timescale of hours or less.

astro-ph.HE↗

arXiv2014

Statistical Properties of Multi-epoch Spectral Variability of SDSS Stripe82 Quasars

We investigate the UV-optical (longward of Ly$α$ 1216Å) spectral variability of nearly 9000 quasars ($0<z<4$) using multi-epoch photometric data within the SDSS Stripe 82 region. The regression slope in the flux-flux space of a quasar light curve directly measures the color of the flux difference spectrum, then the spectral shape of the flux difference spectra can be derived by taking a careful look at the redshift dependence of the regression slopes. First, we confirm that the observed quasar spectrum becomes bluer when the quasar becomes brighter. We infer the spectral index of the composite difference spectrum as $α_ν^{\text{dif}}\sim +1/3$ (in the form of $f_ν\propto ν^{α_ν}$), which is significantly bluer than that of the composite spectrum $α_ν^{\text{com}}\sim -0.5$. We also show that the continuum variability cannot be explained by the accretion disk models with varying mass accretion rate. Second, we examine the effects of broad emission line variability on the color-redshift space. The variability of the "Small Blue Bump" is extensively discussed. We show that the low-ionization lines of MgII and FeII are less variable compared to Balmer emission lines and high-ionization lines, and the Balmer continuum is the dominant variable source around $\sim 3000$Å. These results are compared with previous studies, and the physical mechanisms of the variability of the continuum and emission lines are discussed.

astro-ph.CO↗

arXiv2011

Ultraviolet Continuum Color Variability of Luminous Sloan Digital Sky Survey QSOs

We examine whether the spectral energy distribution of UV continuum emission of active galactic nuclei changes during flux variation. We used multi-epoch photometric data of QSOs in the Stripe 82 observed by the SDSS Legacy Survey and selected 10 bright QSOs observed with high photometric accuracies, in the redshift range of z = 1.0-2.4 where strong broad emission lines such as Lyαand CIV do not contaminate SDSS filters, to examine spectral variation of the UV continuum emission with broadband photometries. All target QSOs showed clear flux variations during the monitoring period 1998-2007, and the multi-epoch flux data in two different bands obtained on the same night showed a linear flux-to-flux relationship for all target QSOs. Assigning the flux in the longer wavelength to the x-axis in the flux-to-flux diagram, the x-intercept of the best-fit linear regression line was positive for most targets, which means that their colors in the observing bands become bluer as they become brighter. Then, the host-galaxy flux was estimated on the basis of the correlation between the stellar mass of the bulge of the host galaxy and the central black hole mass. We found that the longer-wavelength flux of the host galaxy was systematically smaller than that of the fainter extension of the best-fit regression line at the same shorter-wavelength flux for most targets. This result strongly indicates that the spectral shape of the continuum emission of QSOs in the UV region usually becomes bluer as it becomes brighter. We found that the multi-epoch flux-to-flux plots could be fitted well with the standard accretion disk model changing the mass accretion rate with a constant black hole mass for most targets. This finding strongly supports the standard accretion disk model for UV continuum emission of QSOs.

astro-ph.GA↗

arXiv2009

Ultraviolet Fe II emission in z ~ 2 quasars

We present spectra of six luminous quasars at z ~ 2, covering rest wavelengths 1600-3200 A. The fluxes of the UV Fe II emission lines and Mg II 2798 doublet, the line widths of Mg II, and the 3000 A luminosity were obtained from the spectra. These quantities were compared with those of low-redshift quasars at z = 0.06 - 0.55 studied by Tsuzuki et al. In a plot of the Fe II(UV)/Mg II flux ratio as a function of the cental black hole mass, Fe II(UV)/Mg II in our z ~ 2 quasars is systematically greater than in the low-redshift quasars. We confermed that luminosity is not responsible for this excess. It is unclear whether this excess is caused by rich Fe abundance at z ~ 2 over low-redshift or by non-abundance effects such as high gas density, strong radiation field, and high microturbulent velocity.

astro-ph.CO↗