Yunnan Observatory Research Articles

A Geometric Distortion Solution Specifically for Historical Observations and its Implementation

Geometric distortion (GD) critically constrains the precision of astrometry. Using well-established methods to correct GD requires calibration observations, which can only be obtained using a special dithering strategy during the observation period. Unfortunately, this special observation mode is not often used, especially for historical observations before those GD correction methods were presented. As a result, some telescopes have no GD calibration observations for a long period, making it impossible to accurately determine the GD effect. This limits the value of the telescope observations in certain astrometric scenarios, such as using historical observations of moving targets in the solar system to improve their orbits. We investigated a method for handling GD that does not rely on the calibration observations. With this advantage, it can be used to solve the GD models of telescopes which were intractable in the past. The method was implemented in Python and released on GitHub. It was then applied to solve GD in the observations taken with the 1 m and 2.4 m telescopes at Yunnan Observatory. The resulting GD models were compared with those obtained using well-established methods to demonstrate the accuracy. Furthermore, the method was applied in the reduction of observations for two targets, the moon of Jupiter (Himalia) and binary GSC 2038-0293, to show its effectiveness. After GD correction, the astrometric results for both targets show improvements. Notably, the mean residual between the observed and computed position (O − C) for binary GSC 2038-0293 decreased from 36 to 5 mas.

New astrometric positions for six Jovian irregular satellites using Gaia DR3 in 2016 — 2021

The Jovian system is like a miniature solar system, with the system being more than twice as massive as all the other planets combined and having many natural satellites. Its formation and early evolution had a profound influence on our knowledge of the sculpting of the architecture of the solar system. Astrometric observations are of importance, based on these observations, its orbital dynamics, as well as its origin sometimes, of a solar system object can be determined. We aimed to obtain good astrometric positions of some irregular satellites Elara, Pasiphae, Sinope, Lysithea, Carme and Ananke to refine their orbits and ephemerides, and to understand their dynamics. We have taken, processed and reduced 964 ground-based CCD frames obtained between 2016 and 2021 by the 2.4 m telescope at the Lijiang Station of Yunnan Observatory over 27 nights. Among CCD image processing, the image subtraction technique of ISIS is employed to eliminate star images that are close to or/and almost overlapped with those of our targets in 2019. The star catalog Gaia DR3 is utilized for astrometric calibration, in which a weight scheme is applied to solve more accurate plate model. For all targets, their theoretical positions are retrieved from the Institut de Mécanique Céleste et de Calcul des Éphémérides (IMCCE) ephemeris. Our results show the mean (O - C)s (observed minus computed) of the positional residuals of all targets are −0′′.007 and 0′′.011 in R.A. and Dec., respectively. Their corresponding standard deviations are about 0′′.05 in each direction, except for Ananke (the faintest satellite) with its standard deviation about 0′′.08 in each direction.

Analysis of the Effect of Tilted Corner Cube Reflector Arrays on Lunar Laser Ranging

This paper primarily investigates the effect of the tilt of corner cube reflector (CCR) arrays on lunar laser ranging (LLR). A mathematical model was established to study the random errors caused by the tilt of the CCR arrays. The study found that, ideally, when the laser ranging pulse width is 10 picoseconds or less, it is possible to distinguish from which specific corner cubes within the CCR array each peak in the echo signal originates. Consequently, partial data from the echo can be extracted for signal processing, significantly reducing random errors and improving the single-shot precision of LLR. The distance obtained by extracting part of the echo can be reduced to the center position of the array, thereby providing multiple higher-precision ranging results from each measurement. This not only improves the precision of LLR but also increases the data volume. A simulation experiment based on the 1.2 m laser ranging system at Yunnan Observatories was conducted. By extracting one peak for signal processing, the single-shot precision improved from 32.24 mm to 2.52 mm, validating the theoretical analysis results. Finally, an experimental laser ranging system based on a 53 cm binocular telescope system was established for ground experiments. The experimental results indicated that the echo signal could identify the tilt state of the CCR array. By extracting the peak returned by the central CCR for signal processing, the ranging precision was greatly improved. Through theoretical analyses, simulation experiments, and ground experiments, a solution to reduce the random errors caused by the tilt of the CCR array was provided. This offers an approach to enhance the single-shot precision of future LLR and provides a reference for upgrading ground-based equipment at future laser ranging stations.

