Zhengyang Li

Bio: Zhengyang Li is an academic researcher from Chinese Academy of Sciences. The author has contributed to research in topics: Telescope & Physics. The author has an hindex of 11, co-authored 36 publications receiving 336 citations.

Optical observations of LIGO source GW 170817 by the Antarctic Survey Telescopes at Dome A, Antarctica

Abstract: The LIGO detection of gravitational waves (GW) from merging black holes in 2015 marked the beginning of a new era in observational astronomy. The detection of an electromagnetic signal from a GW source is the critical next step to explore in detail the physics involved. The Antarctic Survey Telescopes (AST3), located at Dome A, Antarctica, is uniquely situated for rapid response time-domain astronomy with its continuous night-time coverage during the austral winter. We report optical observations of the GW source (GW 170817) in the nearby galaxy NGC 4993 using AST3. The data show a rapidly fading transient at around 1 day after the GW trigger, with the i-band magnitude declining from 17:23 ± 0:13 magnitude to 17:72 ± 0:09 magnitude in ~ 1:8 h. The brightness and time evolution of the optical transient associated with GW 170817 are broadly consistent with the predictions of models involving merging binary neutron stars. We infer from our data that the merging process ejected about ~10−2 solar mass of radioactive material at a speed of up to 30% the speed of light.

Deep learning aided decision support for pulmonary nodules diagnosing: a review.

Abstract: Deep learning techniques have recently emerged as promising decision supporting approaches to automatically analyze medical images for different clinical diagnosing purposes. Diagnosing of pulmonary nodules by using computer-assisted diagnosing has received considerable theoretical, computational, and empirical research work, and considerable methods have been developed for detection and classification of pulmonary nodules on different formats of images including chest radiographs, computed tomography (CT), and positron emission tomography in the past five decades. The recent remarkable and significant progress in deep learning for pulmonary nodules achieved in both academia and the industry has demonstrated that deep learning techniques seem to be promising alternative decision support schemes to effectively tackle the central issues in pulmonary nodules diagnosing, including feature extraction, nodule detection, false-positive reduction, and benign-malignant classification for the huge volume of chest scan data. The main goal of this investigation is to provide a comprehensive state-of-the-art review of the deep learning aided decision support for pulmonary nodules diagnosing. As far as the authors know, this is the first time that a review is devoted exclusively to deep learning techniques for pulmonary nodules diagnosing.

Variable Stars Observed in the Galactic Disk by AST3-1 from Dome A, Antarctica

Abstract: AST3-1 is the second-generation wide-field optical photometric telescope dedicated to time-domain astronomy at Dome A, Antarctica. Here, we present the results of an i-band images survey from AST3-1 toward one Galactic disk field. Based on time-series photometry of 92,583 stars, 560 variable stars were detected with i magnitude <= 16.5 mag during eight days of observations; 339 of these are previously unknown variables. We tentatively classify the 560 variables as 285 eclipsing binaries ( EW, EB, and EA), 27 pulsating variable stars (delta Scuti, gamma Doradus, delta Cephei variable, and RR Lyrae stars), and 248 other types of variables ( unclassified periodic, multiperiodic, and aperiodic variable stars). Of the eclipsing binaries, 34 show O'Connell effects. One of the aperiodic variables shows a plateau light curve and another variable shows a secondary maximum after peak brightness. We also detected a complex binary system with an RS CVn-like light-curve morphology; this object is being followed-up spectroscopically using the Gemini South telescope.

The first release of the AST3-1 Point Source Catalogue from Dome A, Antarctica

Abstract: he three Antarctic Survey Telescopes (AST3) aim to carry out time-domain imaging survey at Dome A, Antarctica. The first of the three telescopes (AST3-1) was successfully deployed in 2012 January. AST3-1 is a 500 mm aperture modified Schmidt telescope with a 680 mm diameter primary mirror. AST3-1 is equipped with a SDSS i filter and a 10k × 10k frame transfer CCD camera, reduced to 5k × 10k by electronic shuttering, resulting in a 4.3 deg2 field of view. To verify the capability of AST3-1 for a variety of science goals, extensive commissioning was carried out between 2012 March and May. The commissioning included a survey covering 2000 deg2 as well as the entire Large and Small Magellanic Clouds. Frequent repeated images were made of the centre of the Large Magellanic Cloud, a selected exoplanet transit field, and fields including some Wolf–Rayet stars. Here, we present the data reduction and photometric measurements of the point sources observed by AST3-1. We have achieved a survey depth of 19.3 mag in 60 s exposures with 5 mmag precision in the light curves of bright stars. The facility achieves sub-mmag photometric precision under stable survey conditions, approaching its photon noise limit. These results demonstrate that AST3-1 at Dome A is extraordinarily competitive in time-domain astronomy, including both quick searches for faint transients and the detection of tiny transit signals

Status of the first Antarctic Survey Telescopes for Dome A

Abstract: The preliminary site testing carried out since the beginning of 2008 shows the Antarctic Dome A is very likely to be the best astronomical site on earth even better than Dome C and suitable for observations ranging from optical wavelength to infrared and sub-millimeter. After the Chinese Small Telescope Array (CSTAR) which is composed of four small fixed telescopes with diameter of 145mm and mounted on Dome A in 2008 for site testing and variable star monitor, three Antarctic Survey Telescopes (AST3) were proposed for observations of supernovas and extrasolar planets searching. AST3 is composed of 3 large field of view catadioptric telescopes with 500mm entrance diameter and G, R, I filter for each. The telescopes can point and track autonomously along with a light and foldable dome to keep the snow and icing build up. A precise auto-focusing mechanism is designed to make the telescope work at the right focus under large temperature difference. The control and tracking components and assembly were successfully tested at from normal temperature down to -80 Celsius degree. Testing observations of the first AST3 showed it can deliver good and uniform images over the field of 8 square degrees. The first telescope was successfully mounted on Dome A in Jan. 2012 and the automatic observations were started from Mar. 2012.

