arXiv subjects

Tomoki Morokuma

Publications and source records attributed to Tomoki Morokuma.

At least 73 records · Page 4Linked to original sources

arXiv2017

OISTER Optical and Near-Infrared Monitoring Observations of a Peculiar Radio-Loud Active Galactic Nucleus SDSS J110006.07+442144.3

We present monitoring campaign observations at optical and near-infrared (NIR) wavelengths for a radio-loud active galactic nucleus (AGN) at z=0.840, SDSS~J110006.07+442144.3 (hereafter, J1100+4421), which was identified during a flare phase in late February, 2014. The campaigns consist of three intensive observing runs from the discovery to March, 2015, mostly within the scheme of the OISTER collaboration. Optical-NIR light curves and simultaneous spectral energy distributions (SEDs) are obtained. Our measurements show the strongest brightening in March, 2015. We found that the optical-NIR SEDs of J1100+4421 show an almost steady shape despite the large and rapid intranight variability. This constant SED shape is confirmed to extend to $\sim5~μ$m in the observed frame using the archival WISE data. Given the lack of absorption lines and the steep power-law spectrum of $α_ν\sim-1.4$, where $f_ν\proptoν^{α_ν}$, synchrotron radiation by a relativistic jet with no or small contributions from the host galaxy and the accretion disk seems most plausible as an optical-NIR emission mechanism. The steep optical-NIR spectral shape and the large amplitude of variability are consistent with this object being a low $ν_{\rm{peak}}$ jet-dominated AGN. In addition, sub-arcsec resolution optical imaging data taken with Subaru Hyper Suprime-Cam does not show a clear extended component and the spatial scales are significantly smaller than the large extensions detected at radio wavelengths. The optical spectrum of a possible faint companion galaxy does not show any emission lines at the same redshift and hence a merging hypothesis for this AGN-related activity is not supported by our observations.

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arXiv2017

Superluminous transients at AGN centers from interaction between black-hole disk winds and broad-line region clouds

We propose that superluminous transients that appear at central regions of active galactic nuclei (AGNs) such as CSS100217:102913+404220 (CSS100217) and PS16dtm, which reach near or super-Eddington luminosities of the central black holes, are powered by the interaction between accretion disk winds and clouds in broad-line regions (BLRs) surrounding them. If the disk luminosity temporary increases by, e.g., limit-cycle oscillations, leading to a powerful radiatively driven wind, strong shock waves propagate in the BLR. Because the dense clouds in the AGN BLRs typically have similar densities to those found in Type IIn supernovae, strong radiative shocks emerge and efficiently convert the ejecta kinetic energy to radiation. As a result, transients similar to Type IIn supernovae can be observed at AGN central regions. Since a typical black-hole disk wind velocity is ~ 0.1c where c is the speed of light, the ejecta kinetic energy is expected to be ~ 1e52 erg when ~ 1 Msun is ejected. This kinetic energy is transformed to radiation energy in a timescale for the wind to sweep up a similar mass to itself in the BLR, which is a few hundred days. Therefore, both luminosities (~ 1e44 erg/s) and timescales (~ 100 days) of the superluminous transients from AGN central regions match to those expected in our interaction model. If CSS100217 and PS16dtm are related to the AGN activities triggered by limit-cycle oscillations, they become bright again in coming years or decades.

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arXiv2017

Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs). II. Discovery of 32 Quasars and Luminous Galaxies at 5.7 < z < 6.8

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.

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arXiv2017

Broad-Lined Supernova 2016coi with Helium Envelope

We present the early-phase spectra and the light curves of the broad-lined supernova (SN) 2016coi from $t=7$ to $67$ days after the estimated explosion date. This SN was initially reported as a broad-lined Type SN Ic (SN Ic-BL). However, we found that spectra up to $t=12$ days exhibited the He~{\sc i} $λ$5876, $λ$6678, and $λ$7065 absorption lines. We show that the smoothed and blueshifted spectra of normal SNe Ib are remarkably similar to the observed spectrum of SN 2016coi. The line velocities of SN 2016coi were similar to those of SNe Ic-BL and significantly faster than those of SNe Ib. Analyses of the line velocity and light curve suggest that the kinetic energy and the total ejecta mass of SN 2016coi are similar to those of SNe Ic-BL. Together with broad-lined SNe 2009bb and 2012ap for which the detection of He~{\sc i} were also reported, these SNe could be transitional objects between SNe Ic-BL and SNe Ib, and be classified as broad-lined Type `Ib' SNe (SNe `Ib'-BL). Our work demonstrates the diversity of the outermost layer in broad-lined SNe, which should be related to the variety of the evolutionary paths.