Optical intraday variability analysis for the BL Lacertae object 1ES 1426+42.8

ABSTRACT The observation data of blazar 1ES 1426+42.8 were obtained using the 1.02 m optical telescope of Yunnan Observatories during 2021 to 2023. Intraday variability (IDV) is detected on seven nights. We use the turbulent model to investigate the mechanism of IDV in 1ES 1426+42.8. The fitting light curves match the actual IDV curves well. Using this model, we obtain the parameters such as the size of turbulent cells and the width of pulses in the jet. A possible short-lived quasi-periodic oscillation (QPO) of $58.55 \pm 8.09$ min was detected on 2022 April 26 whose light curve exhibits eight cycles at $\gt 3\sigma$ global significance and confirmed by several different techniques. Through a more detailed analysis of the light curve of this night, we find that the period is shortened from 54.23 min ($4\sigma$) to 29.71 min ($3\sigma$). The possible QPO and period shortening phenomenon are best explained by the processes of magnetic reconnections.

Observations of Five Major Uranian Satellites during 2013–2020 Based on Gaia DR3

A total of 10,339 positions of the five major Uranian satellites (Ariel, Umbriel, Titania, Oberon and Miranda) were collected at the Yunnan Observatories from 2013 to 2020 with the 1.0 m astrometric reflector. A median filtering method was applied to reduce the influence of the bright Uranus halo. All CCD observational data was analyzed by the newly developed CCD image-processing software with Gaia DR3 as the reference catalog. The impact of different exposure times on satellite positioning accuracy was evaluated. The observational positions of satellites were compared with theoretical positions from Institut de Mécanique Céleste et de Calcul des Éphémérides (DE441 and Lainey et al. 2015) and the mean offsets are from −0.″115 to 0.″103 in both R.A. and decl. for the five satellites. The standard deviation of the offsets are from 0.″008 to 0.″086 for the four greatest Uranian satellites and from 0.″037 to 0.″132 for Miranda in each direction. These precise observations of the five major Uranian satellites over an 8 yr period will be very useful for improving the orbital parameters of both Uranus and its five major satellites.

Identification of Radio Frequency Interference Using Multi-scale TransUNet

Radio observation is a method for conducting astronomical observations using radio waves. A common challenge in radio observations is Radio Frequency Interference (RFI), which refers to the unintentional or intentional interference of radio signals from other wireless sources within the radio frequency band. Such interference contaminates the astronomical signals received by radio telescopes, significantly affecting time–frequency domain astronomical observations and research. Consequently, identifying RFI is crucial. In this paper, we employ a deep learning approach to detect RFI present in observation data and propose an improved network structure based on TransUNet. This network leverages the principles of a multi-scale convolutional attention mechanism. It introduces an auxiliary branch to extract high-dimensional image information and an enhanced coordinate attention mechanism for feature map extraction, enabling more comprehensive and accurate identification of RFI in time–frequency images. We introduce a novel architecture named the Multi-Scale TransUNet Network, abbreviated as MS-TransUNet. We utilized observation data from the 40 m radio telescope at the Yunnan Observatory as a data set for training, validating, and testing the network. Compared with previous deep learning networks (U-Net, RFI-Net, R-Net, DSC, EMSCA-UNet), the recall rate and f2 score have been significantly improved. Specifically, the recall rate is improved by at least 2.99%, and the f2 score is improved by at least 2.46%. Experiments demonstrate that this network is exceptional in identifying RFI more comprehensively while ensuring high precision.

