arXiv subjects

Zhichen Pan

Publications and source records attributed to Zhichen Pan.

53 records · Page 3Linked to original sources

arXiv2020

The FAST discovery of an Eclipsing Binary Millisecond Pulsar in the Globular Cluster M92 (NGC 6341)

We report the discovery of an eclipsing binary millisecond pulsar in the globular cluster M92 (NGC6341) with the Five-hundred-meter Aperture Spherical radio Telescope (FAST). PSR J1717+4308A, or M92A, has a pulse frequency of 316.5~Hz (3.16~ms) and a dispersion measure of 35.45 pc cm$^{-3}$. The pulsar is a member of a binary system with an orbital period of 0.20~days around a low-mass companion which has a median mass of $\sim$0.18~\Ms. From observations so far, at least two eclipsing events have been observed in each orbit. The longer one lasted for ~5000~s in the orbital phase range 0.1--0.5. The other lasted for ~500~s and occurred between 1000--2000~s before or after the longer eclipsing event. The lengths of these two eclipsing events also change. These properties suggest that J1717+4308A is a ``red-back'' system with a low-mass main sequence or sub-giant companion. Timing observations of the pulsar and further searches of the data for additional pulsars are ongoing.

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arXiv2019

A PRESTO-based Parallel Pulsar Search Pipeline Used for FAST Drift Scan Data

We developed a pulsar search pipeline based on PRESTO (PulsaR Exploration and Search Toolkit). This pipeline simply runs dedispersion, FFT (Fast Fourier Transformation), and acceleration search in process-level parallel to shorten the processing time. With two parallel strategies, the pipeline can highly shorten the processing time in both the normal searches or acceleration searches. This pipeline was first tested with PMPS (Parkes Multibeam Pulsar Survery) data and discovered two new faint pulsars. Then, it was successfully used in processing the FAST (Five-hundred-meter Aperture Spherical radio Telescope) drift scan data with tens of new pulsar discoveries up to now. The pipeline is only CPU-based and can be easily and quickly deployed in computing nodes for testing purposes or data processes.

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arXiv2019

A Hybrid Ensemble method for Pulsar Candidate Classification

In this paper, three ensemble methods: Random Forest, XGBoost, and a Hybrid Ensemble method were implemented to classify imbalanced pulsar candidates. To assist these methods, tree models were used to select features among 30 features of pulsar candidates from references. The skewness of the integrated pulse profile, chi-squared value for sine-squared fit to amended profile and best S/N value play important roles in Random Forest, while the skewness of the integrated pulse profile is one of the most significant features in XGBoost. More than 20 features were selected by their relative scores and then applied in three ensemble methods. In the Hybrid Ensemble method, we combined Random Forest and XGBoost with EasyEnsemble. By changing thresholds, we tried to make a trade-off between Recall and Precision to make them approximately equal and as high as possible. Experiments on HTRU 1 and HTRU 2 datasets show that the Hybrid Ensemble method achieves higher Recall than the other two algorithms. In HTRU 1 dataset, Recall, Precision, and F-Score of the Hybrid Ensemble method are $0.967$, $0.971$, and $0.969$, respectively. In HTRU 2 dataset, the three values of that are $0.920$, $0.917$, and $0.918$, respectively.

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arXiv2019

Pulsar Candidates Classification with Deep Convolutional Neural Networks

As performance of dedicated facilities continually improved, massive pulsar candidates are being received, which makes selecting valuable pulsar signals from candidates challenging. In this paper, we designed a deep convolutional neural network (CNN) with 11 layers for classifying pulsar candidates. Compared to artificial designed features, CNN chose sub-integrations plot and sub-bands plot in each candidate as inputs without carrying biases. To address the imbalanced problem, data augmentation method based on synthetic minority samples is proposed according to characteristics of pulsars. The maximum pulses of pulsar candidates were first translated to the same position, then new samples were generated by adding up multiple subplots of pulsars. The data augmentation method is simple and effective for obtaining varied and representative samples which keep pulsar characteristics. In the experiments on HTRU 1 dataset, it shows that this model can achieve recall as 0.962 while precision as 0.963.

