Search

The characteristics of Low Surface Brightness Galaxies (LSBGs) are very important for understanding the overall characteristics of galaxies. It is of great significance to search and expand the samples of Low Surface Brightness Galaxies by modern machine learning, especially deep learning algorithm. LSBGs are difficult to discern automatically and accurately with traditional methods because of their obscure features. However, deep learning does have the advantage of automatically identifying complex and effective features. To solve this problem, an algorithm named You Only Look Once version X-CS (YOLOX-CS) is proposed to search LSBG in large sample sky survey. Firstly, five classical target detection algorithms are compared through experiments and the optimal YOLOX algorithm is selected as the basic algorithm. Then, the YOLOX-CS framework is constructed by combining different attention mechanisms and different optimizers. The data set uses images from the Sloan Digital Sky Survey (SDSS), labelled from LSBG in the α. 40 -SDSS DR7 (the cross coverage area of 40% HI Arecibo Legacy Fast ALFA Survey and SDSS Data Release7) survey. Due to the small number of samples in this data set, Deep Convolutional Generative Adversarial Networks (DCGAN) model is used to expand the experimental test data. After comparing with a series of target detection algorithms, YOLOX-CS has a good test result in searching LSBG recall rate and Average Precision (AP) value in two data sets before and after expansion. The recall rate and AP value in the test set without expansion data set reach 97.75% and 97.83%, respectively. In the expanded data set of DCGAN model, under the same test set, the recall rate reaches 99.10% and the AP value reaches 98.94%, which proves that the algorithm has excellent performance in LSBG search. Finally, the algorithm is applied to SDSS photometric data, and 765 LSBG candidates are obtained. [ABSTRACT FROM AUTHOR]

Gamma-ray bursts (GRBs) are brief, intense pulses of high-energy emission associated with extreme astrophysical phenomena, e.g. the death of massive stars or the coalescence of compact objects. They have been observed at high energies by the Fermi Large Area Telescope (LAT), which detects GRBs in the 20 MeV–300 GeV energy range. The Fermi-LAT Second GRB Catalog (2FLGC) presents information on 186 GRBs observed from 2008 to 2018. We consider the GRBs that have been fitted in the 2FLGC with a broken (21 GRBs) or simple power law (65 GRBs), compiling a total sample of 86 GRBs. We analyze the relationship between the spectral and temporal indices using closure relations according to the synchrotron forward-shock model evolving in stratified environments ( n ∝ r − k ). We find that the model without energy injection is preferred over the one with energy injection. There is a clear preference for the cooling conditions ν > max{ ν c , ν m } and ν m < ν < ν c (where ν c and ν m are the cooling and characteristic frequencies, namely the frequency at the spectral break). Within these cooling conditions, density profiles r − k with values of k = 1.5 and 2 generally have a higher rate of occurrence when considering relations with and without energy injection. [ABSTRACT FROM AUTHOR]

The YORP (Yarkovsky-O'Keefe-Radzievskii-Paddack) effect is one of the mechanisms of the long-term dynamical evolution of asteroids. Compared with factors such as collision and gravitational perturbation, the YORP is of small magnitude, and the short-time scale observation effect is inconspicuous, which brings great difficulties to the direct measurement of the YORP. From the Asteroid Lightcurve Database, asteroids having a high confidence rotation period were selected for this study. Two subsample groups for identifying potential asteroids slowed by the YORP effect are provided by using the kernel density estimation method and the Kolmogorov-Smirnov test to analyze the rotation rate distribution characteristics of near-Earth asteroids and main belt asteroids; a screening model is proposed based on the light-curve data of seven YORP asteroids with YORP rotation acceleration, combined with the YORP intensity estimation method and the detection conditions of the YORP effect. Finally, ten candidates that can directly detect the YORP effect through light-curve data in the future are listed based on the screening model. [ABSTRACT FROM AUTHOR]

