4,383 research outputs found
Optical Monitoring of the Seyfert Galaxy NGC 4151 and Possible Periodicities in the Historical Light Curve
We report B, V, and R band CCD photometry of the Seyfert galaxy NGC 4151
obtained with the 1.0-m telescope at Weihai Observatory of Shandong University
and the 1.56-m telescope at Shanghai Astronomical Observatory from 2005
December to 2013 February. Combining all available data from literature, we
have constructed a historical light curve from 1910 to 2013 to study the
periodicity of the source using three different methods (the Jurkevich method,
the Lomb-Scargle periodogram method and the Discrete Correlation Function
method). We find possible periods of P_1=4\pm0.1, P_2=7.5\pm0.3 and
P_3=15.9\pm0.3 yr.Comment: 8 pages, 5 figures, Accepted by Research in Astronomy and
Astrophysic
An Inherent Optical Properties Data Processing System for Achieving Consistent Ocean Color Products From Different Ocean Color Satellites
We used field measurements and multimission satellite data to evaluate how well an inherent optical properties (IOPs) data processing system performed at correcting the residual error of the atmospheric correction in satellite remote sensing reflectance (R-rs) and how well the system simultaneously minimized intermission biases between different remote sensing systems. We developed the IOPs data processing system as a semianalytical algorithm called IDAS. Our results show that IDAS generates accurate and consistent IOPs products from two ocean color missions: Sea-viewing Wide Field-of-View Sensor (SeaWiFS) and Moderate Resolution Imaging Spectroradiometer Aqua (MODISA). Specifically, with "high-quality" SeaWiFS and MODISA R-rs data, IDAS provided temporally consistent IOPs products for the oligotrophic open ocean resulting in an annual mean intermission difference of less than 3%, which is significantly lower than what a quasi-analytical algorithm (QAA) provided. We used IDAS to generate a long time series of b(b)(555) from the Northwest Atlantic Subtropical Gyre using SeaWiFS (1998 to 2002) and MODISA (2003 to 2017) images. Our results show that the IDAS-derived annual b(b)(555) decreased monotonically by 2.81% per decade from 1998 to 2017. Comparing the IDAS-generated annual trend for b(b)(555) to the same data processed with the QAA algorithm, we found that the QAA results differed because of impacts of the residual errors of the atmospheric correction and intermission biases. The differences in the annual trends existed despite the same temporal changing patterns of in situ particulate organic carbon existing in the Sargasso Sea and in the satellite chlorophyll-a concentration in the Northwest Atlantic Subtropical Gyre
Gate-tunable anomalous Hall effect in stacked van der Waals ferromagnetic insulator - topological insulator heterostructures
The search of novel topological phases, such as the quantum anomalous Hall
insulator (QAHI) or the axion insulator, has motivated different schemes to
introduce magnetism into topological insulators. One scheme is to introduce
ferromagnetic dopants in topological insulators. However, it is generally
challenging and requires carefully engineered growth/heterostructures or
relatively low temperatures to observe the QAHI due to issues such as the added
disorder with ferromagnetic dopants. Another promising scheme is using the
magnetic proximity effect with a magnetic insulator to magnetize the
topological insulator. Most of these heterostructures are synthesized so far by
growth techniques such as molecular beam epitaxy and metallic organic chemical
vapor deposition. These are not readily applicable to allow mixing and matching
many of the available ferromagnetic and topological insulators due to
difference in growth conditions and lattice mismatch. Here, we demonstrate that
the magnetic proximity effect can still be obtained in stacked heterostructures
assembled via the dry transfer of exfoliated micrometer-sized thin flakes of
van der Waals topological insulator and magnetic insulator materials
(BiSbTeSe2/Cr2Ge2Te6), as evidenced in the observation of an anomalous Hall
effect (AHE). Furthermore, devices made from these heterostructures can allow
modulation of the AHE when controlling the carrier density via electrostatic
gating. These results show that simple mechanical transfer of magnetic van der
Waals materials provides another possible avenue to magnetize topological
insulators by magnetic proximity effect, a key step towards further realization
of novel topological phases such as QAHI and axion insulators.Comment: 22 pages, 10 figure
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