Imaging and variability studies of CTA~102 during the 2016 January γ\gamma-ray flare

The $\gamma$-ray bright blazar CTA 102 is studied using imaging (new 15 GHz and archival 43 GHz Very Long Baseline Array, VLBA data) and time variable optical flux density, polarization degree and electric vector position angle (EVPA) spanning between 2015 June 1 and 2016 October 1, covering a prominent $\gamma$-ray flare during 2016 January. The pc-scale jet indicates expansion with oscillatory features upto 17 mas. Component proper motions are in the range 0.04 - 0.33 mas/yr with acceleration upto 1.2 mas followed by a slowing down beyond 1.5 mas. A jet bulk Lorentz factor $\geq$ 17.5, position angle of 128.3 degrees, inclination angle $\leq$ 6.6 degrees and intrinsic half opening angle $\leq$ 1.8 degrees are derived from the VLBA data. These inferences are employed in a helical jet model to infer long term variability in flux density, polarization degree, EVPA and a rotation of the Stokes Q and U parameters. A core distance of $r_{\rm core,43 \ GHz}$ = 22.9 pc, and a magnetic field strength at 1 pc and the core location of 1.57 G and 0.07 G respectively are inferred using the core shift method. The study is useful in the context of estimating jet parameters and in offering clues to distinguish mechanisms responsible for variability over different timescales.Comment: 20 pages, 7 figures, 3 tables; accepted for publication in Ap

SecureAD: A secure video anomaly detection framework on convolutional neural network in edge computing environment

National Research Foundation (NRF) Singapore under Strategic Capability Research Centres Funding Intiatives; Ministry of Education, Singapore under its Academic Research Funding Tier

Pyrite-Type CoS2 Nanoparticles Supported on Nitrogen-Doped Graphene for Enhanced Water Splitting

It is extremely meaningful to develop cheap, highly efficient, and stable bifunctional electrocatalysts for both hydrogen and oxygen evolution reactions (HER and OER) to promote large-scale application of water splitting technology. Herein, we reported the preparation of CoS2 nanoparticles supported on nitrogen-doped graphene (CoS2@N-GN) by one-step hydrothermal method and the enhanced electrochemical efficacy for catalyzing hydrogen and oxygen in water electrolysis. The CoS2@N-GN composites are composed of nitrogen-doped graphene and CoS2 nanocrystals with the average size of 73.5 nm. Benefitting from the improved electronic transfer and synergistic effect, the as-prepared CoS2@N-GN exhibits remarkable OER and HER performance in 1.0 M KOH, with overpotentials of 243 mV for OER and 204 mV for HER at 10 mA cm−2, and the corresponding Tafel slopes of 51.8 and 108 mV dec−1, respectively. Otherwise, the CoS2@N-GN hybrid also presents superior long-term catalytic durability. Moreover, an alkaline water splitting device assembled by CoS2@N-GN as both anode and cathode can achieve a low cell voltage of 1.53 V at 60 °C with a high faraday efficiency of 100% for overall water splitting. The tremendously enhanced electrochemical behaviors arise from favorable factors including small sized, homogenously dispersed novel CoS2 nanocrystals and coupling interaction with the underlying conductive nitrogen-doped graphene, which would provide insight into the rational design of transition metal chalcogenides for highly efficient and durable hydrogen and oxygen-involved electrocatalysis

Toward Microarcsecond Astrometry for the Innermost Wobbling Jet of the BL Lacertae Object OJ 287

