Search

This study investigates the properties of two gas structures of X-ray selected active galactic nuclei (AGNs), that is, dusty and dust-free gas components, by separating them with the line-of-sight dust extinction ($A_V$) and the neutral gas column density ($N_{\mathrm{H}}$). The typical column density of the dusty and dust-free gas differs depending on the Seyfert type, indicating that both structures have anisotropic column density distributions. The number of targets with the dusty gas column density ($N_{\mathrm{H,d}}$) of $\log N_{\mathrm{H,d}}\ [\mathrm{cm^{-2}}]>23$ is much smaller than that with the same column density of the dust-free gas. This result indicates that the optically-thick part of the dusty gas structure is very thin. There are very few targets with a larger Eddington ratio ($f_{\mathrm{Edd}}$) than the effective Eddington limit of the dusty gas and the covering factor of the dusty gas with $22\leq \log N_{\mathrm{H,d}}\ [\mathrm{cm^{-2}}]<24$ exhibits a clear drop at the effective Eddington limit. These results support the scenario wherein the covering factor of the dusty torus decreases in a high Eddington ratio owing to the radiation-driven dusty gas outflow. The covering factor of the dust-free gas with the column density ($N_{\mathrm{H,df}}$) of $22\leq \log N_{\mathrm{H,df}}\ [\mathrm{cm^{-2}}]<24$ similarly exhibits the decrease in high Eddington ratio, although it may be owing to the dust-free gas outflow driven by certain other mechanisms than the radiation pressure. Finally, we propose an updated picture of the AGN gas structure based on our results and the literature., Comment: 16 pages, 9 figures, accepted for publication in MNRAS

As an inaugural investigation under the X-ray Winds In Nearby-to-distant Galaxies (X-WING) program, we assembled a dataset comprising 132 active galactic nuclei (AGNs) spanning redshifts $z \sim 0$-$4$ characterized by blueshifted absorption lines indicative of X-ray winds. Through an exhaustive review of previous research, we compiled the outflow parameters for 573 X-ray winds, encompassing key attributes such as outflow velocities ($V_{\rm out}$), ionization parameters ($\xi$), and hydrogen column densities. By leveraging the parameters $V_{\rm out}$ and $\xi$, we systematically categorized the winds into three distinct groups: ultrafast outflows (UFOs), low-ionization parameter (low-IP) UFOs, and warm absorbers. Strikingly, a discernible absence of linear correlations in the outflow parameters, coupled with distributions approaching instrumental detection limits, was observed. Another notable finding was the identification of a velocity gap around $V_{\rm out} \sim 10,000~{\rm km~s^{-1}}$. This gap was particularly evident in the winds detected via absorption lines within the $\lesssim$2 keV band, indicating disparate origins for low-IP UFOs and warm absorbers. In cases involving Fe XXV/Fe XXVI lines, where the gap might be attributed to potential confusion between emission/absorption lines and the Fe K-edge, the possibility of UFOs and galactic-scale warm absorbers being disconnected is considered. An examination of the outflow and dust sublimation radii revealed a distinction: UFOs appear to consist of dust-free material, whereas warm absorbers likely comprise dusty gas. From 2024, the X-Ray Imaging and Spectroscopy Mission (XRISM) is poised to alleviate observational biases, providing insights into the authenticity of the identified gap, a pivotal question in comprehending AGN feedback from UFOs., Comment: 34 pages, 4 tables, 7 figures, accepted in ApJS. The CSV files of Tables B2-B4 are available from the URL: http://www.kusastro.kyoto-u.ac.jp/~styamada/yamada2024bApJS/XWING-PaperI_tables.zip

In this study, we performed high-resolution near-infrared (NIR) spectroscopy (R = 28,000; ${\lambda} = 0.90$-1.35 ${\mu}$m) with a high signal-to-noise ratio on HD 200775, a very young (${\sim}$0.1 Myr old) and massive intermediate-mass star (a binary star with a mass of about 10 $M_{\odot}$ each) with a protoplanetary disk. The obtained spectra show eight forbidden lines of three elements: two of [S II] (10289 and 10323 {\AA}), two of [N I] (10400 and 10410 {\AA}), and four of [Fe II] (12570, 12946, 12981, and 13209 {\AA}). This is the first time that the [N I] lines are detected in a young stellar object with a doublet deblended. Gaussian fitting of the spectra indicates that all line profiles have low-velocity components and exhibit blueshifted features, suggesting that all lines originate from the disk winds (magnetohydrodynamic disk wind and/or photoevaporative wind). Based on the fit, the [N I] and [Fe II] lines are categorized into narrow components, while the [S II] lines are at the boundary between broad and narrow components. These forbidden lines are suggested to be very promising disk wind tracers among the existing ones because they are in the NIR-wavelength range, which can be observed from early stages with high sensitivities. Among these lines, [N I] lines would be a rather powerful probe for deriving the basic physical parameters of disk wind gases. However, the study of these lines herein is limited to one object; thus, further studies are needed to examine their properties., Comment: Accepted for publication in ApJ

The identity of dark matter has been a mystery in astronomy, cosmology, and particle theory for about a century. Bessho, Ikeda, and Yin (2022), three of the current authors, proposed using the state-of-the-art infrared spectrographs, including WINERED at $6.5$m Magellan Clay telescope and NIRSpec at James Webb Space Telescope, as efficient detectors for the indirect detection of dark matter with the mass around eV by measuring the line photons from the dark matter two body decays. Applying this concept, we have performed spectrographic observations of dwarf spheroidal galaxies (dSphs) Leo V and Tucana II using WINERED by utilizing an object-sky-object nodding observation technique for background subtraction. We present the first result from this dark matter search. Employing zero consistent flux data after the sky subtraction, we have established one of the most stringent limits to date on dark matter decaying into line photons in the mass range of $1.8-2.7\,$eV. Our data can also be applied to constrain other spectra of photons from the dSphs., Comment: 15 pages, 4 figures, 1 table, 6 data files attached