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arXiv2019

PSR J1926-0652: A Pulsar with Interesting Emission Properties Discovered at FAST

We describe PSR J1926-0652, a pulsar recently discovered with the Five-hundred-meter Aperture Spherical radio Telescope (FAST). Using sensitive single-pulse detections from FAST and long-term timing observations from the Parkes 64-m radio telescope, we probed phenomena on both long and short time scales. The FAST observations covered a wide frequency range from 270 to 800 MHz, enabling individual pulses to be studied in detail. The pulsar exhibits at least four profile components, short-term nulling lasting from 4 to 450 pulses, complex subpulse drifting behaviours and intermittency on scales of tens of minutes. While the average band spacing P3 is relatively constant across different bursts and components, significant variations in the separation of adjacent bands are seen, especially near the beginning and end of a burst. Band shapes and slopes are quite variable, especially for the trailing components and for the shorter bursts. We show that for each burst the last detectable pulse prior to emission ceasing has different properties compared to other pulses. These complexities pose challenges for the classic carousel-type models.

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arXiv2019

The First Pulsar Discovered by FAST

To assist with the commissioning (Jiang et al. 2019) of the Five-hundred-meter Aperture Spherical radio Telescope (FAST), we performed a pulsar search, with the primary goal of developing and testing the pulsar data acquisition and processing pipelines. We tested and used three pipelines, two (P1 and P2 hereafter) searched for the periodic signature of pulsars whereas the other one was used to search for bright single pulses (P3 hereafter). A pulsar candidate was discovered in the observation on the 22nd August, 2017, and later confirmed by the Parkes radio telescope on the 10th September, 2017. This pulsar, named PSR J1900-0134, was the first pulsar discovered by FAST. The pulsar has a pulse period of 1.8 s and a dispersion measure (DM) of 188\,pc\,cm$^{-3}$.

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arXiv2019

Pulsar Candidate Selection Using Ensemble Networks for FAST Drift-Scan Survey

The Commensal Radio Astronomy Five-hundred-meter Aperture Spherical radio Telescope (FAST) Survey (CRAFTS) utilizes the novel drift-scan commensal survey mode of FAST and can generate billions of pulsar candidate signals. The human experts are not likely to thoroughly examine these signals, and various machine sorting methods are used to aid the classification of the FAST candidates. In this study, we propose a new ensemble classification system for pulsar candidates. This system denotes the further development of the pulsar image-based classification system (PICS), which was used in the Arecibo Telescope pulsar survey, and has been retrained and customized for the FAST drift-scan survey. In this study, we designed a residual network model comprising 15 layers to replace the convolutional neural networks (CNNs) in PICS. The results of this study demonstrate that the new model can sort >96% of real pulsars to belong the top 1% of all candidates and classify >1.6 million candidates per day using a dual--GPU and 24--core computer. This increased speed and efficiency can help to facilitate real-time or quasi-real-time processing of the pulsar-search data stream obtained from CRAFTS. In addition, we have published the labeled FAST data used in this study online, which can aid in the development of new deep learning techniques for performing pulsar searches.

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arXiv2018

Studies of Turbulence Dissipation in Taurus Molecular Cloud with Core Velocity Dispersion (CVD)

Turbulence dissipation is an important process affecting the energy balance in molecular clouds, the birth place of stars. Previously, the rate of turbulence dissipation is often estimated with semi-analytic formulae from simulation. Recently we developed a data analysis technique called core-velocity-dispersion (CVD), which, for the first time, provides direct measurements of the turbulence dissipation rate in Taurus, a star forming cloud. The thus measured dissipation rate of $(0.45\pm 0.05)\times 10^{33} {\rm erg\ s^{-1}}$ is similar to those from dimensional analysis and also consistent with the previous energy injection rate based on molecular outflows and bubbles.

