Search

We report the discovery of PSR J1909+0122 by the Five-hundred-meter Aperture Spherical radio Telescope (FAST) as part of the Commensal Radio Astronomy FAST Survey (CRAFTS). PSR J1909+0122 has a spin period of 1.257 seconds, a dispersion measure (DM) of 186.2 pc cm-3. The averaged pulse profile shows two distinct components. We performed single-pulse study based on one-hour observation at 1.25 GHz on August 23rd, 2021. We used a threshold of 5 σep to measure the nulling fraction (NF) as 63% ± 1.5%. The longitude resolved fluctuation spectra (LRFS) and FFT spectra of the binary sequences revealed the 30-rotation-period quasi-periodicity of nulling. We examined the reliability of the periodicity by comparing it to random noise injection. The NF, E-dot, and modulation periodicity PM of PSR J1909+0122 were compared with other periodic nulling pulsars, showing the source of J1909+0122 has the second largest NF in the population. Long-term timing observations over six months were used to derive the phase-connected ephemeris of this pulsar. The measured P and P-dot values disfavor dipolar geometry for polar gap models, and the prediction for a space-charge limited flow model in the case of inverse Compton scattering is only just above the death line. In this work, PSR J1909+0122 has revealed possible correlations between nulling behavior and pulsar properties, which will help to shed light on the pulsar emission mechanism and its temporal evolution in future observations.

Small-signal modeling of grid-tied inverters (GTIs) is an essential work in stability analysis of GTIs-weak grid coupling system. Different from previous small-signal modeling methods of GTIs with black-box property, a reduced-order modeling method has been proposed in this paper to provide an impedance model with white-box property. Firstly, Impedance and admittance are seamlessly incorporated into the modeling method, where concepts of partial admittances, partial impedance, active power admittance and reactive power admittance are especially introduced. As a result, simplified expressions of impedance model of GTIs have been obtained with physical interpretations of dq impedances strengthened. After that, dominated relationships between terms of GTIs impedance matrix and its internal physical parts are explicitly revealed to represent white-box property of the proposed impedance model. Specifically, the influence and effect mechanism of feedforward voltage compensation (FVC), current controller (CC), phase lock loop (PLL) and power points (Id, Iq) are emphasized based on the proposed impedance model. Finally, perturbations generated independent with PLL are injected into a hardware in-loop experiment platform, and impedance measurement results are provided to verify the conclusions drawn from the improved modeling method.

Electrochemical water splitting is a facile and effective route to generate pure hydrogen and oxygen. However, the sluggish kinetics of hydrogen evolution reaction (HER) and especially oxygen evolution reaction (OER) hinder the water splitting efficiency. Meanwhile, the high-cost of noble-metal catalysts limit their actual application. It is thus highly urgent to exploit an economical and earth-abundant bifunctional HER and OER electrocatalyst to simplify procedure and reduce cost. Herein, we synthesize the three-dimensionally ordered macro-/mesoporous (3DOM/m) NixCo100-x alloys with distinctive structure and large surface area via a dual-templating technique. Among them, the 3DOM/m Ni61Co39 shows the lowest overpotentials of 121 mV and 241 mV at 10 mA/cm2 for HER and OER, respectively. Furthermore, when employed for water splitting, the Ni61Co39 only requires 1.60 V to approach 10 mA/cm2 and presents excellent stability. These encouraging performances of the Ni61Co39 render it a promising bifunctional catalyst for overall water splitting.

The polarization of fast radio bursts (FRBs), which are bright astronomical transient phenomena, contains information about their environments. Using wide-band observations with two telescopes, we report polarization measurements of five repeating FRBs and find a trend of lower polarization at lower frequencies. This behavior is modeled as multipath scattering, characterized by a single parameter, σ RM , the rotation measure (RM) scatter. Sources with higher σ RM have higher RM magnitude and scattering time scales. The two sources with the highest σ RM , FRB 20121102A and FRB 20190520B, are associated with compact persistent radio sources. These properties indicate a complex environment near the repeating FRBs, such as a supernova remnant or a pulsar wind nebula, consistent with their having arisen from young stellar populations.

Fast radio bursts (FRBs) are brief, intense flashes of radio waves from unidentified extragalactic sources. Polarized FRBs originate in highly magnetized environments. We report observations of the repeating FRB 20190520B spanning seventeen months , which show its amount of Faraday rotation is highly variable and twice changes its sign. The FRB also depolarizes below radio frequencies around 1 to 3 GHz. We interpret these properties as due to change in the parallel component of the integrated magnetic field along the line-of-sight, including reversals. This could result from propagation through a turbulent, magnetized screen of plasma located between $10^{-5}$ to 100 parsecs of the FRB source. This is consistent with the bursts passing through the stellar wind of a binary companion of the FRB source., Comment: arXiv:2203.08151 has been merged into this

The Tianlai Dish Pathfinder Array is a radio interferometer array consisting of 16 six-meter dish antennas. The original digital backend integration time is at the seconds level, designed for H i intensity mapping experiment. A new digital backend with millisecond response is added to enable it to search for fast radio burst during its observations. The design and calibration of this backend, and the real time search pipeline for it are described in this paper. It is capable of forming 16 digital beams for each linear polarization, covering an area of 19.6 square degrees. The search pipeline is capable of searching for, recording and classifying FRBs automatically in real time. In commissioning, we succeeded in capturing the signal pulses from the pulsars PSR B0329+54 and B2021+51.