X-ray flares are frequently detected in the X-ray afterglow light curves and are highly correlated with the prompt emission of gamma-ray bursts (GRBs). We compile a comprehensive sample of X-ray flares up to 2021 April, comprising 697 flares. We classify the total sample into four types: early flares ( t p ≤ 10 3 s), late flares ( t p > 10 3 s), long gamma-ray burst (LGRB) flares and short gamma-ray burst (SGRB) flares, and analyze the distributions and relationships of the flare parameters. It is found that the early flares have a higher frequency, shorter duration, and more asymmetrical structure. In addition, the distributions of the morphological parameters of the SGRB flares are similar to those of the LGRB flares. We also find that the durations and rising (decay) times of the early flares are positively correlated with the peak times, but the late flares follow the different dependent relations. There is a strong anti-correlation between the peak luminosities ( L X , P ) and the peak times of the flares, e.g., L X , p ∝ t p , z − 1.45 for the LGRB flares, and L X , p ∝ t p , z − 1.27 for the SGRB flares, respectively. Furthermore, the peak luminosity is highly dependent on the isotropic energy ( E X , i s o ) for the early LGRB flares, the best fit is L X , p ∝ E X , i s o 1.06 ( r = 0.89 ). We also find a tight three-parameter correlation, L X , p ∝ t p , z − 1.03 E X , i s o 0.92 ( r = 0.96 ). All the late flares fall into the 3 σ confidence region defined by the early flares. In terms of the point of kinematic arguments, both the SGRB and LGRB flares support a common scheme of internal origin. The SGRB flares have similar properties to the LGRB flares, suggesting that both of them share a similar physical mechanism from the late-time activity of central engine. [ABSTRACT FROM AUTHOR]

Gamma-ray bursts (GRBs) are brief, intense pulses of high-energy emission associated with extreme astrophysical phenomena, e.g. the death of massive stars or the coalescence of compact objects. They have been observed at high energies by the Fermi Large Area Telescope (LAT), which detects GRBs in the 20 MeV–300 GeV energy range. The Fermi-LAT Second GRB Catalog (2FLGC) presents information on 186 GRBs observed from 2008 to 2018. We consider the GRBs that have been fitted in the 2FLGC with a broken (21 GRBs) or simple power law (65 GRBs), compiling a total sample of 86 GRBs. We analyze the relationship between the spectral and temporal indices using closure relations according to the synchrotron forward-shock model evolving in stratified environments (n∝r−k). We find that the model without energy injection is preferred over the one with energy injection. There is a clear preference for the cooling conditions ν> max{νc,νm} and νm<ν<νc (where νc and νm are the cooling and characteristic frequencies, namely the frequency at the spectral break). Within these cooling conditions, density profiles r−k with values of k=1.5 and 2 generally have a higher rate of occurrence when considering relations with and without energy injection.

X-ray flares are frequently detected in the X-ray afterglow light curves and are highly correlated with the prompt emission of gamma-ray bursts (GRBs). We compile a comprehensive sample of X-ray flares up to 2021 April, comprising 697 flares. We classify the total sample into four types: early flares (tp≤103 s), late flares (tp>103 s), long gamma-ray burst (LGRB) flares and short gamma-ray burst (SGRB) flares, and analyze the distributions and relationships of the flare parameters. It is found that the early flares have a higher frequency, shorter duration, and more asymmetrical structure. In addition, the distributions of the morphological parameters of the SGRB flares are similar to those of the LGRB flares. We also find that the durations and rising (decay) times of the early flares are positively correlated with the peak times, but the late flares follow the different dependent relations. There is a strong anti-correlation between the peak luminosities (LX,P) and the peak times of the flares, e.g., LX,p∝tp,z−1.45 for the LGRB flares, and LX,p∝tp,z−1.27 for the SGRB flares, respectively. Furthermore, the peak luminosity is highly dependent on the isotropic energy (EX,iso) for the early LGRB flares, the best fit is LX,p∝EX,iso1.06(r=0.89). We also find a tight three-parameter correlation, LX,p∝tp,z−1.03EX,iso0.92(r=0.96). All the late flares fall into the 3σ confidence region defined by the early flares. In terms of the point of kinematic arguments, both the SGRB and LGRB flares support a common scheme of internal origin. The SGRB flares have similar properties to the LGRB flares, suggesting that both of them share a similar physical mechanism from the late-time activity of central engine.