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arXiv2018

Considerations for a Multi-beam Multi-purpose Survey with FAST

Having achieved 'first-light' right before the opening ceremony on September 25, 2016, the Five-hundred-meter Aperture Spherical radio Telescope (FAST) is being busily commissioned. Its innovative design requires ~1000 points to be measured and driven instead of just the two axes of motion, e.g. Azimuth and Elevation for most of the conventional antennae, to realize pointing and tracking. We have devised a survey plan to utilized the full sensitivity of FAST, while minimizing the complexities in operation the system. The 19-beam L band focal plan array will be rotated to specific angles and taking continuous data streams while the surface shape and the focal cabin stay fixed. Such a survey will cover the northern sky in about 220 full days. Our aim is to obtain data for pulsar search, HI (neutral hydrogen) galaxies, HI imaging, and radio transients, simultaneously, through multiple backends. These data sets could be a significant contribution to all related fields in radio astronomy and remain relevant for decades.

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arXiv2018

Where is OH and Does It Trace the Dark Molecular Gas (DMG)?

Hydroxyl (OH) is expected to be abundant in diffuse interstellar molecular gas as it forms along with $H_2$ under similar conditions and within a similar extinction range. We have analyzed absorption measurements of OH at 1665 MHz and 1667 MHz toward 44 extragalactic continuum sources, together with the J=1-0 transitions of $^{12}$CO, $^{13}$CO , and C$^{18}$O, and the J=2-1 of $^{12}$CO. The excitation temperature of OH were found to follow a modified log-normal distribution, $ f(T{\rm_{ex}}) \propto \frac{1}{ \sqrt{2π}σ} \rm{exp}\left[-\frac{[ln(\textit{T}_{ex})-ln(3.4\ K)]^2}{2σ^2}\right] $, the peak of which is close to the temperature of the Galactic emission background (CMB+synchron). In fact, 90% of the OH has excitation temperature within 2 K of the Galactic background at the same location, providing a plausible explanation for the apparent difficulty to map this abundant molecule in emission. The opacities of OH were found to be small and peak around 0.01. For gas at intermediate extinctions (A$\rm_V$ $\sim$ 0.05-2 mag), the detection rate of OH with detection limit $N(\mathrm{OH})\simeq 10^{12}$ cm$^{-2}$ is approximately independent of $A\rm_V$. We conclude that OH is abundant in the diffuse molecular gas and OH absorption is a good tracer of `dark molecular gas (DMG)'. The measured fraction of DMG depends on assumed detection threshold of the CO data set. The next generation of highly sensitive low frequency radio telescopes, FAST and SKA, will make feasible the systematic inventory of diffuse molecular gas, through decomposing in velocity the molecular (e.g. OH and CH) absorption profiles toward background continuum sources with numbers exceeding what is currently available by orders of magnitude.

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arXiv2017

Large-Scale Spectroscopic Mapping of the $ρ$ Ophiuchi Molecular Cloud Complex I. The C$_{2}$H to N$_2$H$^+$ Ratio as a Signpost of Cloud Characteristics

We present 2.5-square-degree C$_{2}$H N=1-0 and N$_2$H$^+$ J=1-0 maps of the $ρ$ Ophiuchi molecular cloud complex. These are the first large-scale maps of the $ρ$ Ophiuchi molecular cloud complex with these two tracers. The C$_{2}$H emission is spatially more extended than the N$_2$H$^+$ emission. One faint N$_2$H$^+$ clump Oph-M and one C$_{2}$H ring Oph-RingSW are identified for the first time. The observed C$_{2}$H to N$_{2}$H$^{+}$ abundance ratio ([C$_{2}$H]/[N$_{2}$H$^{+}$]) varies between 5 and 110. We modeled the C$_{2}$H and N$_2$H$^+$ abundances with 1-D chemical models which show a clear decline of [C$_2$H]/[N$_2$H$^+$] with chemical age. Such an evolutionary trend is little affected by temperatures when they are below 40 K. At high density (n$_H$ $>$ 10$^5$ cm$^{-3}$), however, the time it takes for the abundance ratio to drop at least one order of magnitude becomes less than the dynamical time (e.g., turbulence crossing time $\rm \sim$10$^5$ years). The observed [C$_2$H]/[N$_2$H$^+$] difference between L1688 and L1689 can be explained by L1688 having chemically younger gas in relatively less dense regions. The observed [C$_{2}$H]/[N$_{2}$H$^{+}$] values are the results of time evolution, accelerated at higher densities. For the relative low density regions in L1688 where only C$_2$H emission was detected, the gas should be chemically younger.