We present a data reduction pipeline written in Python for data obtained with the near-infrared cross-dispersed echelle spectrograph, WINERED, which yields a 0.91$-$1.35 $\mu$m spectrum with the resolving power of $R_{\text{max}} \equiv \lambda / \Delta \lambda = 28,000$ or 70,000 depending on the observing mode. The pipeline was developed to efficiently extract the spectrum from the raw data with high quality. It comprises two modes: the calibration and the science mode. The calibration mode automatically produces the flat-fielding image, bad pixel map, echellogram distortion map and the dispersion solution from the set of the calibration data. Using calibration images and parameters, the science data of astronomical objects can be reduced automatically using the science mode. The science mode is also used for the real-time quick look at the data during observations. An example of the spectra reduced with WARP is presented. The effect of the highly inclined slit image on the spectral resolution is discussed., Comment: 20 pages, 13 figures, 1 table, accepted for publication in Publications of the Astronomical Society of the Pacific

Outflows in the Active Galactic Nucleus (AGN) are considered to play a key role in the host galaxy evolution through transfer of a large amount of energy. A Narrow Line Region (NLR) in the AGN is composed of ionized gas extending from pc-scales to kpc-scales. It has been suggested that shocks are required for ionization of the NLR gas. If AGN outflows generate such shocks, they will sweep through the NLR and the outflow energy will be transferred into a galaxy-scale region. In order to study contribution of the AGN outflow to the NLR-scale shock, we measure the [\ion{Fe}{2}]$\lambda12570$/[\ion{P}{2}]$\lambda11886$ line ratio, which is a good tracer of shocks, using near-infrared spectroscopic observations with WINERED (Warm INfrared Echelle spectrograph to Realize Extreme Dispersion and sensitivity) mounted on the New Technology Telescope. Among 13 Seyfert galaxies we observed, the [\ion{Fe}{2}] and [\ion{P}{2}] lines were detected in 12 and 6 targets, respectively. The [\ion{Fe}{2}]/[\ion{P}{2}] ratios in 4 targets were found to be higher than 10, which implies the existence of shocks. We also found that the shock is likely to exist where an ionized outflow, i.e., a blue wing in [\ion{S}{3}]$\lambda9533$, is present. Our result implies that the ionized outflow present over a NLR-scale region sweeps through the interstellar medium and generates a shock., Comment: Accepted for ApJ, 20 pages, 11 figures

We present the quasar luminosity function (LF) at $z = 7$, measured with 35 spectroscopically confirmed quasars at $6.55 < z < 7.15$. The sample of 22 quasars from the Subaru High-$z$ Exploration of Low-Luminosity Quasars (SHELLQs) project, combined with 13 brighter quasars in the literature, covers an unprecedentedly wide range of rest-frame ultraviolet magnitudes over $-28 < M_{1450} < -23$. We found that the binned LF flattens significantly toward the faint end populated by the SHELLQs quasars. A maximum likelihood fit to a double power-law model has a break magnitude $M^*_{1450} = -25.60^{+0.40}_{-0.30}$, a characteristic density $\Phi^* = 1.35^{+0.47}_{-0.30}$ Gpc$^{-3}$ mag$^{-1}$, and a bright-end slope $\beta = -3.34^{+0.49}_{-0.57}$, when the faint-end slope is fixed to $\alpha = -1.2$ as observed at $z \le 6$. The overall LF shape remains remarkably similar from $z = 4$ to $7$, while the amplitude decreases substantially toward higher redshifts, with a clear indication of an accelerating decline at $z \ge 6$. The estimated ionizing photon density, $10^{48.2 \pm 0.1}$ s$^{-1}$ Mpc$^{-3}$, is less than 1 % of the critical rate to keep the intergalactic medium ionized at $z = 7$, and thus indicates that quasars are not a major contributor to cosmic reionization., Comment: The Astrophysical Journal Letters, in press

"Changing-look active galactic nuclei" (CLAGNs) are known to change their apparent types between types 1 and 2, usually accompanied by a drastic change in their luminosity on timescales of years. However, it is still unclear whether materials in broad-line regions (BLRs) in CLAGNs appear and disappear during the type-transition or remain at the same location while the line production is simply activated or deactivated. Here we present our X-ray-optical monitoring results of a CLAGN, NGC 3516, by Suzaku, Swift, and ground telescopes, with our primary focus on the narrow Fe-K$\alpha$ emission line, which is an effective probe of the BLR materials. We detected significant variations of the narrow Fe-K$\alpha$ line on a timescale of tens of days during the type-2 (faint) phase in 2013-2014, and conducted "narrow Fe-K$\alpha$ reverberation mapping," comparing its flux variation with those of the X-ray continuum from a corona and $B$-band continuum from an accretion disk. We derived, as a result, a time lag of $10.1^{+5.8}_{-5.6}$ days ($1\sigma$ errors) for the Fe-K$\alpha$ line behind the continuum, which is consistent with the location of the BLR determined in optical spectroscopic reverberation mapping during the type-1 (bright) phase. This finding shows that the BLR materials remained at the same location without emitting optical broad-lines during the type-2 phase. Considering the drastic decrease of the radiation during the type-transition, our result is possibly inconsistent with the hotly-discussed formation models of the BLR which propose that the radiative pressure from an accretion disk should be the main driving force., Comment: 15 pages, 7 figures, 3 tables, accepted for the publication in ApJ

