22 research outputs found
Sacrificial Catalyst of Carbothermal-Shock Synthesized 1T-MoS2 Layers for Ultralong-Lifespan Seawater Battery
School of Energy and Chemical Engineering (Energy Engineering (Battery Science and Technology))The development of efficient electrochemical catalysts for seawater batteries has been of great interest to improve its battery performance with long service life. However, the escalating charge potentials during the battery cycle erode the cell performance and remain a huge challenge. In this work, we demonstrate the sacrificial electrocatalyst of Pt nanoparticles decorated 1T-MoS2 layer triggered by carbothermal shock (CTS) treatment for improving the charge/discharge overpotential and the lifespan of the seawater battery. The simple but potent process of CTS treatment enables us to observe the phase transition of MoS2 crystal from semiconducting 2H to metallic 1T phase, resulting in improved ORR activity under seawater catholyte. In particular, the sacrificial catalyst of MoS2 during the charging process effectively reduces the charging potential due to the oxidation of MoS2, which extends the cycle stability. Furthermore, the increasing portion of 1T-MoS2 induced by the subsequent CTS process of developing Pt nanoparticles on CTS treated MoS2 exhibits a significantly low charge/discharge potential gap of ??0.39 V, a high power density of 6.56 mW cm???2, and long cycle life of up to approximately 800 hours. Thus, high throughput CTS triggered Pt decorated 1T-MoS2 offers a novel strategy for developing efficient bifunctional electrocatalysts to facilitate the long service life of seawater batteries.clos
Deficit of Hot Dust in Low-redshift Active Galactic Nuclei
We assemble a broad-band spectral energy distribution (SED) ranging from
optical to mid-infrared of nearby active galactic nuclei at . SED
fitting analysis is performed using semi-empirical templates derived from
Palomar-Green quasars to classify the sample into normal, warm-dust-deficient
(WDD), and hot-dust-deficient (HDD) AGNs. Kolmogorov-Smirnov tests reveal that
HDD AGNs exhibit, on average higher AGN luminosity than normal and WDD AGNs.
HDD fraction, on the other hand, is only weakly correlated with black hole mass
and inversely correlated with Eddington ratio. By fixing the other parameters,
we conclude that the HDD fraction is primarily connected with the AGN
luminosity. It implies that there is a causal connection between the covering
factor of the hot dust component and AGN luminosity, possibly due to the
sublimation of the innermost dust or the thickening of the intervening gas in
the broad-line region. Analysis of the outflow properties traced by the wing of
[O III] suggests that outflows may be related to the formation and
maintenance of the hot dust component. Finally, we demonstrate through
comparison with previous studies that the classification of HDD AGNs requires
careful subtraction of the host galaxy light.Comment: Accepted for publication in Ap
The Structure Function of Mid-infrared Variability in Low-redshift Active Galactic Nuclei
Using the multi-epoch mid-infrared (MIR) photometry from the Wide-field
Infrared Survey Explorer spanning a baseline of yr, we extensively
investigate the MIR variability of nearby active galactic nuclei (AGNs) at
. We find that the ensemble structure function in the W1 band
(m) can be modeled with a broken power law. Type 1 AGNs tend to
exhibit larger variability amplitudes than type 2 AGNs, possibly due to the
extinction by the torus. The variability amplitude is inversely correlated with
the AGN luminosity, consistent with a similar relation known in the optical.
Meanwhile, the slope of the power law increases with AGN luminosity. This trend
can be attributed to the fact that the inner radius of the torus is
proportional to the AGN luminosity, as expected from the sizeluminosity
relation of the torus. Interestingly, low-luminosity type 2 AGNs, unlike
low-luminosity type 1 AGNs, tend to exhibit smaller variability amplitude than
do high-luminosity AGNs. We argue that either low-luminosity type 2 AGNs have
distinctive central structures due to their low luminosity or their MIR
brightness is contaminated by emission from the cold dust in the host galaxy.
Our findings suggest that the AGN unification scheme may need to be revised. We
find that the variability amplitude of dust-deficient AGNs is systematically
larger than that of normal AGNs, supporting the notion that the hot and warm
dust in dust-deficient AGNs may be destroyed and reformed according to the
strength of the ultraviolet radiation from the accretion disk.Comment: Accepted for publication in ApJ, 17 pages, 13 figure
A Mid-infrared Flare in the Seyfert Galaxy NGC 3786: A Changing-look Event Triggered by an Obscured Tidal Disruption Event?