The possibility that primordial black holes (PBHs) form a part of dark matter has been considered for a long time but poorly constrained over a wide mass range. Fast radio bursts (FRBs) are bright radio transients with millisecond duration. Lensing effect of them has been proposed as one of the cleanest probes for constraining the presence of PBHs in the stellar mass window. In this paper, we first apply the normalised cross-correlation algorithm to search and identify candidates of lensed FRBs in the latest public FRB observations, i.e. $593$ FRBs which mainly consist of the first Canadian Hydrogen Intensity Mapping Experiment FRB catalog, and then derive constraints on the abundance of PBHs from the null search result of lensing signature. For a monochromatic mass distribution, the fraction of dark matter made up of PBHs could be constrained to $\leq87\%$ for $\geq500~M_{\odot}$ at 95\% confidence level by assuming flux ratio thresholds dependent signal-to-noise ratio for each FRB and that apparently one-off events are intrinsic single bursts. This result would be improved by a three times factor when a conventional constant flux ratio threshold is considered. Moreover, we derive constraints on PBHs with a log-normal mass function naturally predicted by some popular inflation models and often investigated with gravitational wave detections. We find that, in this mass distribution scenario, the constraint from currently public FRB observations is relatively weaker than the one from gravitational wave detections. It is foreseen that upcoming complementary multi-messenger observations will yield considerable constraints on the possibilities of PBHs in this intriguing mass window., 13 Pages, 9 Figures, 2 Tables, Accepted by ApJ. arXiv admin note: text overlap with arXiv:2103.08510

The event rate, energy distribution, and time-domain behaviour of repeating fast radio bursts (FRBs) contains essential information regarding their physical nature and central engine, which are as yet unknown. As the first precisely-localized source, FRB 121102 has been extensively observed and shows non-Poisson clustering of bursts over time and a power-law energy distribution. However, the extent of the energy distribution towards the fainter end was not known. Here we report the detection of 1652 independent bursts with a peak burst rate of 122~hr^{-1}, in 59.5 hours spanning 47 days. A peak in the isotropic equivalent energy distribution is found to be ~4.8 x 10^{37} erg at 1.25~GHz, below which the detection of bursts is suppressed. The burst energy distribution is bimodal, and well characterized by a combination of a log-normal function and a generalized Cauchy function. The large number of bursts in hour-long spans allow sensitive periodicity searches between 1 ms and 1000 s. The non-detection of any periodicity or quasi-periodicity poses challenges for models involving a single rotating compact object. The high burst rate also implies that FRBs must be generated with a high radiative efficiency, disfavoring emission mechanisms with large energy requirements or contrived triggering conditions., 69 pages, 14 figures, 2 tables; Updates for author correction

Fast radio bursts (FRBs) are cosmic sources that emit millisecond-duration radio pulses with a wide range of luminosities and yet unknown origin(s) (Petroff et al. 2019; cordes et al. 2019). A subset of FRBs were found to repeat, the prototype of which is the first precisely-located FRB 121102 (Spitler et al. 2016), residing in a dwarf galaxy at redshift z=0.193 (Chatterjee 2017; Tendulkar et al. 2017). The source has been observed by most major telescopes and shows non-Poisson clustering of bursts over time, the hitherto highest burst rate, and a burst isotropic equivalent energy largely consistent with a power-law (Law et al. 2017; zhang et al. 2018; Gourdji et al. 2019), all of which are crucial characteristics to be compared to non-repeating sources. However, due to sensitivity limits, no true energy distribution of any FRB is known. Here we report the detection of 1652 independent bursts, more than quadruple the total of all previously published ones combined, in a total of 59.5 observing hours spanning 47 days using the Five-hundred-meter Aperture Spherical radio Telescope (FAST). The peak burst rate of 122 hr-1is by far the highest ever observed of any FRB. A characteristic peak in the isotropic equivalent energy distribution is found to be ~4.8×1037 erg at 1.25 GHz, suggesting a possible threshold for producing abundant coherent radio bursts from FRBs. The burst energy distribution is optimally described by a bimodal distribution consisting of a log-normal function plus a Cauchy function. While no periodicity was found between 1 ms and 1000 s, and the majority of the burst arrival times are consistent with being random, there exists a visible peak in the waiting time distribution at about 3.4 ms, corresponding to significant clustering. Our results start to reveal the stochastic nature of abundant weaker bursts, which could be present in other FRB sources, apparently repeating or not. FRB generation mechanisms must be efficient and economical. Expensive triggers and/or contrived conditions for burst production seem unlikely.