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arXiv2016

Isophote Shapes of Early-Type Galaxies in Massive Clusters at $z\sim1$ and 0

We compare the isophote shape parameter $a_{4}$ of early-type galaxies (ETGs) between $z\sim1$ and 0 as a proxy for dynamics to investigate the epoch at which the dynamical properties of ETGs are established, using cluster ETG samples with stellar masses of $\log(M_{*}/M_{\odot})\geq10.5$ which have spectroscopic redshifts. We have 130 ETGs from the Hubble Space Telescope Cluster Supernova Survey for $z\sim1$ and 355 ETGs from the Sloan Digital Sky Survey for $z\sim0$. We have developed an isophote shape analysis method which can be used for high-redshift galaxies and has been carefully compared with published results. We have applied the same method for both the $z\sim1$ and $0$ samples. We find similar dependence of the $a_{4}$ parameter on the mass and size at $z\sim1$ and 0; the main population of ETGs changes from disky to boxy at a critical stellar mass of $\log(M_{*}/M_{\odot})\sim11.5$ with the massive end dominated by boxy. The disky ETG fraction decreases with increasing stellar mass both at $z\sim1$ and $0$, and is consistent between these redshifts in all stellar mass bins when the Eddington bias is taken into account. Although uncertainties are large, the results suggest that the isophote shapes and probably dynamical properties of ETGs in massive clusters are already in place at $z>1$ and do not significantly evolve in $z<1$, despite significant size evolution in the same galaxy population. The constant disky fraction favors less violent processes than mergers as a main cause of the size and morphological evolution of intermediate mass ETGs in $z<1$.

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arXiv2016

J-GEM Follow-Up Observations of The Gravitational Wave Source GW151226

We report the results of optical--infrared follow-up observations of the gravitational wave (GW) event GW151226 detected by the Advanced LIGO in the framework of J-GEM (Japanese collaboration for Gravitational wave ElectroMagnetic follow-up). We performed wide-field optical imaging surveys with Kiso Wide Field Camera (KWFC), Hyper Suprime-Cam (HSC), and MOA-cam3. The KWFC survey started at 2.26 days after the GW event and covered 778 deg$^2$ centered at the high Galactic region of the skymap of GW151226. We started the HSC follow-up observations from 12 days after the event and covered an area of 63.5 deg$^2$ of the highest probability region of the northern sky with the limiting magnitudes of 24.6 and 23.8 for i band and z band, respectively. MOA-cam3 covered 145 deg$^2$ of the skymap with MOA-red filter 2.5 months after the GW alert. Total area covered by the wide-field surveys was 986.5 deg$^2$. The integrated detection probability of all the observed area was $\sim$29%. We also performed galaxy-targeted observations with six optical and near-infrared telescopes from 1.61 days after the event. Total of 238 nearby (<100 Mpc) galaxies were observed with the typical I band limiting magnitude of $\sim$19.5. We detected 13 supernova candidates with the KWFC survey, and 60 extragalactic transients with the HSC survey. Two third of the HSC transients were likely supernovae and the remaining one third were possible active galactic nuclei. With our observational campaign, we found no transients that are likely to be associated with GW151226.

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arXiv2016

Machine-learning Selection of Optical Transients in Subaru/Hyper Suprime-Cam Survey

We present an application of machine-learning (ML) techniques to source selection in the optical transient survey data with Hyper Suprime-Cam (HSC) on the Subaru telescope. Our goal is to select real transient events accurately and in a timely manner out of a large number of false candidates, obtained with the standard difference-imaging method. We have developed the transient selector which is based on majority voting of three ML machines of AUC Boosting, Random Forest, and Deep Neural Network. We applied it to our observing runs of Subaru-HSC in 2015 May and August, and proved it to be efficient in selecting optical transients. The false positive rate was 1.0% at the true positive rate of 90% in the magnitude range of 22.0--25.0 mag for the former data. For the latter run, we successfully detected and reported ten candidates of supernovae within the same day as the observation. From these runs, we learned the following lessons: (1) the training using artificial objects is effective in filtering out false candidates, especially for faint objects, and (2) combination of ML by majority voting is advantageous.