The Combined Ultraviolet, Optical, and Near-Infrared Light Curves of the Kilonova Associated with the Binary Neutron Star Merger GW170817: Unified Data Set, Analytic Models, and Physical Implications

Abstract: We present the first effort to aggregate, homogenize, and uniformly model the combined ultraviolet, optical, and near-infrared dataset for the electromagnetic counterpart of the binary neutron star merger GW170817. By assembling all of the available data from 18 different papers and 46 different instruments, we are able to identify and mitigate systematic offsets between individual datasets, and to identify clear outlying measurements, with the resulting pruned and adjusted dataset offering an opportunity to expand the study of the kilonova. The unified dataset includes 647 individual flux measurements, spanning 0.45 to 29.4 days post-merger, and thus has greater constraining power for physical models than any single dataset. We test a number of semi-analytical models and find that the data are well modeled with a three-component kilonova model: a "blue" lanthanide-poor component with Mej~0.020 Msol and vej~0.27c; an intermediate opacity "purple" component with Mej~0.047 Msol and vej~0.15c; and a "red" lanthanide-rich component with Mej~0.011 Msol and vej~0.14c. We further explore the possibility of ejecta asymmetry and its impact on the estimated parameters. From the inferred parameters we draw conclusions about the physical mechanisms responsible for the various ejecta components, the properties of the neutron stars, and, combined with an up-to-date merger rate, the implications for r-process enrichment via this channel. To facilitate future studies of this keystone event we make the unified dataset and our modeling code public.

Optimized transit detection algorithm to search for periodic transits of small planets

Abstract: We present a new method to detect planetary transits from time-series photometry, the transit least squares (TLS) algorithm. TLS searches for transit-like features while taking the stellar limb darkening and planetary ingress and egress into account. We have optimized TLS for both signal detection efficiency (SDE) of small planets and computational speed. TLS analyses the entire, unbinned phase-folded light curve. We compensated for the higher computational load by (i.) using algorithms such as “Mergesort” (for the trial orbital phases) and by (ii.) restricting the trial transit durations to a smaller range that encompasses all known planets, and using stellar density priors where available. A typical K2 light curve, including 80 d of observations at a cadence of 30 min, can be searched with TLS in ∼10 s real time on a standard laptop computer, as fast as the widely used box least squares (BLS) algorithm. We perform a transit injection-retrieval experiment of Earth-sized planets around sun-like stars using synthetic light curves with 110 ppm white noise per 30 min cadence, corresponding to a photometrically quiet K P = 12 star observed with Kepler . We determine the SDE thresholds for both BLS and TLS to reach a false positive rate of 1% to be SDE = 7 in both cases. The resulting true positive (or recovery) rates are ∼93% for TLS and ∼76% for BLS, implying more reliable detections with TLS. We also test TLS with the K2 light curve of the TRAPPIST-1 system and find six of seven Earth-sized planets using an iterative search for increasingly lower signal detection efficiency, the phase-folded transit of the seventh planet being affected by a stellar flare. TLS is more reliable than BLS in finding any kind of transiting planet but it is particularly suited for the detection of small planets in long time series from Kepler , TESS, and PLATO. We make our python implementation of TLS publicly available.

Follow Up of GW170817 and Its Electromagnetic Counterpart by Australian-Led Observing Programmes

Abstract: The discovery of the first electromagnetic counterpart to a gravitational wave signal has generated follow-up observations by over 50 facilities world-wide, ushering in the new era of multi-messenger astronomy. In this paper, we present follow-up observations of the gravitational wave event GW170817 and its electromagnetic counterpart SSS17a/DLT17ck (IAU label AT2017gfo) by 14 Australian telescopes and partner observatories as part of Australian-based and Australian-led research programs. We report early- to late-time multi-wavelength observations, including optical imaging and spectroscopy, mid-infrared imaging, radio imaging, and searches for fast radio bursts. Our optical spectra reveal that the transient source emission cooled from approximately 6 400 K to 2 100 K over a 7-d period and produced no significant optical emission lines. The spectral profiles, cooling rate, and photometric light curves are consistent with the expected outburst and subsequent processes of a binary neutron star merger. Star formation in the host galaxy probably ceased at least a Gyr ago, although there is evidence for a galaxy merger. Binary pulsars with short (100 Myr) decay times are therefore unlikely progenitors, but pulsars like PSR B1534+12 with its 2.7 Gyr coalescence time could produce such a merger. The displacement (~2.2 kpc) of the binary star system from the centre of the main galaxy is not unusual for stars in the host galaxy or stars originating in the merging galaxy, and therefore any constraints on the kick velocity imparted to the progenitor are poor.