The Tianlai Cylinder Pathfinder is a radio interferometer array designed to test techniques for 21 cm intensity mapping in the post-reionization Universe, with the ultimate aim of mapping the large scale structure and measuring cosmological parameters such as the dark energy equation of state. Each of its three parallel cylinder reflectors is oriented in the north-south direction, and the array has a large field of view. As the Earth rotates, the northern sky is observed by drift scanning. The array is located in Hongliuxia, a radio-quiet site in Xinjiang, and saw its first light in September 2016. In this first data analysis paper for the Tianlai cylinder array, we discuss the sub-system qualification tests, and present basic system performance obtained from preliminary analysis of the commissioning observations during 2016-2018. We show typical interferometric visibility data, from which we derive the actual beam profile in the east-west direction and the frequency band-pass response. We describe also the calibration process to determine the complex gains for the array elements, either using bright astronomical point sources, or an artificial on site calibrator source, and discuss the instrument response stability, crucial for transit interferometry. Based on this analysis, we find a system temperature of about 90 K, and we also estimate the sensitivity of the array., 28 pages, 30 figures

We report the discovery of a highly dispersed fast radio burst, FRB~181123, from an analysis of $\sim$1500~hr of drift-scan survey data taken using the Five-hundred-meter Aperture Spherical radio Telescope (FAST). The pulse has three distinct emission components, which vary with frequency across our 1.0--1.5~GHz observing band. We measure the peak flux density to be $>0.065$~Jy and the corresponding fluence $>0.2$~Jy~ms. Based on the observed dispersion measure of 1812~cm$^{-3}$~pc, we infer a redshift of $\sim 1.9$. From this, we estimate the peak luminosity and isotropic energy to be $\lesssim 2\times10^{43}$~erg~s$^{-1}$ and $\lesssim 2\times10^{40}$~erg, respectively. With only one FRB from the survey detected so far, our constraints on the event rate are limited. We derive a 95\% confidence lower limit for the event rate of 900 FRBs per day for FRBs with fluences $>0.025$~Jy~ms. We performed follow-up observations of the source with FAST for four hours and have not found a repeated burst. We discuss the implications of this discovery for our understanding of the physical mechanisms of FRBs., 7 pages, 3 figures, accepted for publication in ApJL

The Tianlai Dish Pathfinder Array is a radio interferometer designed to test techniques for 21~cm intensity mapping in the post-reionization universe as a means for measuring large-scale cosmic structure. It performs drift scans of the sky at constant declination. We describe the design, calibration, noise level, and stability of this instrument based on the analysis of about $\sim 5 \%$ of 6,200 hours of on-sky observations through October, 2019. Beam pattern determinations using drones and the transit of bright sources are in good agreement, and compatible with electromagnetic simulations. Combining all the baselines, we make maps around bright sources and show that the array behaves as expected. A few hundred hours of observations at different declinations have been used to study the array geometry and pointing imperfections, as well as the instrument noise behaviour. We show that the system temperature is below 80~K for most feed antennas, and that noise fluctuations decrease as expected with integration time, at least up to a few hundred seconds. Analysis of long integrations, from 10 nights of observations of the North Celestial Pole, yielded visibilities with amplitudes of 20-30~mK, consistent with the expected signal from the NCP radio sky with $, 30 pages, 38 figures

The digital correlator is a crucial element in a modern radio telescope. In this paper we describe a scalable design of the correlator system for the Tianlai pathfinder array, which is an experiment dedicated to test the key technologies for conducting 21cm intensity mapping survey. The correlator is of the FX design, which firstly performs Fast Fourier Transform (FFT) including Polyphase Filter Bank (PFB) computation using a Collaboration for Astronomy Signal Processing and Electronics Research (CASPER) Reconfigurable Open Architecture Computing Hardware-2 (ROACH2) board, then computes cross-correlations using Graphical Processing Units (GPUs). The design has been tested both in laboratory and in actual observation., 11 page, 10 figures

Fast radio burst (FRB) is an occasional and instantaneous radio burst. In order to detect more samples for research, this paper develops a real-time search digital backend based on multi-beam receiver. The system includes data acquisition and processing, single-beam signal processing, multi-beam signal synthesis and analysis, detection results storage and distribution. It can collect and analyze multi-beam signals simultaneously. Multi-beam signal reference effect is used to eliminate radio frequency interference and improve detection accuracy. Observation experiments were carried out on a radio telescope. The function of FRB detection was tested by capturing the single pulse signal of a pulsar. The result shows that the digital backend can achieve accurate and real-time FRB detection.