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arXiv2016

Subaru high-z exploration of low-luminosity quasars (SHELLQs). I. Discovery of 15 quasars and bright galaxies at 5.7 < z < 6.9

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.

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arXiv2016

X-ray and Optical Correlation of Type I Seyfert NGC 3516 Studied with Suzaku and Japanese Ground-Based Telescopes

From 2013 April to 2014 April, we performed an X-ray and optical simultaneous monitoring of the type 1.5 Seyfert galaxy NGC 3516. It employed Suzaku, and 5 Japanese ground-based telescopes, the Pirka, Kiso Schmidt, Nayuta, MITSuME, and the Kanata telescopes. The Suzaku observations were conducted seven times with various intervals ranging from days, weeks, to months, with an exposure of $\sim50$ ksec each. The optical $B$-band observations not only covered those of Suzaku almost simultaneously, but also followed the source as frequently as possible. As a result, NGC 3516 was found in its faint phase with the 2-10 keV flux of $0.21-2.70 \times 10^{-11}$ erg s$^{-1}$ cm$^{-2}$. The 2-45 keV X-ray spectra were composed of a dominant variable hard power-law continuum with a photon index of $\sim1.7$, and a non-relativistic reflection component with a prominent Fe-K$α$ emission line. Producing the $B$-band light curve by differential image photometry, we found that the $B$-band flux changed by $\sim2.7 \times 10^{-11}$ erg s$^{-1}$ cm$^{-2}$, which is comparable to the X-ray variation, and detected a significant flux correlation between the hard power-law component in X-rays and the $B$-band radiation, for the first time in NGC 3516. By examining their correlation, we found that the X-ray flux preceded that of $B$ band by $2.0^{+0.7}_{-0.6}$ days ($1σ$ error). Although this result supports the X-ray reprocessing model, the derived lag is too large to be explained by the standard view which assumes a "lamppost"-type X-ray illuminator located near a standard accretion disk. Our results are better explained by assuming a hot accretion flow and a truncated disk.

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arXiv2016

Constraining the Age and Distance of the Galactic Supernova Remnant G156.2+5.7 by H-alpha Expansion Measurements

We present deep H-alpha images of portions of the X-ray bright but optically faint Galactic supernova remnant G156.2+5.7, revealing numerous and delicately thin nonradiative filaments which mark the location of the remnant's forward shock. These new images show that these filaments have a complex structure not visible on previous lower resolution optical images. By comparing H-alpha images taken in 2004 at the McDonald Observatory and in 2015-2016 at the Kiso Observatory, we set a stringent 1-sigma upper limit of expansion to be 0.06 arcsec/yr. This proper motion, combined with a shock speed of 500 km/s inferred from X-ray spectral analyses, gives a distance of > 1.7 kpc. In addition, a simple comparison of expansion indices of several SNRs allows us to infer the age of the remnant to be a few 10,000 yr old. These estimates are more straightforward and reliable than any other previous studies, and clearly rule out a possibility that G156.2+5.7 is physically associated with part of the Taurus-Auriga cloud and dust complex at a distance of 200-300 pc.

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arXiv2016

J-GEM Follow-Up Observations to Search for an Optical Counterpart of The First Gravitational Wave Source GW150914

We present our optical follow-up observations to search for an electromagnetic counterpart of the first gravitational wave source GW150914 in the framework of the Japanese collaboration for Gravitational wave ElectroMagnetic follow-up (J-GEM), which is an observing group utilizing optical and radio telescopes in Japan, as well as those in New Zealand, China, South Africa, Chile, and Hawaii. We carried out a wide-field imaging survey with Kiso Wide Field Camera (KWFC) on the 1.05-m Kiso Schmidt telescope in Japan and a galaxy-targeted survey with Tripole5 on the B&C 61-cm telescope in New Zealand. Approximately 24 deg2 regions in total were surveyed in i-band with KWFC and 18 nearby galaxies were observed with Tripole5 in g-, r-, and i-bands 4-12 days after the gravitational wave detection. Median 5-sigma depths are i~18.9 mag for the KWFC data and g~18.9 mag, r~18.7 mag, and i~18.3 mag for the Tripole5 data. Probability for a counterpart to be in the observed area is 1.2% in the initial skymap and 0.1% in the final skymap. We do not find any transient source associated to an external galaxy with spatial offset from its center, which is consistent with the local supernova rate. We summarize future prospects and ongoing efforts to pin down electromagnetic counterparts of binary black hole mergers as well as neutron star mergers.