We report an exceptional mid-infrared flare in the Seyfert 1.8 NGC 3786. In
the multi-epoch data from the Wide-field Infrared Survey Explorer, the nuclear
mid-infrared brightness of NGC 3786 appears to vary substantially up to
mag around mid-2020. However, there is no evidence of significant
variation in the corresponding light curve of the optical band from the Zwicky
Transient Facility. This implies that the flare may have been heavily obscured
by nuclear dust. Through follow-up spectroscopic observations with Gemini-North
after the flare, we find that broad emission lines in and
newly appear, while the broad emission is
marginally detected in the post-flare spectrum. In addition, their central
wavelengths are systematically redshifted up to 900 km s with respect to
the narrow emission lines. This reveals that the flare is associated with the
changing-look phenomenon from type 1.8 to type 1. Based on these findings, we
argue that the flare is likely to originate from an obscured tidal disruption
event, although extreme variation in the accretion rate may not be ruled out
completely.Comment: Accepted to Ap
Exploring the relationship between the spatial distribution of roads and universal pattern of travel-route efficiency in urban road networks
Urban road networks are well known to have universal characteristics and
scale-invariant patterns, despite the different geographical and historical
environments of cities. Previous studies on universal characteristics of the
urban road networks mostly have paid attention to their network properties but
often ignored the spatial networked structures. To fill the research gap, we
explore the underlying spatial patterns of road networks. In doing so, we
inspect the travel-route efficiency in a given road network across 70 global
cities which provides information on the usage pattern and functionality of the
road structure. The efficiency is quantified by the detour patterns of the
travel routes, estimated by the detour index (DI). The DI is a long-standing
popular measure, but its spatiality has been barely considered so far. In this
study, we probe the behavior of DI with respect to spatial variables by
scanning the network radially from a city center. Through empirical analysis,
we first discover universal properties in DI throughout most cities, which are
summarized as a constant behavior of DI regardless of the radial position from
a city center and clear collapse into a single curve for DIs for various radii
with respect to the angular distance. Especially, the latter enables us to know
the scaling factor in the length scale. We also reveal that the core-periphery
spatial structure of the roads induces the universal pattern, which is
supported by an artificial road network model. Furthermore, we visualize the
spatial DI pattern on the city map to figure out the city-specific
characteristics. The most and least efficient connections of several
representative cities show the potential for practical implications in
analyzing individual cities.Comment: 11 pages, 6 figure
Photometric Selection of Unobscured QSOs in the Ecliptic Poles: KMTNet in the South Field and Pan-STARRS in the North Field
We search for quasi-stellar objects (QSOs) in a wide area of the south
ecliptic pole (SEP) field, which has been and will continue to be intensively
explored through various space missions. For this purpose, we obtain deep
broadband optical images of the SEP field covering an area of
deg with the Korea Microlensing Telescope Network.
The 5 detection limits for point sources in the bands are
estimated to be 22.59, 22.60, 22.98, and 21.85 mag, respectively.
Utilizing data from Wide-field Infrared Survey Explorer, unobscured QSO
candidates are selected among the optically point-like sources using the
mid-infrared (MIR) and optical-MIR colors. To further refine our selection and
eliminate any contamination not adequately removed by the color-based
selection, we perform the spectral energy distribution fitting with archival
photometric data ranging from optical to MIR. As a result, we identify a total
of 2,383 unobscured QSO candidates in the SEP field. We also apply a similar
method to the north ecliptic pole field using the Pan-STARRS data and obtain a
similar result of identifying 2,427 candidates. The differential number count
per area of our QSO candidates is in good agreement with those measured from
spectroscopically confirmed ones in other fields. Finally, we compare the
results with the literature and discuss how this work will be implicated in
future studies, especially with the upcoming space missions.Comment: 14 pages, 9 figures, accepted for publication in ApJ
The Seoul National University AGN Monitoring Project. II. BLR Size and Black Hole Mass of Two AGNs
Active galactic nuclei (AGNs) show a correlation between the size of the broad line region and the monochromatic continuum luminosity at 5100 Å, allowing black hole mass estimation based on single-epoch spectra. However, the validity of the correlation is yet to be clearly tested for high-luminosity AGNs. We present the first reverberation mapping results of the Seoul National University AGN Monitoring Project (SAMP), which is designed to focus on luminous AGNs for probing the high end of the size–luminosity relation. We report time lag measurements of two AGNs, namely, 2MASS J10261389+5237510 and SDSS J161911.24+501109.2, using the light curves obtained over an ∼1000 days period with an average cadence of 10 and 20 days, respectively, for photometry and spectroscopy monitoring. Based on a cross-correlation analysis and Hβ line width measurements, we determine the Hβ lag as and days in the observed frame, and black hole mass as and , respectively, for 2MASS J1026 and SDSS J1619
Hα Reverberation Mapping of the Intermediate-mass Active Galactic Nucleus in NGC 4395
We present the results of a high-cadence spectroscopic and imaging monitoring campaign of the active galactic nucleus (AGN) of NGC 4395. High signal-to-noise-ratio spectra were obtained at the Gemini-N 8 m telescope using the GMOS integral field spectrograph (IFS) on 2019 March 7 and at the Keck I 10 m telescope using the Low-Resolution Imaging Spectrometer with slit masks on 2019 March 3 and April 2. Photometric data were obtained with a number of 1 m-class telescopes during the same nights. The narrow-line region (NLR) is spatially resolved; therefore, its variable contributions to the slit spectra make the standard procedure of relative flux calibration impractical. We demonstrate that spatially resolved data from the IFS can be effectively used to correct the slit-mask spectral light curves. While we obtained no reliable lag owing to the lack of a strong variability pattern in the light curves, we constrain the broad-line time lag to be less than 3 hr, consistent with the photometric lag of ∼80 minutes reported by Woo et al. By exploiting the high-quality spectra, we measure the second moment of the broad component of the Hα emission line to be 586 ± 19 km s−1, superseding the lower value reported by Woo et al. Combining the revised line dispersion and the photometric time lag, we update the black hole mass to (1.7 ± 0.3) × 104 M⊙