In this paper, main different grid-connected photovoltaic (PV) standard systems, included IEC system, western country system, American system and China system have been summarized, analyzed and compared. Based on comparison between different standard systems, relative merits, existing difference, and reasons of difference have been analyzed in gird-connected PV standard system. Furthermore, combined with relevant PV generation policy and technology developing status, the prospect analyze has been proposed for future grid-connected PV standard system development in each system.

With the rapid increasing of solar power installed capacity, the utility has higher requirement to the Low voltage ride-through (LVRT) capability of grid-connected PV power station. The PV power station is expected to continue operation during the fault, especially the voltage drop of the grid. Therefore, the effective on-site test platform has been desired and needed thereby matching latest standards not only in China but also in the worldwide. In this paper, based on latest standards, one suitable and convenience on-site test platform and one on-site testing technique for Medium and large PV power plant is proposed and introduced. Meanwhile, the corresponding simulation result is verified the effectiveness and practicality.

This article provides a specially designed low voltage ride-through (LVRT) detection device, which uses the transformer tap switch to not only switch the taps of the reactors but also change the structure of the main circuit structure. The cost of this set of design solutions is only 10% of traditional testing device. This set of design can reduce the using number of reactors, increase testing device reliability, and improve the detection efficiency.

This article is a comparative assessment of grid-connected PV system standards between China, the U.S and Europe, analysis of the difference between the various standards. On the one hand, to provide a reference with the standards preparation and improvement of grid-connected PV system in China; on the other hand, to increase the understanding of Chinese grid-connected PV system standards, and to promote the common progress of the international PV standards.

This paper presents a miniature RF MEMS (Micro—Electro-Mechanical System) switch design optimized for the high residual stress and stress gradient available in a thin metal layer process. The switch demonstrates a stress gradient tolerance of ±100 MPa/µm, with

For large-scale photovoltaic power station connected to medium-voltage grid, it is not proper to evaluate its power quality by traditional limitation in China. In this paper, it proposes testing technology and research in order to evaluate the power quality for large-scale photovoltaic power station according to latest photovoltaic standards. The tests will focus on flicker, harmonics, inter-harmonics and high frequency components. For voltage flicker under either continuous operation or switching operation, it gives one method which simulates the voltage fluctuation imposed by photovoltaic power station and calculates flicker emission on a fictitious grid, thereby avoiding the direct influence of the real voltage fluctuation of the grid at the measurement point on flicker. For current harmonics, interharmonics and high frequency components over continuous operation, it specifies the individual current components as sub grouped values in accordance with IEC 61000-4-7][1]. For the purpose of including the effect of voltage fluctuations during actual measurement, the choice of sub grouping method is applied.

A new type LVRT test device, basing on power electronics technology, has been developed. Compared with traditional reactor VGS, this device overcomes many drawbacks, such as bulky size, difficult to control precision, and difficult to continuously testing. Considering the characteristics of distributed system accessing to the grid by photovoltaic power station, the designed testing platform has the capability of flexible and mobile performance. In this paper, working principle, hardware and software structures, and features have been introduced. Meanwhile, Testing verification and data analysis also have been conducted in laboratory. Based on those procedures, the device has been verified for having excellent effectiveness and practicality. Over that period, the device has been applied and tested on site in the Qinghai, which had got the ideal result as well as in laboratory.

In recent years, Chinese government has published many subsidy policies for photovoltaic power, such as "BIPV" "Golden Sun Demonstration Projecti" and "Bidding for Concession". Meanwhile, several local governments also published their own subsidy policies to increase the investment subsidies or to publish electricity price in tariff for photovoltaic power projects. The publishing of these policies have been promoting the rapid growth in total amount of installed photovoltaic power. Furthermore, PV power permeability has reached an upper value in some region. The instability, random city, and upper permeability acting on PV power, bring serious influence to safe and stable operation of the grid in China. Therefore, State Grid Corporation of China (SGCC) has published two enterprise standards, Q/GDW 617-2011 "Technical specifications for photovoltaic power station connected to the grid" and Q/GDW 618-2011 "Testing procedures for photovoltaic power station connected to the grid", which specifies the prospective technical parameters at different voltage rating, capacity and grid-connected types, and required testing items and methods. Focused on grid connected PV power station, the standards specifies the requirement of testing parameters, including active power control, voltage/reactive power regulation, response to abnormal voltage/frequency, power quality, low voltage ride through (LVRT), anti-islanding protection, back-feed protection, reconnection, lightening & ear thing protection, electromagnetic compatibility, surge immunity, electric energy metering, and communication & signal. In this article, it will describe those two standards in details, and analyse specific parameter and established basis required by each technical clause.