We present the measurement of the line-of-sight extinction of the dusty torus for a large number of obscured active galactic nuclei (AGNs) based on the reddening of the colour of the variable flux component in near-infrared (NIR) wavelengths. We collected long-term monitoring data by $\textit{Wide-field Infrared Survey Explorer (WISE)}$ for 513 local AGNs catalogued by the $\mathit{Swift/}$BAT AGN Spectroscopic Survey (BASS) and found that the multi-epoch NIR flux data in two different bands (WISE $W1$ and $W2$) are tightly correlated for more than 90% of the targets. The flux variation gradient (FVG) in the $W1$ and $W2$ bands was derived by applying linear regression analysis, and we reported that those for unobscured AGNs fall in a relatively narrow range, whereas those for obscured AGNs are distributed in a redder and broader range. The AGN's line-of-sight dust extinction ($A_V$) is calculated using the amount of the reddening in the FVG and is compared with the neutral hydrogen column density ($N_{\rm{}H}$) of the BASS catalogue. We found that the $N_{\rm{}H}/A_V$ ratios of obscured AGNs are greater than those of the Galactic diffuse interstellar medium (ISM) and are distributed with a large scatter by at most two orders of magnitude. Furthermore, we found that the lower envelope of the $N_{\rm{}H}/A_V$ of obscured AGNs is comparable to the Galactic diffuse ISM. These properties of the $N_{\rm{}H}/A_V$ can be explained by increase in the $N_{\rm{}H}$ attributed to the dust-free gas clouds covering the line of sight in the broad-line region., Comment: 11 pages, 7 figures, published in MNRAS

The search for Population III (Pop III) stars has fascinated and eluded astrophysicists for decades. One promising place for capturing evidence of their presence must be high-redshift objects; signatures should be recorded in their characteristic chemical abundances. We deduce the Fe and Mg abundances of the broad-line region (BLR) from the intensities of ultraviolet Mg II and Fe II emission lines in the near-infrared spectrum of UKIDSS Large Area Survey (ULAS) J1342+0928 at $z = 7.54$, by advancing our novel flux-to-abundance conversion method developed for $z\sim 1$ quasars. We find that the BLR of this quasar is extremely enriched, by a factor of 20 relative to the solar Fe abundance, together with a very low Mg/Fe abundance ratio: $[\mathrm{Fe/H}]=+1.36\pm0.19$ and $[\mathrm{Mg/Fe}]=-1.11\pm0.12$, only 700 million years after the Big Bang. We conclude that such an unusual abundance feature cannot be explained by the standard view of chemical evolution that considers only the contributions from canonical supernovae. While there remains uncertainty in the high-mass end of the Pop III IMF, here we propose that the larger amount of iron in ULAS J1342+0928 was supplied by a pair-instability supernova (PISN) caused by the explosion of a massive Pop III star in the high-mass end of the possible range of 150-300 $M_\odot$. Chemical-evolution models based on initial PISN enrichment well explain the trend in [Fe/Mg]-$z$ all the way from $z < 3$ to $z = 7.54$. We predict that stars with very low [Mg/Fe] at all metallicities are hidden in the Galaxy, and they will be efficiently discovered by ongoing new-generation photometric surveys., Comment: 17 pages, 4 figures, 3 tables. Accepted for publication in ApJ

We searched for diffuse interstellar bands (DIBs) in the 0.91$<\lambda<$1.33 $\mu$m region by analyzing the near-infrared (NIR) high-resolution ($R=20,000$ and 28,000) spectra of 31 reddened early-type stars ($0.04

We present the spectroscopic discovery of 69 quasars at 5.8 < z < 7.0, drawn from the Hyper Suprime-Cam (HSC) Subaru Strategic Program (SSP) imaging survey data. This is the 16th publication from the Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs) project, and completes identification of all but the faintest candidates (i.e., i-band dropouts with zAB < 24 and y-band detections, and z-band dropouts with yAB < 24) with Bayesian quasar probability Pq > 0.1 in the HSC-SSP third public data release (PDR3). The sample reported here also includes three quasars with Pq < 0.1 at z ~ 6.6, which we selected in an effort to completely cover the reddest point sources with simple color cuts. The number of high-z quasars discovered in SHELLQs has now grown to 162, including 23 type-II quasar candidates. This paper also presents identification of seven galaxies at 5.6 < z < 6.7, an [O III] emitter at z = 0.954, and 31 Galactic cool stars and brown dwarfs. High-z quasars and galaxies comprise 75 % and 16 % respectively of all the spectroscopic SHELLQs objects that pass our latest selection algorithm with the PDR3 photometry. That is, a total of 91 % of the objects lie at z > 5.6. This demonstrates that the algorithm has very high efficiency, even though we are probing an unprecedentedly low-luminosity population down to M1450 ~ -21 mag., Comment: Accepted for publication in The Astrophysical Journal Supplement Series

Determining the effective temperatures (Teff) of red supergiants (RSGs) observationally is important in many fields of stellar physics and galactic astronomy, yet some significant difficulties remain due to model uncertainty originating majorly in the extended atmosphere of RSGs. Here we propose the line-depth ratio (LDR) method in which we use only Fe I lines. As opposed to the conventional LDR method with lines of multiple species involved, the LDR of this kind is insensitive to the surface gravity effects and expected to circumvent the uncertainty originating in the upper atmosphere of RSGs. Therefore, the LDR--Teff relations that we calibrated empirically with red giants may be directly applied to RSGs, though various differences, e.g., caused by the three-dimensional non-LTE effects, between the two groups of objects need to be kept in mind. Using the near-infrared YJ-band spectra of nine well-known solar-metal red giants observed with the WINERED high-resolution spectrograph, we selected 12 pairs of Fe I lines least contaminated with other lines. Applying their LDR--Teff relations to ten nearby RSGs, the resultant Teff with the internal precision of 30--70 K shows good agreement with previous observational results assuming one-dimensional LTE and with Geneva's stellar evolution model. We found no evidence of significant systematic bias caused by various differences, including those in the size of the non-LTE effects, between red giants and RSGs except for one line pair which we rejected because the non-LTE effects may be as large as ~250 K. Nevertheless, it is difficult to evaluate the systematic bias, and further study is required, e.g., with including the three-dimensional non-LTE calculations of all the lines involved., Comment: 19 pages, 9 figures, 9 tables, accepted for publication in MNRAS