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arXiv2016

OISTER Optical and Near-Infrared Observations of the Super-Chandrasekhar Supernova Candidate SN 2012dn: Dust Emission from the Circumstellar Shell

We present extensively dense observations of the super-Chandrasekhar supernova (SC SN) candidate SN 2012dn from $-11$ to $+140$ days after the date of its $B$-band maximum in the optical and near-infrared (NIR) wavelengths conducted through the OISTER ToO program. The NIR light curves and color evolutions up to 35 days after the $B$-band maximum provided an excellent match with those of another SC SN 2009dc, providing a further support to the nature of SN 2012dn as a SC SN. We found that SN 2012dn exhibited strong excesses in the NIR wavelengths from $30$ days after the $B$-band maximum. The $H$ and $K_{s}$-band light curves exhibited much later maximum dates at $40$ and $70$ days after the $B$-band maximum, respectively, compared with those of normal SNe Ia. The $H$ and $K_{s}$-band light curves subtracted by those of SN 2009dc displayed plateaued evolutions, indicating a NIR echo from the surrounding dust. The distance to the inner boundary of the dust shell is limited to be $4.8 - 6.4\times10^{-2}$ pc. No emission lines were found in its early phase spectrum, suggesting that the ejecta-CSM interaction could not occur. On the other hand, we found no signature that strongly supports the scenario of dust formation. The mass loss rate of the pre-explosion system is estimated to be $10^{-6}-10^{-5}$ M$_{\odot}$ yr$^{-1}$, assuming that the wind velocity of the system is $10-100$ km~s$^{-1}$, which suggests that the progenitor of SN 2012dn could be a recurrent nova system. We conclude that the progenitor of this SC SN could be explained by the single-degenerate scenario.

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arXiv2016

Optical Variability Properties of Mini-BAL and NAL Quasars

While narrow absorption lines (NALs) are relatively stable, broad absorption lines (BALs) and mini-BAL systems usually show violent time variability within a few years via a mechanism that is not yet understood. In this study, we examine variable ionization state (VIS) scenario as a plausible mechanism, as previously suspected. Over three years, we performed photometric monitoring observations of four mini-BAL and five NAL quasars at $z_{\rm em}$ $\sim$ 2.0 - 3.1 using the 105 cm Kiso Schmidt Telescope in $u$, $g$, and $i$-bands. We also performed spectroscopic monitoring observation of one of our mini-BAL quasar (HS1603+3820) using the 188-cm Okayama Telescope over the same period as the photometric observations. Our main results are as follows: (1) Structure function (SF) analysis revealed that the quasar UV flux variability over three years was not large enough to support the VIS scenario, unless the ionization condition of outflow gas is very low. (2) There was no crucial difference between the SFs of mini-BAL and NAL quasars. (3) The variability of the mini-BAL and quasar light curves was weakly synchronized with a small time delay for HS1603+3820. These results suggest that the VIS scenario may need additional mechanisms such as a variable shielding by X-ray warm absorbers.

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arXiv2016

An Effective Selection Method for Low-Mass Active Black Holes and First Spectroscopic Identification

We present a new method to effectively select objects which may be low-mass active black holes (BHs) at galaxy centers using high-cadence optical imaging data, and our first spectroscopic identification of an active 2.7x10^6 Msun BH at z=0.164. This active BH was originally selected due to its rapid optical variability, from a few hours to a day, based on Subaru Hyper Suprime-Cam~(HSC) g-band imaging data taken with 1-hour cadence. Broad and narrow H-alpha and many other emission lines are detected in our optical spectra taken with Subaru FOCAS, and the BH mass is measured via the broad H-alpha emission line width (1,880 km s^{-1}) and luminosity (4.2x10^{40} erg s^{-1}) after careful correction for the atmospheric absorption around 7,580-7,720A. We measure the Eddington ratio to be as low as 0.05, considerably smaller than those in a previous SDSS sample with similar BH mass and redshift, which indicates one of the strong potentials of our Subaru survey. The g-r color and morphology of the extended component indicate that the host galaxy is a star-forming galaxy. We also show effectiveness of our variability selection for low-mass active BHs.