Study of the Intensity Distribution of the Green Line in the Inner Corona

The corona is a key region of solar atmospheric activity, and the source of solar-terrestrial space weather. Limited by observation, research on the plasma structure and magnetic field state of the lower coronal atmosphere is still very lacking, and there are few international studies on the brightness stratification of the lower coronal atmosphere in the visible light band. In this paper, the coronal green line (FeXIV5303 Å) observation data of Lijiang coronagraph YOGIS (Yunnan Observatories Green-line Imaging System) is used to analyze the bright structure and mid-coronal loops in the inner coronal region (1.03R⊙–1.25R⊙, where R⊙ represents the solar radius) effectively. By fitting the exponential decay of the intensity of the bright structures at the radial height of the sun and comparing these fitting results, it is found that the decay index of the static inner coronal loops obtained is around a fixed value. Then, the more obvious coronal loops are extracted, and by performing the same exponential fitting on the intensities of different heights, the obtained decay index is also relatively similar to that of the bright structure, which provides a reference for further study of the evolution of physical parameters in the corona.

Radio frequency interference detection using efficient multiscale convolutional attention UNet

ABSTRACT Studying the Universe through radio telescope observation is crucial. However, radio telescopes capture not only signals from the universe but also various interfering signals, known as radio frequency interference (RFI). The presence of RFI can significantly impact data analysis. Ensuring the accuracy, reliability, and scientific integrity of research findings by detecting and mitigating or eliminating RFI in observational data, presents a persistent challenge in radio astronomy. In this study, we proposed a novel deep learning model called EMSCA-UNet for RFI detection. The model employs multiscale convolutional operations to extract RFI features of various scale sizes. Additionally, an attention mechanism is utilized to assign different weights to the extracted RFI feature maps, enabling the model to focus on vital features for RFI detection. We evaluated the performance of the model using real data observed from the 40 m radio telescope at Yunnan Observatory. Furthermore, we compared our results to other models, including U-Net, RFI-Net, and R-Net, using four commonly employed evaluation metrics: precision, recall, F1 score, and IoU. The results demonstrate that our model outperforms the other models on all evaluation metrics, achieving an average improvement of approximately 5 per cent compared to U-Net. Our model not only enhances the accuracy and comprehensiveness of RFI detection but also provides more detailed edge detection while minimizing the loss of useful signals.

Research on a 44 Day Quasi-periodic Oscillation of Optical Bands for BL Lac S5 0716+714

By means of astronomical observation data from the 60 cm Telescope at Yunnan Observatory, optical data in the g, r, and i bands were collected for BL Lac S5 0716+714, spanning from 2017 November 10 to 2018 May 15. The original data set contains 21,396 quasi-simultaneous multiband points, with 7132 data points for each band. The Lomb–Scargle periodogram method and the weighted wavelet Z-transform method were used to search for a quasi-periodic oscillation (QPO) signal in the data. For the first time, we report a QPO signal at 44 ± 6 days with a final significance of 3.98σ. Further analysis of the spectrum index reveals that the 44 day QPO signal is most likely explained by a helical motion of a blob with velocity β in the jet, where the viewing angle of the emission region in the jet undergoes periodic variations. In addition, we employed the hypothesis testing method (the null hypothesis) to analyze the flux distribution and determined that a double log-normal distribution provides a better fit; thus, there may be two radiative mini-regions within a jet in this source, so this 44 day QPO signal may be superimposed on a longer-term outburst.