To investigate the chemical abundance of broad-line region clouds in quasars at high redshifts, we performed near-infrared spectroscopy of six luminous quasars at z ~ 2.7 with the WINERED spectrograph mounted on the New Technology Telescope (NTT) at the La Silla Observatory, Chile. The measured Fe II/Mg II flux ratios nearly matched with the published data for 0.7 < z < 1.6, suggesting that there is no evolution over a long period of cosmic time, which is consistent with previous studies. To derive the chemical abundances from the measured equivalent widths (EWs), their dependence on nonabundance factors must be corrected. In our previous paper, we proposed a method to derive the [Mg/Fe] abundance ratio and the [Fe/H] abundance by correcting the dependence of EW(Mg II) and EW(Fe II) on the Eddington ratio. To the best of our knowledge, that was the first report to discuss the star-formation history through a direct comparison with chemical evolution models. In the present study, we further investigated the dependence of EWs on luminosity, which is known as the Baldwin effect (BEff). Additional correction for the BEff significantly affects the derived chemical abundances for the six luminous quasars at z ~ 2.7, and the resultant abundances agree well with the prediction of chemical evolution models. Given that most distant quasars found thus far are biased toward luminous ones, the correction of the measured EWs for the BEff is crucial for extending the chemical evolution study to higher redshifts., Comment: 15 pages, 10 figures, accepted for publication in ApJ

A line-depth ratio (LDR) of two spectral lines with different excitation potentials is expected to be correlated with the effective temperature ($T_\mathrm{eff}$). It is possible to determine $T_\mathrm{eff}$ of a star with a precision of tens of Kelvin if dozens or hundreds of tight LDR-$T_\mathrm{eff}$ relations can be used. Most of the previous studies on the LDR method were limited to optical wavelengths, but Taniguchi and collaborators reported 81 LDR relations in the $YJ$ band, 0.97-1.32 $\mu\mathrm{m}$, in 2018. However, with their sample of only 10 giants, it was impossible to account for the effects of surface gravity and metallicity on the LDRs well. Here we investigate the gravity effect based on $YJ$-band spectra of 63 stars including dwarfs, giants, and supergiants observed with the WINERED spectrograph. We found that some LDR-$T_\mathrm{eff}$ relations show clear offsets between the sequence of dwarfs and those of giants/supergiants. The difference between the ionization potentials of the elements considered in each line pair and the corresponding difference in the depths can, at least partly, explain the dependency of the LDR on the surface gravity. In order to expand the stellar parameter ranges that the LDR method can cover with high precision, we obtained new sets of LDR-$T_\mathrm{eff}$ relations for solar-metal G0-K4 dwarfs and F7-K5 supergiants, respectively. The typical precision that can be achieved with our relations is 10-30 K for both dwarfs and supergiants., Comment: 12 pages, 7 figures, accepted for publication in Monthly Notices of the Royal Astronomical Society

Stellar absorption lines of heavy elements can give us various insights into the chemical evolution of the Galaxy and nearby galaxies. Recently developed spectrographs for the near-infrared wavelengths are becoming more and more powerful for producing a large number of high-quality spectra, but identification and characterization of the absorption lines in the infrared range remain to be fulfilled. We searched for lines of the elements heavier than the iron group, i.e., those heavier than Ni, in the Y (9760--11100 AA) and J (11600--13200 AA) bands. We considered the lines in three catalogs, i.e., Vienna Atomic Line Database (VALD), the compilation by R. Kurucz, and the list published in 1999 by Melendez and Barbuy. Candidate lines were selected based on synthetic spectra and the confirmation was done by using WINERED spectra of 13 supergiants and giants within FGK spectral types (spanning 4000--7200 K in the effective temperature). We have detected lines of Zn I, Sr II, Y II, Zr I, Ba II, Sm II, Eu II, and Dy II, in the order of atomic number. Although the number of the lines is small, 23 in total, they are potentially useful diagnostic lines of the Galactic chemical evolution, especially in the regions for which interstellar extinction hampers detailed chemical analyses with spectra in shorter wavelengths. We also report the detection of lines whose presence was not predicted by the synthetic spectra created with the above three line lists., Comment: 19 pages plus 18 pages with additional plots, 5 figures, 3 tables, to be published in the Astrophysical Journal Supplement Series

We used the WINERED spectrograph to perform near-infrared high-resolution spectroscopy (resolving power $R = 28$,000) of 13 young intermediate-mass stars in the Taurus star-forming region. Based on the presence of near- and mid-infrared continuum emission, young intermediate-mass stars can be classified into three different evolutionary stages: Phases I, II, and III in the order of evolution. Our obtained spectra ($\lambda = 0.91$--1.35\,$\mu$m) depict \HeI $\lambda$10830 and P$\beta$ lines that are sensitive to magnetospheric accretion and winds. We also investigate five sources each for P$\beta$ and \ion{He}{1} lines that were obtained from previous studies along with our targets. We observe that the P$\beta$ profile morphologies in Phases I and II corresponded to an extensive variety of emission features; however, these features are not detected in Phase III. We also observe that the He I profile morphologies are mostly broad subcontinuum absorption lines in Phase I, narrow subcontinuum absorption lines in Phase II, and centered subcontinuum absorption features in Phase III. Our results indicate that the profile morphologies exhibit a progression of the dominant mass flow processes: stellar wind and probably magnetospheric accretion in the very early stage, magnetospheric accretion and disk wind in the subsequent stage, and no activities in the final stage. These interpretations further suggest that opacity in protoplanetary disks plays an important role in mass flow processes. Results also indicate that \ion{He}{1} absorption features in Phase III sources, associated with chromospheric activities even in such young phases, are characteristics of intermediate-mass stars., Comment: Accepted for publication in the Astrophysical Journal