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arXiv2016

The Five-hundred-meter Aperture Spherical Radio Telescope (FAST) Project

The Five-hundred-meter Aperture Spherical radio Telescope (FAST) is a Chinese mega-science project funded by the National Development and Reform Commission (NDRC) of the People's Republic of China. The National Astronomical Observatories of China (NAOC) is in charge of its construction and subsequent operation. Upon its expected completion in September 2016, FAST will surpass the 300-meter Arecibo Telescope and the 100-meter Green Bank Telescope in terms of absolute sensitivity in the 70 MHz to 3 GHz bands. In this paper, we report on the project, its current status, the key science goals, and plans for early science.

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arXiv2016

Physical properties of CO-dark molecular gas traced by C$^+$

Neither HI nor CO emission can reveal a significant quantity of so-called dark gas in the interstellar medium (ISM). It is considered that CO-dark molecular gas (DMG), the molecular gas with no or weak CO emission, dominates dark gas. We identified 36 DMG clouds with C$^+$ emission (data from Galactic Observations of Terahertz C+ (GOT C+) project) and HINSA features. Based on uncertainty analysis, optical depth of HI $τ\rm_{HI}$ of 1 is a reasonable value for most clouds. With the assumption of $τ\rm_{HI}=1$, these clouds were characterized by excitation temperatures in a range of 20 K to 92 K with a median value of 55 K and volume densities in the range of $6.2\times10^1$ cm$^{-3}$ to $1.2\times 10^3$ cm$^{-3}$ with a median value of $2.3\times 10^2$ cm$^{-3}$. The fraction of DMG column density in the cloud ($f\rm_{DMG}$) decreases with increasing excitation temperature following an empirical relation $f\rm_{DMG}=-2.1\times 10^{-3}T_(ex,τ_{HI}=1)$+1.0. The relation between $f\rm_{DMG}$ and total hydrogen column density $N_H$ is given by $f\rm_{DMG}$=$1.0-3.7\times 10^{20}/N_H$. The values of $f\rm_{DMG}$ in the clouds of low extinction group ($A\rm_V \le 2.7$ mag) are consistent with the results of the time-dependent, chemical evolutionary model at the age of ~ 10 Myr. Our empirical relation cannot be explained by the chemical evolutionary model for clouds in the high extinction group ($A\rm_V > 2.7$ mag). Compared to clouds in the low extinction group ($A\rm_V \le 2.7$ mag), clouds in the high extinction group ($A\rm_V > 2.7$ mag) have comparable volume densities but excitation temperatures that are 1.5 times lower. Moreover, CO abundances in clouds of the high extinction group ($A\rm_V > 2.7$ mag) are $6.6\times 10^2$ times smaller than the canonical value in the Milky Way. #[Full version of abstract is shown in the text.]#

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arXiv2016

Discovery of Two New Pulsars in 47 Tucanae (NGC 104)

We report the discovery of two new millisecond pulsars (PSRs J0024$-$7204aa and J0024$-$7204ab) in the globular cluster 47\,Tucanae (NGC 104). Our results bring the total number of pulsars in 47\,Tucanae to 25. These pulsars were discovered by reprocessing archival observations from the Parkes radio telescope. We reprocessed the data using a standard search procedure based on the PRESTO software package as well as using a new method in which we incoherently added the power spectra corresponding to $\sim$1100\,hr of observations. The newly discovered PSR~J0024$-$7204aa, has a pulse frequency of $\rm \sim$541\,Hz (corresponding to a $\rm \sim$1.84 ms period), which is higher than any other pulsars currently known in the cluster and ranks 12$^{\rm{th}}$ amongst all the currently known pulsars. The dispersion measure of this pulsar, 24.941(7)\,cm$^{-3}$ pc, is the highest in the cluster. The second discovered pulsar, PSR~J0024$-$7204ab, is an isolated pulsar with a pulse frequency of $\rm \sim$270\,Hz (corresponding to a period of $\rm \sim$3.70 ms).