Astrometry via close approach events: applications to main-belt asteroid (702) Alauda

ABSTRACT The release of Gaia catalogue is revolutionary to the astronomy of Solar system objects. After some effects such as atmospheric refraction and CCD geometric distortion have been taken into account, the astrometric precision for ground-based telescopes can reach the level of tens of milli-arcseconds (mas). If an object approaches a reference star in a small relative angular distance (less than 100 arcsec), which is called close approach event in this work, the relative positional precision between the object and reference star will be further improved since the systematic effects of atmospheric turbulence and local telescope optics can be reduced. To obtain the precise position of a main-belt asteroid in an close approach event, a second-order angular velocity model with time is supposed in the sky plane. By fitting the relationship between the relative angular distance and observed time, we can derive the time of maximum approximation and calculate the corresponding position of the asteroid. In practice, five nights’ CCD observations including 15 close approach events of main-belt asteroid (702) Alauda are taken for testing by the 1 m telescope at Yunnan Observatory, China. Compared with conventional solutions, our results show that the positional precision significantly improves, which reaches better than 4 mas, and 1 mas in the best case when referenced for the Jet Propulsion Laboratory (JPL) ephemeris in both right ascension and declination.

Revised physical parameters and study on magnetic activities for the eclipsing binary USNO-B1.0 1387-0467554

ABSTRACT We present optimized physical parameters of the active eclipsing binary system USNO-B1.0 1387-0467554 discovered recently in Yunnan–Hong Kong wide-field photometric (YNHK) survey, based on the analysis of its two colour light curves and low resolution spectra obtained at Yunnan Observatories, China. Its spot distributions are derived by means of the Wilson–Devinney code, using photometric data of the YNHK survey from 2016 to 2021. There exist active longitude belts for the spot activities on the primary star, for which the effect of tidal force seems dominant comparing with the one of common magnetic field of the binary system. We have investigated its chromospheric activities through the indicators including Hα, Hβ, CaII H&K, and HeI D3 lines covered by the low resolution spectroscopic observations. There are obvious spatial correlations between the magnetic activities in its photosphere and chromosphere. The EHα/EHβ values suggest that both plages and prominences led to the emissions seen in the Balmer lines at most phases. A prominence structure appeared around the phase of 0.8 on 2018 October 25 and evolved in November and December, 2018. The spectra demonstrate that there were frequent flare events in the system. Moreover, the decay part of a white-light flare event was hunted by the YNHK survey on 2021 October 26.

The study on transiting systems HAT-P-13, HAT-P-16, and WASP-32 through combining ground-based and TESS photometry

ABSTRACT High-precision transit photometry supplies ideal opportunities for detecting new exoplanets and characterizing their physical properties, which usually encode valuable information for unveiling the planetary structure, atmosphere, and dynamical history. We present revised properties of three transiting systems (i.e. HAT-P-13, HAT-P-16, and WASP-32) through analyzing TESS photometry and ground-based transit observations, which were obtained at the 1-m and 2.4-m telescopes of Yunnan Observatories, China, and the 1.2-m telescope of Hamburg Observatory, Germany, as well as the data in the literature. During modelling the transit light curves, Gaussian process is employed to account for the potential systematic errors. Through comprehensive timing analysis, we find that both HAT-P-13b and HAT-P-16b show significant timing variations (TTVs) that can be explained by apsidal precession. TTVs of WASP-32b may be led by a decaying orbit due to tidal dissipation or apsidal precession. However, the current observations can not rule out the origins of three systems’ TTVs from gravitational perturbations of close planetary companions conclusively.

Propagating and Stationary Bright Knots in the Quiet Sun

The question of what heats the solar chromosphere and corona remains one of the most important puzzles in solar physics and astrophysics. Up to now, two mechanisms are considered to work in heating the chromosphere and corona: magnetic reconnection and wave (turbulent flow) dissipation. But it is still not understood which mechanism is dominant. To solve the heating problem, one important topic at this stage is that we should understand how much energy is contributing from the two mechanisms respectively to the heating. In the quiet Sun, the thermal energy signal is observed as brightenings. Here we report two kinds of bright knots with a total of 3605 in the chromosphere of the quiet Sun, using the data from the New Vacuum Solar Telescope at Yunnan Observatories. The first kind of 1537 bright knots, which is first detected in chromospheric fibrils where waves and their dissipation are ubiquitous, propagates along these fibrils with velocities from 5 to 69 km s−1. The second kind of 2068 knots keeps stationary, and always appears at the footpoints of these fibrils where network magnetic fields exist, suggesting that magnetic reconnection locally produces these stationary knots. Based on the observations of thousands of bright knots, we display the different distribution patterns of the two kinds of bright knots in the quiet Sun, and deduce that half of the energy for heating the chromosphere is supplied by wave dissipation, and the other half by magnetic reconnection.