We report the discovery of 28 quasars and 7 luminous galaxies at 5.7 $\le$ z $\le$ 7.0. This is the tenth in a series of papers from the Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs) project, which exploits the deep multi-band imaging data produced by the Hyper Suprime-Cam (HSC) Subaru Strategic Program survey. The total number of spectroscopically identified objects in SHELLQs has now grown to 93 high-z quasars, 31 high-z luminous galaxies, 16 [O III] emitters at z ~ 0.8, and 65 Galactic cool dwarfs (low-mass stars and brown dwarfs). These objects were found over 900 deg2, surveyed by HSC between 2014 March and 2018 January. The full quasar sample includes 18 objects with very strong and narrow Ly alpha emission, whose stacked spectrum is clearly different from that of other quasars or galaxies. While the stacked spectrum shows N V 1240 emission and resembles that of lower-z narrow-line quasars, the small Ly alpha width may suggest a significant contribution from the host galaxies. Thus these objects may be composites of quasars and star-forming galaxies., Comment: ApJ, in press. arXiv admin note: text overlap with arXiv:1803.01861

We report the first detection of C$_2$ $A^1\Pi_u$--$X^1\Sigma_g^+$ (0,0) and CN $A^2\Pi_u$--$X^2\Sigma^+$ (0,0) absorption bands in the interstellar medium. The detection was made using the near-infrared (0.91--1.35 $\mu$m) high-resolution ($R=20,000$ and 68,000) spectra of Cygnus OB2 No.\,12 collected with the WINERED spectrograph mounted on the 1.3 m Araki telescope. The $A$--$X$ (1,0) bands of C$_2$ and CN were detected simultaneously. These near-infrared bands have larger oscillator strengths, compared with the $A$--$X$ (2,0) bands of C$_2$ and CN in the optical. In the spectrum of the C$_2$ (0,0) band with $R=68,000$, three velocity components in the line of sight could be resolved and the lines were detected up to high rotational levels ($J''\sim20$). By analyzing the rotational distribution of C$_2$, we could estimate the kinetic temperature and gas density of the clouds with high accuracy. Furthermore, we marginally detected weak lines of $^{12}$C$^{13}$C for the first time in the interstellar medium. Assuming that the rotational distribution and the oscillator strengths of the relevant transitions of $^{12}$C$_2$ and $^{12}$C$^{13}$C are the same, the carbon isotope ratio was estimated to be $^{12}\text{C}/^{13}\text{C}=50$--100, which is consistent with the ratio in the local interstellar medium. We also calculated the oscillator strength ratio of the C$_2$ (0,0) and (1,0) bands from the observed band strengths. Unfortunately, our result could not discern theoretical and experimental results because of the uncertainties. High-resolution data to resolve the velocity components will be necessary for both bands in order to put stronger constraints on the oscillator strength ratios., Comment: 15 pages, 6 figures. Accepted to ApJ

For a detailed analysis of stellar chemical abundances, high-resolution spectra in the optical have mainly been used, while the development of near-infrared (NIR) spectrograph has opened new wavelength windows. Red giants have a large number of resolved absorption lines in both the optical and NIR wavelengths, but the characteristics of the lines in different wave passbands are not necessarily the same. We present a selection of FeI lines in the $z^{\prime}$, $Y$, and $J$ bands (0.91-1.33 $\mu$m). On the basis of two different lists of lines in this range, the Vienna Atomic Line Database (VALD) and the catalog published by Mel\'endez & Barbuy in 1999 (MB99), we selected sufficiently strong lines that are not severely blended and compiled lists with 107 FeI lines in total (97 and 75 lines from VALD and MB99, respectively). Combining our lists with high-resolution ($\lambda/\Delta\lambda = 28,000$) and high signal-to-noise ($>500$) spectra taken with a NIR spectrograph, WINERED, we present measurements of the iron abundances of two prototype red giants: Arcturus and $\mu$ Leo. A bootstrap method for determining the microturbulence and abundance together with their errors is demonstrated. The standard deviations of $\log\epsilon_{\rm Fe}$ values from individual FeI lines are significantly smaller when we use the lines from MB99 instead of those from VALD. With the MB99 list, we obtained $\xi=1.20\pm0.11\ {\rm km~s^{-1}}$ and $\log\epsilon_{\rm Fe}=7.01\pm0.05$ dex for Arcturus, and $\xi=1.54\pm0.17\ {\rm km~s^{-1}}$ and $\log\epsilon_{\rm Fe}=7.73\pm0.07$ dex for $\mu$ Leo. These final values show better agreements with previous values in the literature than the corresponding values we obtained with VALD., Comment: 14 pages, 8 figures, 5 tables, accepted for publication in ApJ

We report the discovery of a quasar at z = 7.07, which was selected from the deep multi-band imaging data collected by the Hyper Suprime-Cam (HSC) Subaru Strategic Program survey. This quasar, HSC J124353.93+010038.5, has an order of magnitude lower luminosity than do the other known quasars at z > 7. The rest-frame ultraviolet absolute magnitude is M1450 = -24.13 +/- 0.08 mag and the bolometric luminosity is Lbol = (1.4 +/- 0.1) x 10^{46} erg/s. Its spectrum in the optical to near-infrared shows strong emission lines, and shows evidence for a fast gas outflow, as the C IV line is blueshifted and there is indication of broad absorption lines. The Mg II-based black hole mass is Mbh = (3.3 +/- 2.0) x 10^8 Msun, thus indicating a moderate mass accretion rate with an Eddington ratio 0.34 +/- 0.20. It is the first z > 7 quasar with sub-Eddington accretion, besides being the third most distant quasar, known to date. The luminosity and black hole mass are comparable to, or even lower than, those measured for the majority of low-z quasars discovered by the Sloan Digital Sky Survey, and thus this quasar likely represents a z > 7 counterpart to quasars commonly observed in the low-z universe., Comment: Accepted for publication in ApJ Letters