The BL Lacertae object OJ 287 is a very unusual quasar producing a wobbling radio jet and some double-peaked optical outbursts with a possible period of about 12 yr for more than one century. This variability is widely explained by models of binary supermassive black hole (SMBH) or precessing jet/disk from a single SMBH. To enable an independent and nearly bias-free investigation on these possible scenarios, we explored the feasibility of extremely high-precision differential astrometry on its innermost restless jet at mm-wavelengths. Through re-visiting some existing radio surveys and very long baseline interferometry (VLBI) data at frequencies from 1.4 to 15.4 GHz and performing new Very Long Baseline Array (VLBA) observations at 43.2 GHz, we find that the radio source J0854$+$1959, 7.1 arcmin apart from OJ 287 and no clearly-seen optical and infrared counterparts, could provide a nearly ideal reference point to track the complicated jet activity of OJ 287. The source J0854$+$1959 has a stable GHz-peaked radio spectrum and shows a jet structure consisting of two discrete, mas-scale-compact and steep-spectrum components and showing no proper motion over about 8 yr. The stable VLBI structure can be interpreted by an episodic, optically thin and one-sided jet. With respect to its 4.1-mJy peak feature at 43.2 GHz, we have achieved an astrometric precision at the state-of-art level, about 10 $\mu$as. These results indicate that future VLBI astrometry on OJ 287 could allow us to accurately locate its jet apex and activity boundary, align its restless jet structure over decades without significant systematic bias, and probe various astrophysical scenarios.Comment: 10 pages, 3 figures, 2 tables, accepted for publication in Astrophysical Journal Letter

High-precision adaptive slope compensation circuit for DC-DC converter in wearable devices

This paper presents a high precision adaptive slope compensation circuit for for DC-DC converter in wearable devices. Compared with the traditional adaptive slope compensation circuit, the comparator is used to sample the output voltage and input voltage, which greatly improves the accuracy.In this paper, the circuit is designed in UMC 0.18-μm CMOS Technology and verified by Virtuoso Spectre Circuit Simulator. The simulation results show that the accuracy of the adaptive slope compensation circuit in this paper can reach more than 96%

VLBI observations of a sample of Palomar-Green quasars - I. Parsec-scale morphology

We observed 20 Palomar-Green quasars at low redshi ft (z < 0.5) with total flux density >1 mJy, including four radio-loud quasars (RLQs) and 16 radio-quiet quasars (RQQs), using the Very Long Baseline Array (VLBA) at 5 GHz. 10 RQQs are clearly detected in the VLBA images, and a compact radio core is identified in eight of them, indicating the prevalence of active galactic nucleus (AGN)-related radio emission in this flux-density-limited RQQ sample. The RQQs and RLQs in our sample have a division at similar to 30 mJy. The radio emission from RQQs appears to be the result of a combination of star formation and AGN-associated activities. All RQQs in our sample have a 5 GHz flux density ratio of Very Large Array (VLA) A-array to D-array f(c) = S-A(VLA)/S-D(VLA) above 0.2. The RQQs with f(a)(VLBA and VLA flux density ratio S-VLBA/S-A(VLA)) > 0.2 versus f(a) < 0.2 show significant differences in morphology, compactness, and total flux density. f(a) of RQQs is systematically lower than that of RLQs, probably due to the extended jets or relic jets of RQQs on tens to hundreds parsecs that are resolved out in VLBA images. Future larger samples, especially with the addition of milliarcsec resolution radio images of RQQs with total flux densities below 1 mJy, can test the conclusions of this paper and contribute to the understanding of the radio emission mechanism of RQQs, and the dichotomy and physical connection between RQQs and RLQs

VLBI Observations of a sample of Palomar-Green quasars II: characterising the parsec-scale radio emission

This study uses multi-frequency Very Long Baseline Interferometry (VLBI) to study the radio emission from 10 radio-quiet quasars (RQQs) and four radio-loud quasars (RLQs). The diverse morphologies, radio spectra, and brightness temperatures observed in the VLBI images of these RQQs, together with the variability in their GHz spectra and VLBI flux densities, shed light on the origins of their nuclear radio emission. The total radio emission of RQQs appears to originate from non-thermal synchrotron radiation due to a combination of active galactic nuclei and star formation activities. However, our data suggest that the VLBI-detected radio emission from these RQQs is primarily associated with compact jets or corona, with extended emissions such as star formation and large-scale jets being resolved by the high resolution of the VLBI images. Wind emission models are not in complete agreement the VLBI observations. Unlike RLQs, where the parsec-scale radio emission is dominated by a relativistically boosted core, the radio cores of RQQs are either not dominant or are mixed with significant jet emission. RQQs with compact cores or core-jet structures typically have more pronounced variability, with flat or inverted spectra, whereas jet-dominated RQQs have steep spectra and unremarkable variability. Future high-resolution observations of more RQQs could help to determine the fraction of different emission sources and their associated physical mechanisms.Comment: This paper has been accepted by MNRA