Yunnan–Hong Kong wide field photometric survey

AbstractWe are running a wide field photometric survey under the collaboration between Yunnan Observatories and Hong Kong Astronomical Society with a Centurion 18‐inch telescope at the Lijiang station of Yunnan Observatories, China. Its scientific goals are discovering new transiting exoplanetary systems and other types of variable stars, and studying asteroids of the solar system. The survey was designed to monitor several fixed sky areas through time series photometry with short cadences. Since 2016, numerous observing data have been accumulated and reduced by means of dedicated pipelines to deliver the light curves for tens of thousands objects, which are cross‐matched through the Carlsberg Meridian Catalog 15 or USNO‐B1.0 catalog. Systematic errors and trends that existed in the raw photometric results are simulated through selected reference stars and then removed to improve signal‐to‐noise ratios of the light curves, which is crucial to detect weak signals like exoplanet transits and other variable stars of low amplitudes in brightness variations. The final data products own the photometric precision better than 0.01 magnitude for stars brighter than 13.8 magnitude in the V band. By means of generalized Lomb–Scargle, phase dispersion minimization, and box‐fitting least squares techniques, we have already identified about a thousand of variable stars from the survey, including transiting exoplanet candidates, spotted stars, eclipsing binaries, pulsating stars, and so on. In addition, we have also captured around a thousand of asteroids of the solar system in the survey fields and produced their light curves due to spins.

Chinese Sunspot Drawings and Their Digitization—(VII) Sunspot Penumbra to Umbra Area Ratio Using the Hand-Drawing Records from Yunnan Observatories

The ratio of penumbral to umbra area of sunspots plays a crucial role in the solar physics fields, especially for understanding the origin and evolution of the solar activity cycle. By analyzing the recently digitized sunspot drawings observed from Yunnan Observatories (1957–2021), we investigate the long-term variation of the penumbral to umbra area ratio of sunspots. An automatic extraction method, based on the maximum between-class variance and the morphological discrimination, is used to accurately extract penumbra and umbra and to calculate the ratio over six solar cycles (cycle 19–24). The expected value of the ratio of penumbra to umbra area is found to be 6.63 ± 0.98, and it does not exhibit any systematic variation with sunspot latitudes and phases. The average ratio fluctuates from 5 to 7.5 per year and the overall trend has decreased after 1999 compared to the previous one. The ratio of sunspot penumbra to umbra area satisfies the log-normal distribution, implying that its variation is related to the evolution of the photospheric magnetic field. Our results are consistent with previous works.