We present new measurements of the quasar luminosity function (LF) at $z \sim 6$, over an unprecedentedly wide range of the rest-frame ultraviolet luminosity $M_{1450}$ from $-30$ to $-22$ mag. This is the fifth in a series of publications from the Subaru High-$z$ Exploration of Low-Luminosity Quasars (SHELLQs) project, which exploits the deep multi-band imaging data produced by the Hyper Suprime-Cam (HSC) Subaru Strategic Program survey. The LF was calculated with a complete sample of 110 quasars at $5.7 \le z \le 6.5$, which includes 48 SHELLQs quasars discovered over 650 deg$^2$, and 63 brighter quasars discovered by the Sloan Digital Sky Survey and the Canada-France-Hawaii Quasar Survey (including one overlapping object). This is the largest sample of $z \sim 6$ quasars with a well-defined selection function constructed to date, and has allowed us to detect significant flattening of the LF at its faint end. A double power-law function fit to the sample yields a faint-end slope $\alpha = -1.23^{+0.44}_{-0.34}$, a bright-end slope $\beta = -2.73^{+0.23}_{-0.31}$, a break magnitude $M_{1450}^* = -24.90^{+0.75}_{-0.90}$, and a characteristic space density $\Phi^* = 10.9^{+10.0}_{-6.8}$ Gpc$^{-3}$ mag$^{-1}$. Integrating this best-fit model over the range $-18 < M_{1450} < -30$ mag, quasars emit ionizing photons at the rate of $\dot{n}_{\rm ion} = 10^{48.8 \pm 0.1}$ s$^{-1}$ Mpc$^{-3}$ at $z = 6.0$. This is less than 10 % of the critical rate necessary to keep the intergalactic medium ionized, which indicates that quasars are not a major contributor to cosmic reionization., Comment: Accepted for publication in ApJ

We present a high-resolution ($R \simeq 20,000$) near-infrared (9,100-13,500 $\AA$) long-slit spectrum of P Cygni obtained with the newly commissioned WINERED spectrograph in Japan. In the obtained spectrum, we have found that the velocity profiles of the [Fe II] emission lines are resolved into two peaks at a velocity of $\simeq220$ km s$^{-1}$ with a moderate dip in between and with additional sub-peaks at $\simeq\pm100$~km~s$^{-1}$. The sub-peak component is confirmed with the long-slit echellogram to originate in the known shell with a radius of $\simeq10^{\prime\prime}$, which was originally created by the outburst in 1600 AD. On the other hand, the $\simeq220$ km s$^{-1}$ component, which dominates the [Fe II] flux from P Cygni, is found to be concentrated closer to the central star with an apparent spatial extent of $\simeq3^{\prime\prime}$. The extent is much larger than the compact ($<0^{\prime\prime}.1$) regions traced with hydrogen, helium, and metal permitted lines. The velocity, estimated mass, and dynamical time of the extended emission-line region suggest that the region is an outer part of the stellar wind region. We suggest that the newly-identified emission-line region may trace a reverse shock due to the stellar wind overtaking the outburst shell., Comment: 16 pages, 8 figures, accepted in MNRAS

We report a method of correcting a near-infrared (0.90-1.35 $\mu$m) high-resolution ($\lambda/\Delta\lambda\sim28,000$) spectrum for telluric absorption using the corresponding spectrum of a telluric standard star. The proposed method uses an A0\,V star or its analog as a standard star from which on the order of 100 intrinsic stellar lines are carefully removed with the help of a reference synthetic telluric spectrum. We find that this method can also be applied to feature-rich objects having spectra with heavily blended intrinsic stellar and telluric lines and present an application to a G-type giant using this approach. We also develop a new diagnostic method for evaluating the accuracy of telluric correction and use it to demonstrate that our method achieves an accuracy better than 2\% for spectral parts for which the atmospheric transmittance is as low as $\sim$20\% if telluric standard stars are observed under the following conditions: (1) the difference in airmass between the target and the standard is $\lesssim 0.05$; and (2) that in time is less than 1 h. In particular, the time variability of water vapor has a large impact on the accuracy of telluric correction and minimizing the difference in time from that of the telluric standard star is important especially in near-infrared high-resolution spectroscopic observation., Comment: 20 pages, 16 figures, accepted for publication in Publications of the Astronomical Society of the Pacific

The effective temperature, one of the most fundamental atmospheric parameters of a star, can be estimated using various methods, and here we focus on the method using line-depth ratios (LDRs). This method combines low- and high-excitation lines and makes use of relations between LDRs of these line pairs and the effective temperature. It has an advantage, for example, of being minimally affected by interstellar reddening, which changes stellar colours. We report 81 relations between LDRs and the effective temperature established with high-resolution, $\lambda /\Delta \lambda \sim 28,000$, spectra of nine G- to M-type giants in Y and J bands. Our analysis gives the first comprehensive set of LDR relations for this wavelength range. The combination of all these relations can be used to determine the effective temperatures of stars that have 3700

We present spectroscopic identification of 32 new quasars and luminous galaxies discovered at 5.7 < z < 6.8. This is the second in a series of papers presenting the results of the Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs) project, which exploits the deep multi-band imaging data produced by the Hyper Suprime-Cam (HSC) Subaru Strategic Program survey. The photometric candidates were selected by a Bayesian probabilistic algorithm, and then observed with spectrographs on the Gran Telescopio Canarias and the Subaru Telescope. Combined with the sample presented in the previous paper, we have now identified 64 HSC sources over about 430 deg2, which include 33 high-z quasars, 14 high-z luminous galaxies, 2 [O III] emitters at z ~ 0.8, and 15 Galactic brown dwarfs. The new quasars have considerably lower luminosity (M1450 ~ -25 to -22 mag) than most of the previously known high-z quasars. Several of these quasars have luminous (> 10^(43) erg/s) and narrow (< 500 km/s) Ly alpha lines, and also a possible mini broad absorption line system of N V 1240 in the composite spectrum, which clearly separate them from typical quasars. On the other hand, the high-z galaxies have extremely high luminosity (M1450 ~ -24 to -22 mag) compared to other galaxies found at similar redshift. With the discovery of these new classes of objects, we are opening up new parameter spaces in the high-z Universe. Further survey observations and follow-up studies of the identified objects, including the construction of the quasar luminosity function at z ~ 6, are ongoing., Comment: Accepted for publication in PASJ (HSC special issue); minor revisions made since ver. 1 and 2