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arXiv2015

A New Method for Constraining Molecular Cloud Thickness: A study of Taurus, Perseus and Ophiuchus

The core velocity dispersion (CVD) is a potentially useful tool for studying the turbulent velocity field of molecular clouds. CVD is based on centroid velocities of dense gas clumps, thus is less prone to density fluctuation and reflects more directly the cloud velocity field. Prior work demonstrated that the Taurus molecular cloud CVD resembles the well-known Larson's linewidth-size relation of molecular clouds. In this work, we studied the dependence of the CVD on the line-of-sight thickness of molecular clouds, a quantity which cannot be measured by direct means. We produced a simple statistical model of cores within clouds and analyzed the CVD of a variety of hydrodynamical simulations. We show that the relation between the CVD and the 2D projected separation of cores ($L_{2D}$) is sensitive to the cloud thickness. When the cloud is thin, the index of CVD-$L_{2D}$ relation ($γ$ in the relation CVD$\sim L_{2D}^γ$) reflects the underlying energy spectrum ($E(k)\sim k^{-β}$) in that $γ\sim(β-1)/2$. The CVD-$L_{2D}$ relation becomes flatter ($γ\to 0$) for thicker clouds. We used this result to constrain the thicknesses of Taurus, Perseus, and Ophiuchus. We conclude that Taurus has a ratio of cloud depth to cloud length smaller than about 1/10-1/8, i.e. it is a sheet. A simple geometric model fit to the linewidth-size relation indicates that the Taurus cloud has a $\sim 0.7$ pc line-of-sight dimension. In contrast, Perseus and Ophiuchus are thicker and have ratios of cloud depth to cloud length larger than about 1/10-1/8.

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arXiv2015

Quantifying Dark Gas

A growing body of evidence has been supporting the existence of so-called "dark molecular gas" (DMG), which is invisible in the most common tracer of molecular gas, i.e., CO rotational emission. DMG is believed to be the main gas component of the intermediate extinction region between A$\rm_v$$\sim$0.05-2, roughly corresponding to the self-shielding threshold of H$_2$ and $^{13}$CO. To quantify DMG relative to HI and CO, we are pursuing three observational techniques, namely, HI self-absorption, OH absorption, and TeraHz C$^+$ emission. In this paper, we focus on preliminary results from a CO and OH absorption survey of DMG candidates. Our analysis show that the OH excitation temperature is close to that of the Galactic continuum background and that OH is a good DMG tracer co-existing with molecular hydrogen in regions without CO. Through systematic "absorption mapping" by Square Kilometer Array (SKA) and ALMA, we will have unprecedented, comprehensive knowledge of the ISM components including DMG in terms of their temperature and density, which will impact our understanding of galaxy evolution and star formation profoundly.

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arXiv2012

The Five-hundred-meter Aperture Spherical Radio Telescope Project and its Early Science Opportunities

The National Astronomical Observatories, Chinese Academy of Science (NAOC), has started building the largest antenna in the world. Known as FAST, the Five-hundred-meter Aperture Spherical radio Telescope is a Chinese mega-science project funded by the National Development and Reform Commission (NDRC). FAST also represents part of Chinese contribution to the international efforts to build the square kilometer array (SKA). Upon its finishing around September of 2016, FAST will be the most sensitive single-dish radio telescope in the low frequency radio bands between 70 MHz and 3 GHz. The design specifications of FAST, its expected capabilities, and its main scientific aspirations were described in an overview paper by Nan et al. (2011). In this paper, we briefly review the design and the key science goals of FAST, speculate the likely limitations at the initial stages of FAST operation, and discuss the opportunities for astronomical discoveries in the so-called early science phase.

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