YFPOL: A Linear Polarimeter of Lijiang 2.4 m Telescope

Polarimetry plays an important role in investigating physical properties for celestial objects. We present a polarimeter named YFPOL for the Cassegrain focus of the Lijiang 2.4 m Telescope (LJT) of Yunnan Observatories, Chinese Academy of Sciences. YFPOL is a traditional single-beam polarimeter with a rotating polarizer. As the focal-reducer instrument Yunnan Faint Object Spectrograph and Camera (YFOSC) is always positioned on the Cassegrain focal plane of LJT, we develop two sets of ultra-thin (thickness <12 mm) polarizer rotation control systems with wireless charging and control functions, which are suitable for mounting on the two front-wheels of YFOSC. One set is used as the polarimetric calibration unit, and the other is for the polarimetric modulation unit. Both of the polarizers have an ultra-high contrast ratio of 1,000,000:1 in the optical band. We investigate the instrumental polarization characteristics (IPCs) in the full field of view that is transferred from YFOSC. Furthermore, we identify that the IPCs change when the Cassegrain axis rotates. The spurious polarization from the IPCs can be effectively minimized by flat-fielding using the unpolarized domeflat, when the Cassegrain rotation angle is the same or nearest to that of the polarization observation. We develop a quasi-automatic pipeline for YFPOL and its effectiveness has been verified by tests of the polarimetric observation with blazar S5 0716+714. The calibration is performed by observing the zero-polarized and highly-polarized standard stars. We successfully reach high precision polarization in the field of view, and the systematic uncertainty is below 0.8% for a V = 11.68 target with a 10 s exposure. The instrument polarization angle offset is 2.°6. YFPOL is not only a simple polarimeter, but also a spectropolarimeter with grisms that can be considered in the future.

Growth of a filament channel by intermittent small-scale magnetic reconnection

Context. A filament channel (FC), a plasma volume where the magnetic field is primarily aligned with the polarity inversion line, is believed to be the pre-eruptive configuration of coronal mass ejections. Nevertheless, evidence for how the FC is formed is still elusive. Aims. In this paper, we present a detailed study of the build-up of a FC in order to understand its formation mechanism. Methods. The New Vacuum Solar Telescope (NVST) of the Yunnan Observatory and the Optical and Near-infrared Solar Eruption Tracer (ONSET) of Nanjing University, as well as the Atmospheric Imaging Assembly (AIA) and Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO), are used to study the growth process of the FC. Furthermore, we reconstruct the nonlinear force-free field (NLFFF) of the active region using the regularized Biot-Savart laws (RBSL) and the magnetofrictional method to reveal the three-dimensional (3D) magnetic field properties of the FC. Results. We find that partial filament materials are quickly transferred to longer magnetic field lines formed by small-scale magnetic reconnection, as evidenced by dot-like Hα and extreme ultraviolet (EUV) brightenings and subsequent bidirectional outflow jets, as well as untwisting motions. The Hα and EUV bursts appear repeatedly at the same location and are closely associated with flux cancelation, which occurs between two small-scale opposite polarities and is driven by shearing and converging motions. The 3D NLFFF model reveals that the reconnection takes place in a hyperbolic flux tube that is located above the flux-cancelation site and below the FC. Conclusions. The FC is gradually built up toward a twisted flux rope via a series of small-scale reconnection events that occur intermittently prior to the eruption.

On the Relation Between Coronal Green Line Brightness and Magnetic Fields Intensity

Two-dimensional (2D) solar coronal magnetogram is difficult to be measured directly until now. From the previous knowledge, a general relation has been noticed that the brighter green-line brightness for corona, the higher coronal magnetic field intensity may correspond to. To try to further reveal the relationship between coronal green line brightness and magnetic field intensity, we use the 2D coronal images observed by Yunnan Observatories Green-line Imaging System (YOGIS) of the 10 cm Lijiang coronagraph and the coronal magnetic field maps calculated from the current-free extrapolations with the photospheric magnetograms taken by Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO) spacecraft. In our analysis, we identified the coronal loop structures and construct two-dimensional maps of the corresponding magnetic field intensity in the plane of the sky (POS) above the limb. We derive the correlation coefficients between the coronal brightness and the magnetic field intensity for different heights of coronal layers. We further use a linear combination of a Gaussian and a quadratic profile to fit the correlation coefficients distribution, finding a largest correlation coefficient of 0.82 near 1.1 R ⊙ (solar radii) where is almost the top of the closed loop system. For the small closed loop system identified, the correlation coefficient distributions crossing and covering the loop are calculated. We also investigate the correlation with extended heliocentric latitude zones and long period of one whole Carrington Rotation, finding again that the maximum correlation coefficient occurs at the same height. It is the first time for us to find that the correlation coefficients are high (all are larger than 0.8) at the loop-tops and showing poor correlation coefficients with some fluctuations near the feet of the coronal loops. Our findings indicate that, for the heating of the low-latitude closed loops, both DC (dissipation of currents) and AC (dissipation of Alfvén and magnetosonic waves) mechanisms should act simultaneously on the whole closed loop system while the DC mechanisms dominate in the loop-top regions. Therefore, in the distributions of the correlation coefficients with different heights of coronal layers, for both large- and small-scale latitude ranges, the coefficients can reach their maximum values at the same coronal height of 1.1 R ⊙, which may indicate the particular importance of the height of closed loops for studying the coupling of the local emission mechanism and the coronal magnetic fields, which maybe helpful for studying the origin of the low-speed solar wind.