We present an analysis of Mg II $\lambda2798$ and Fe II UV emission lines for archival Sloan Digital Sky Survey (SDSS) quasars to explore diagnostics of the magnesium-to-iron abundance ratio in a broad-line region cloud. Our sample consists of 17,432 quasars selected from the SDSS Data Release 7 with a redshift range of $0.72 < z < 1.63$. A strong anticorrelation between Mg II equivalent width (EW) and the Eddington ratio is found, while only a weak positive correlation is found between Fe II EW and the Eddington ratio. To investigate the origin of these differing behaviors of Mg II and Fe II emission lines, we have performed photoionization calculations using the Cloudy code, where constraints from recent reverberation mapping studies are considered. We find from calculations that (i) Mg II and Fe II emission lines are created at different regions in a photoionized cloud, and (ii) their EW correlations with the Eddington ratio can be explained by just changing the cloud gas density. These results indicate that the Mg II/Fe II flux ratio, which has been used as a first-order proxy for the Mg/Fe abundance ratio in chemical evolution studies with quasar emission lines, depends largely on the cloud gas density. By correcting this density dependence, we propose new diagnostics of the Mg/Fe abundance ratio for a broad line region cloud. Comparing the derived Mg/Fe abundance ratios with chemical evolution models, we suggest that $\alpha$-enrichment by mass loss from metal-poor intermediate-mass stars occurred at $z\sim2$ or earlier., Comment: 18 pages, 17 figures, accepted for publication in The Astrophysical Journal

High-resolution X-ray spectroscopy with Hitomi was expected to resolve the origin of the faint unidentified E=3.5 keV emission line reported in several low-resolution studies of various massive systems, such as galaxies and clusters, including the Perseus cluster. We have analyzed the Hitomi first-light observation of the Perseus cluster. The emission line expected for Perseus based on the XMM-Newton signal from the large cluster sample under the dark matter decay scenario is too faint to be detectable in the Hitomi data. However, the previously reported 3.5 keV flux from Perseus was anomalously high compared to the sample-based prediction. We find no unidentified line at the reported high flux level. Taking into account the XMM measurement uncertainties for this region, the inconsistency with Hitomi is at a 99% significance for a broad dark-matter line and at 99.7% for a narrow line from the gas. We do not find anomalously high fluxes of the nearby faint K line or the Ar satellite line that were proposed as explanations for the earlier 3.5 keV detections. We do find a hint of a broad excess near the energies of high-n transitions of Sxvi (E=3.44 keV rest-frame) -- a possible signature of charge exchange in the molecular nebula and another proposed explanation for the unidentified line. While its energy is consistent with XMM pn detections, it is unlikely to explain the MOS signal. A confirmation of this interesting feature has to wait for a more sensitive observation with a future calorimeter experiment., Comment: Discussion of systematics significantly expanded. 9 pages, 5 figures; ApJ Lett. in press

Clusters of galaxies are the most massive gravitationally-bound objects in the Universe and are still forming. They are thus important probes of cosmological parameters and a host of astrophysical processes. Knowledge of the dynamics of the pervasive hot gas, which dominates in mass over stars in a cluster, is a crucial missing ingredient. It can enable new insights into mechanical energy injection by the central supermassive black hole and the use of hydrostatic equilibrium for the determination of cluster masses. X-rays from the core of the Perseus cluster are emitted by the 50 million K diffuse hot plasma filling its gravitational potential well. The Active Galactic Nucleus of the central galaxy NGC1275 is pumping jetted energy into the surrounding intracluster medium, creating buoyant bubbles filled with relativistic plasma. These likely induce motions in the intracluster medium and heat the inner gas preventing runaway radiative cooling; a process known as Active Galactic Nucleus Feedback. Here we report on Hitomi X-ray observations of the Perseus cluster core, which reveal a remarkably quiescent atmosphere where the gas has a line-of-sight velocity dispersion of 164+/-10 km/s in a region 30-60 kpc from the central nucleus. A gradient in the line-of-sight velocity of 150+/-70 km/s is found across the 60 kpc image of the cluster core. Turbulent pressure support in the gas is 4% or less of the thermodynamic pressure, with large scale shear at most doubling that estimate. We infer that total cluster masses determined from hydrostatic equilibrium in the central regions need little correction for turbulent pressure., Comment: 31 pages, 11 Figs, published in Nature July 8

The Narrow-line Seyfert 1 galaxy (NLS1) IRAS 13224$-$3809 is known to exhibit significant X-ray spectral variation, a sharp spectral drop at $\sim$ 7 keV, strong soft excess emission, and a hint of iron L-edge feature, which is very similar to the NLS1 1H 0707$-$495. We have proposed the "Variable Double Partial Covering (VDPC) model" to explain the energy spectra and spectral variability of 1H 0707$-$495 (Mizumoto, Ebisawa and Sameshima 2014, PASJ, 66, 122). In this model, the observed flux/spectral variations below 10 keV within a $\sim$day are primarily caused by change of the partial covering fraction of patchy clouds composed by double absorption layers in the line of sight. In this paper, we apply the VDPC model to IRAS 13224$-$3809. Consequently, we have found that the VDPC model can explain the observed spectral variations of IRAS 13224$-$3809 in the 0.5$-$10 keV band. In particular, we can explain the observed Root Mean Square (RMS) spectra (energy dependence of the fractional flux variation) in the entire 0.5$-$10 keV band. In addition to the well-known significant drop in the iron K-band, we have found intriguing iron L-peaks in the RMS spectra when the iron L-edge is particularly deep. This feature, which is also found in 1H 0707$-$495, is naturally explained with the VDPC model, such that the RMS variations increase at the energies where optical depths of the partial absorbers are large. The absorbers have a larger optical depth at the iron L-edge than in the adjacent energy bands, thus a characteristic iron L-peak appears. On the other hand, just below the iron K-edge, the optical depth is the lowest and the RMS spectrum has a broad dip., Comment: 18 pages, 10 figures, Accepted for PASJ

