Shintaro Koshida

STATISTICAL PROPERTIES OF MULTI-EPOCH SPECTRAL VARIABILITY OF SDSS STRIPE 82 QUASARS

We investigate the UV-optical (longward of Lyα 1216 A) 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{sub ν}{sup dif}∼+1/3 (in the form of f{sub ν}∝ν{sup α{sub ν}}), which is significantly bluer than that of the composite spectrum α{sub ν}{sup com}∼−0.5. We also show that the continuum variability cannot be explained by accretion disk models with varying mass accretion rates. 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 Mg II and Fe II are less variable compared to Balmer emission lines and high-ionization lines, and the Balmer continuum is the dominant variablemorexa0» source around ∼3000 A. These results are compared with previous studies, and the physical mechanisms of the variability of the continuum and emission lines are discussed.«xa0less

J-GEM observations of an electromagnetic counterpart to the neutron star merger GW170817

GW170817 is the first detected gravitational wave source from a neutron star merger. We present the Japanese collaboration for gravitational-wave electro-magnetic (J-GEM) follow-up observations of SSS17a, an electromagnetic counterpart of GW170817. SSS17a shows a 2.5mag decline in the z band during the period between 1.7 and 7.7 d after the merger. Such a rapid decline is not comparable with supernovae light curves at any epoch. The color of SSS17a also evolves rapidly and becomes redder during later epochs: the z - H color has changed by approximately 2.5mag during the period between 0.7 and 7.7 d. The rapid evolutions of both the color and the optical brightness are consistent with the expected properties of a kilonova that is powered by the radioactive decay of newly synthesized r-process nuclei. Kilonova models with Lanthanide elements can reproduce the aforementioned observed properties well, which suggests that r-process nucleosynthesis beyond the second peak takes place in SSS17a. However, the absolute magnitude of SSS17a is brighter than the expected brightness of the kilonova models with an ejectamass of 0.01M(circle dot), which suggests a more intensemass ejection (similar to 0.03M(circle dot)) or possibly an additional energy source.

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

GROUND-BASED Paα NARROW-BAND IMAGING OF LOCAL LUMINOUS INFRARED GALAXIES. I. STAR FORMATION RATES AND SURFACE DENSITIES

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A WEAK LENSING VIEW OF THE DOWNSIZING OF STAR-FORMING GALAXIES*

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

The University of Tokyo Atacama Observatory 6.5m telescope: project overview and current status

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Development status of the simultaneous two-color near-infrared multi-object spectrograph SWIMS for the TAO 6.5m telescope

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

Calibration of AGN Reverberation Distance Measurements

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Laboratory performance evaluation of the mid-infrared camera and spectrograph MIMIZUKU for the TAO 6.5-m telescope

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

NIR camera and spectrograph SWIMS for TAO 6.5m telescope: overview and development status

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