Nereid is one of the most distinctive natural satellites that we know in the Solar system. The orbit of Nereid is highly eccentric and inclined with respect to the equator of its primary, Neptune. Studying Nereid is one of the inspiring ways to acquire better knowledge of the Solar system. Due to its faintness, the ground-based observations of Nereid have been limited and the observation precisions in the past were generally not high. A total of 150 new observed positions of Nereid in the period 2012–2017 were collected by the 0.8 m reflecting telescope at Xinglong station of National Astronomical Observatory and the 2.4 m reflecting telescope at Lijiang station of Yunnan Astronomical Observatory. Thanks to the high-quality reference catalogue Gaia DR1 and suitable processing methods for images, the precision of our new observations of Nereid is 2–3 times higher than those of the previous observations, and the dispersions of our observations are better than 70 mas. [ABSTRACT FROM AUTHOR]

Given the ever-increasing number of time-domain astronomical surveys, employing robust, interpretative, and automated data-driven classification schemes is pivotal. Based on graph theory, we present new data-driven classification heuristics for spectral data. A spectral classification scheme of Type II supernovae (SNe II) is proposed based on the phase relative to the maximum light in the V band and the end of the plateau phase. We utilize a compiled optical data set that comprises 145 SNe and 1595 optical spectra in 4000–9000 Å. Our classification method naturally identifies outliers and arranges the different SNe in terms of their major spectral features. We compare our approach to the off-the-shelf umap manifold learning and show that both strategies are consistent with a continuous variation of spectral types rather than discrete families. The automated classification naturally reflects the fast evolution of Type II SNe around the maximum light while showcasing their homogeneity close to the end of the plateau phase. The scheme we develop could be more widely applicable to unsupervised time series classification or characterization of other functional data. [ABSTRACT FROM AUTHOR]

We present the automated systemfor estimating the parameters of radio sources observed on all available continuum radiometers (two receiving facilities of secondary mirrors No. 1 and No. 2 with a total of 30 radiometers) developed at RATAN-600 radio telescope and put into normal operation. The system is also used for the monitoring of the parameters of the antenna and receiving systems of RATAN-600 radio telescope, which is carried out using current measurements of calibration radio sources. [ABSTRACT FROM AUTHOR]

We perform revised spectral calibrations for the AKARI near-infrared grism to correct quantitatively for the effect of the wavelength-dependent refractive index. The nearinfrared grism covering the wavelength range of 2.5-5.0μm, with a spectral resolving power of 120 at 3.6μm, is found to be contaminated by second-order light at wavelengths longer than 4.9μm, which is especially serious for red objects. First, we present the wavelength calibration considering the refractive index of the grism as a function of the wavelength for the first time. We find that the previous solution is positively shifted by up to 0.01μm compared with the revised wavelengths at 2.5-5.0μm. In addition, we demonstrate that second-order contamination occurs even with a perfect order-sorting filter owing to the wavelength dependence of the refractive index. Secondly, the spectral responses of the system from the first- and second-order light are simultaneously obtained from two types of standard objects with different colors. The response from the second-order light suggests leakage of the order-sorting filter below 2.5μm. The relations between the output of the detector and the intensities of the first- and second-order light are formalized by a matrix equation that combines the two orders. The removal of the contaminating second-order light can be achieved by solving the matrix equation. The new calibration extends the available spectral coverage of the grism mode from 4.9μm up to 5.0μm. The revision can be used to study spectral features falling in these extended wavelengths, e.g., the carbon monoxide fundamental ro-vibrational absorption within nearby active galactic nuclei. [ABSTRACT FROM AUTHOR]

The Monitor of All-sky X-ray Image (MAXI) Gas Slit Camera (GSC) detects gamma-ray bursts (GRBs), including bursts with soft spectra, such as X-ray flashes (XRFs).MAXI/GSC is sensitive to the energy range from 2 to 30 keV. This energy range is lower than other currently operating instruments which are capable of detecting GRBs. Since the beginning of the MAXI operation on 2009 August 15, GSC observed 35 GRBs up to the middle of 2013. One third of them were also observed by other satellites. The rest of them show a trend to have soft spectra and low fluxes. Because of the contribution of those XRFs, the MAXI GRB rate is about three times higher than those expected from the BATSE log N-log P distribution. When we compare it to the observational results of the Widefield X-ray Monitor on the High Energy Transient Explorer 2, which covers the the same energy range as that of MAXI/GSC, we find the possibility that many of the MAXI bursts are XRFs with Epeak lower than 20 keV. We discuss the source of soft GRBs observed only by MAXI. The MAXI log N-log S distribution suggests that the MAXI XRFs are distributed over a closer distance than hard GRBs. Since the distributions of the hardness of galactic stellar flares and X-ray bursts overlap with those of MAXI GRBs, we discuss the possibility of confusion of such galactic transients with the MAXI GRB samples. [ABSTRACT FROM AUTHOR]