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arXiv2016

Rapidly Rising Transients from Subaru Hyper Suprime-Cam Transient Survey

We present rapidly rising transients discovered by a high-cadence transient survey with Subaru telescope and Hyper Suprime-Cam. We discovered five transients at z=0.384-0.821 showing the rising rate faster than 1 mag per 1 day in the restframe near-ultraviolet wavelengths. The fast rising rate and brightness are the most similar to SN 2010aq and PS1-13arp, for which the ultraviolet emission within a few days after the shock breakout was detected. The lower limit of the event rate of rapidly rising transients is ~9 % of core-collapse supernova rates, assuming a duration of rapid rise to be 1 day. We show that the light curves of the three faint objects agree with the cooling envelope emission from the explosion of red supergiants. The other two luminous objects are, however, brighter and faster than the cooling envelope emission. We interpret these two objects to be the shock breakout from dense wind with the mass loss rate of ~10^{-3} Msun yr^{-1}, as also proposed for PS1-13arp. This mass loss rate is higher than that typically observed for red supergiants. The event rate of these luminous objects is >~1 % of core-collapse supernova rate, and thus, our study implies that more than ~1 % of massive stars can experience an intensive mass loss at a few years before the explosion.

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arXiv2015

Detection of Remnant Dust Cloud Associated with the 2007 Outburst of 17P/Holmes

This paper reports a new optical observation of 17P/Holmes one orbital period after the historical outburst event in 2007. We detected not only a common dust tail near the nucleus, but also a long narrow structure that extended along the position angle 274.6+/- 0.1 degree beyond the field of view of the Kiso Wide Field Camera, i.e., >0.2 degree eastward and >2.0 degree westward from the nuclear position. The width of the structure decreased westward with increasing distance from the nucleus. We obtained the total cross section of the long extended structure in the field of view, C= (2.3 +/- 0.5)x10^10 m^2. From the position angle, morphology and the mass, we concluded that the long narrow structure consists of materials ejected during the 2007 outburst. On the basis of the dynamical behavior of dust grains in the solar radiation field, we estimated that the long narrow structure would be composed of 1 mm-1 cm grains having an ejection velocity of >50 m/s. The velocity was more than one order of magnitude faster than that of millimeter - centimeter grains from typical comets around a heliocentric distance rh of 2.5 AU. We considered that sudden sublimation of a large amount of water ice (about 10^30 mol/s) would be responsible for the high ejection velocity. We finally estimated a total mass of M=(4-8)x10^11 kg and a total kinetic energy of E=(1-6)x10^15 J for the 2007 outburst ejecta, which are consistent with those of previous studies that conducted soon after the outburst.

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arXiv2015

Optical and Near-Infrared Polarimetry for a Highly Dormant Comet 209P/LINEAR

We conducted an optical and near-infrared polarimetric observation of the highly dormant Jupiter-Family Comet, 209P/LINEAR. Because of its low activity, we were able to determine the linear polarization degrees of the coma dust particles and nucleus independently, that is $P_n$=30.3$^{+1.3}_{-0.9}$% at $α$=92.2$^\circ$ and $P_n$=31.0$^{+1.0}_{-0.7}$% at $α$=99.5$^\circ$ for the nucleus, and $P_c$=28.8$^{+0.4}_{-0.4}$% at $α$=92.2$^\circ$ and 29.6$^{+0.3}_{-0.3}$% at $α$=99.5$^\circ$ for the coma. We detected no significant variation in $P$ at the phase angle coverage of 92.2$^\circ$-99.5$^\circ$, which may imply that the obtained polarization degrees are nearly at maximum in the phase-polarization curves. By fitting with an empirical function, we obtained the maximum values of linear polarization degrees $P_\mathrm{max}$=30.8% for the nucleus and $P_\mathrm{max}$=29.6% for the dust coma. The $P_\mathrm{max}$ of the dust coma is consistent with those of dust-rich comets. The low geometric albedo of $P_v$=0.05 was derived from the slope-albedo relationship and was associated with high $P_\mathrm{max}$. We examined $P_\mathrm{max}$-albedo relations between asteroids and 209P, and found that the so-called Umov law seems to be applicable on this cometary surface.

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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.

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