Multicolor Optical Monitoring of the γ -Ray Emitting Narrow-line Seyfert 1 Galaxy PMN J0948+0022 from 2020 to 2021

In order to study the physical properties of the γ-ray emitting narrow-line Seyfert 1 (NLS1) galaxy population, photometric and spectral observations of the γ-ray emitting NLS1 PMN J0948+0022 were made with the Lijiang 2.4 m optical telescope of Yunnan Observatories, Chinese Academy of Sciences. Photometric data in the B and R bands were collected for 50 nights from 2020 January to 2021 December. During the observation epoch, the variability amplitudes are 73.67% in the B band and 79.96% in the R band. Intra-day variability is found in two observation nights, and the duty cycle value is 29% with variability amplitude > 12.9% in the R band, which support the presence of the relativistic jets in the target. The redder-when-brighter (RWB) chromatic trend (or steeper-when-brighter trend) appears on intra-day and long timescales. The RWB trend is dominated by the radiation of accretion disk and jet, and resembles those in flat spectrum radio quasars. When PMN J0948+0022 is brighter than 17.ͫ5 in the R band, there is no color change trend. By analyzing the spectral data of PMN J0948+0022, we obtained the black hole mass of M • = 1.61 × 107 M ⊙ and accretion rate of , and confirmed that PMN J0948+0022 is a super-Eddington accreting NLS1. The redshifts of reverberation mapped super-Eddington accreting active galactic nuclei can be expanded by PMN J0948+0022 up to above 0.5. Super-Eddington accreting NLS1 galaxies were chosen as a new type of cosmological candle in the literature. PMN J0948+0022 may be used as a target for the next step of reverberation mapping monitoring project of super-Eddington accreting massive black holes.

New CCD Astrometric Observations of Himalia Using Gaia DR2 in 2015–2021

Abstract The long arc and high-quality astrometric measurements of outer irregular satellites are prerequisites for improving their orbital theories and increasing the precision of their ephemerides. In order to obtain good astrometric positions of Himalia, the largest irregular satellite of Jupiter, we have processed and reduced 911 ground-based CCD frames obtained between 2015 and 2021 by three telescopes (including 1 and 2.4 m telescopes at Yunnan Observatory, and 0.8 m telescope at Purple Mountain Observatory) over 61 nights. Subtracting off the companion star of our target by constructing an effective point-spread function (ePSF) model in some CCD frames, the ePSF-subtracted technique is used to reduce the centering error. Some additional techniques are applied in data reduction to further improve positional accuracy and precision of Himalia. This includes geometric distortion correction, weighted polynomial plate models, and the precision-premium effect, since their relative positional measurements have better precision when two objects are very near (e.g., less than 60″). The star catalog Gaia DR2 is used for astrometric calibration, and theoretical positions of Himalia are retrieved from JPL Horizons ephemeris, including the satellite ephemeris Jup344 and the newest planetary ephemeris DE441. Our results show the mean (O − C)s (observed minus computed) of the positional residuals of Himalia are −0."004 and 0.″005 in R.A. and decl., respectively, and their corresponding standard deviations are about 0.″020 in each direction.