We report the discovery of 15 quasars and bright galaxies at 5.7 < z < 6.9. This is the initial result from the Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs) project, which exploits the exquisite multiband imaging data produced by the Subaru Hyper Suprime-Cam (HSC) Strategic Program survey. The candidate selection is performed by combining several photometric approaches including a Bayesian probabilistic algorithm to reject stars and dwarfs. The spectroscopic identification was carried out with the Gran Telescopio Canarias and the Subaru Telescope for the first 80 deg2 of the survey footprint. The success rate of our photometric selection is quite high, approaching 100 % at the brighter magnitudes (zAB < 23.5 mag). Our selection also recovered all the known high-z quasars on the HSC images. Among the 15 discovered objects, six are likely quasars, while the other six with interstellar absorption lines and in some cases narrow emission lines are likely bright Lyman-break galaxies. The remaining three objects have weak continua and very strong and narrow Ly alpha lines, which may be excited by ultraviolet light from both young stars and quasars. These results indicate that we are starting to see the steep rise of the luminosity function of z > 6 galaxies, compared with that of quasars, at magnitudes fainter than M1450 ~ -22 mag or zAB ~24 mag. Follow-up studies of the discovered objects as well as further survey observations are ongoing., Comment: Published in ApJ (828:26, 2016)

We obtained the near-infrared (NIR) high-resolution ($R\equiv\lambda/\Delta\lambda\sim20,000$) spectra of the seven brightest early-type stars in the Cygnus OB2 association for investigating the environmental dependence of diffuse interstellar bands (DIBs). The WINERED spectrograph mounted on the Araki 1.3m telescope in Japan was used to collect data. All 20 of the known DIBs within the wavelength coverage of WINERED ($0.91<\lambda<1.36\mu$m) were clearly detected along all lines of sight because of their high flux density in the NIR wavelength range and the large extinction. The equivalent widths (EWs) of DIBs were not correlated with the column densities of C$_2$ molecules, which trace the patchy dense component, suggesting that the NIR DIB carriers are distributed mainly in the diffuse component. On the basis of the correlations among the NIR DIBs both for stars in Cyg OB2 and stars observed previously, $\lambda\lambda$10780, 10792, 11797, 12623, and 13175 are found to constitute a "family", in which the DIBs are correlated well over the wide EW range. In contrast, the EW of $\lambda$10504 is found to remain almost constant over the stars in Cyg OB2. The extinction estimated from the average EW of $\lambda$10504 ($A_V\sim3.6$mag) roughly corresponds to the lower limit of the extinction distribution of OB stars in Cyg OB2. This suggests that $\lambda$10504 is absorbed only by the foreground clouds, implying that the carrier of $\lambda$10504 is completely destroyed in Cyg OB2, probably by the strong UV radiation field. The different behaviors of the DIBs may be caused by different properties of the DIB carriers., Comment: 13 pages, 9 figures, accepted for the publication in ApJ

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., Comment: 39 pages, 7 figures, accepted by PASJ

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., Comment: 32 pages, 7 figures, accepted for publication in PASJ

The joint JAXA/NASA ASTRO-H mission is the sixth in a series of highly successful X-ray missions developed by the Institute of Space and Astronautical Science (ISAS), with a planned launch in 2015. The ASTRO-H mission is equipped with a suite of sensitive instruments with the highest energy resolution ever achieved at E > 3 keV and a wide energy range spanning four decades in energy from soft X-rays to gamma-rays. The simultaneous broad band pass, coupled with the high spectral resolution of Delta E < 7 eV of the micro-calorimeter, will enable a wide variety of important science themes to be pursued. ASTRO-H is expected to provide breakthrough results in scientific areas as diverse as the large-scale structure of the Universe and its evolution, the behavior of matter in the gravitational strong field regime, the physical conditions in sites of cosmic-ray acceleration, and the distribution of dark matter in galaxy clusters at different redshifts., Comment: 24 pages, 18 figures, Proceedings of the SPIE Astronomical Instrumentation "Space Telescopes and Instrumentation 2014: Ultraviolet to Gamma Ray"

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., Comment: 45 pages, 10 figures, accepted for publication in PASJ

We investigate the UV-optical (longward of Ly$\alpha$ 1216\AA) spectral variability of nearly 9000 quasars ($0 \alpha_\lambda)

The joint JAXA/NASA ASTRO-H mission is the sixth in a series of highly successful X-ray missions initiated by the Institute of Space and Astronautical Science (ISAS). ASTRO-H will investigate the physics of the high-energy universe via a suite of four instruments, covering a very wide energy range, from 0.3 keV to 600 keV. These instruments include a high-resolution, high-throughput spectrometer sensitive over 0.3-2 keV with high spectral resolution of Delta E < 7 eV, enabled by a micro-calorimeter array located in the focal plane of thin-foil X-ray optics; hard X-ray imaging spectrometers covering 5-80 keV, located in the focal plane of multilayer-coated, focusing hard X-ray mirrors; a wide-field imaging spectrometer sensitive over 0.4-12 keV, with an X-ray CCD camera in the focal plane of a soft X-ray telescope; and a non-focusing Compton-camera type soft gamma-ray detector, sensitive in the 40-600 keV band. The simultaneous broad bandpass, coupled with high spectral resolution, will enable the pursuit of a wide variety of important science themes., Comment: 22 pages, 17 figures, Proceedings of the SPIE Astronomical Instrumentation "Space Telescopes and Instrumentation 2012: Ultraviolet to Gamma Ray"

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 Ly\alpha and 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., Comment: 43 pages, 31 figures, accepted for publication in ApJ

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., Comment: 5pages, 5figures, 2tables, accepted for publication in MNRAS