Xiang-Tao He

The Relation between the Amplitude and the Period of Solar Cycles

The maximum amplitudes of solar activity cycles are found to be well anti-correlated ( r = −0.72 ) with the newly defined solar cycle lengths three cycles before (at lag -3) in 13-month running mean sunspot numbers during the past 190 years. This result could be used for predicting the maximum sunspot numbers. The amplitudes of Cycles 24 and 25 are estimated to be 149.5 ± 27.6 and 144.3 ± 27.6, respectively.

A New Method to Determine Epochs of Solar Cycle Extrema

A weighted average method is proposed to determine the epochs of solar cycle extrema and hence the solar cycle lengths. Comparing to the previous methods, this method has the advantage that the extremum epochs are easily and uniquely determined.

The Multiwavelength Quasar Survey. I. Initial Results

This is the first paper in a series about our Multiwavelength Quasar Survey. We give a detailed description of this survey and present the scientific objectives, candidate selection procedures, and initial results of the observations of X-ray candidates in field f1002. Multiwavelength methods are employed in order to reduce the selection effects. In this survey, we hope to get a quasar sample more complete than that of any previous survey.

Two cataclysmic variables identified from ROSAT bright sources

This paper presents the results of optical spectroscopic observations of two ROSAT bright sources, 1RXS J020928.9+283243 and 1RXS J042332.8+745300. The low-dispersion spectra suggest the cataclysmic variable classification for the two objects. Further photometric observations are expected to reveal the variable features and to confirm the classifications.

The multiwavelength Quasar Survey. II. Quasars in the Coma Cluster

This is the second paper of the series about our Multiwavelength Quasar Survey. We present the observational results for the X-ray candidates in the field of the Coma Cluster. Nine new X-ray active galactic nuclei are discovered.

Stability of stellar structure in non-equilibrium thermodynamics. I. the PPI reaction

Abstract Using Prigogines stability criterion of excess entropy production in non-linear and non-equilibrium thermodynamics, we re-examined the stability of stellar structure with respect to the PPI reaction in solar type stars. We show that the star is stable if the relative mass abundence of Hydrogen to Helium is less than 8.9, a condition which is amply satisfied by the normal stars.

The multi-wavelength quasar survey - III. Quasars in field 836

This is the third paper in a series connected with our Multiwavelength Quasar Survey. The survey is aimed to provide a quasar sample more complete than any previous survey by using a combined selection technique to reduce selection effects. We present the observational results for the X-ray candidates in field f836. We found 15 X-ray AGNs in this field of which eight are new discoveries. The X-ray data and optical spectra of these AGNs are given. We give the X-ray candidate selection criteria, which proved to be highly efficient in isolating X-ray AGNs.

M dwarf stars — the by-product of X-ray selected AGN candidates

X-ray loud M dwarfs are a major source of by-products (contamination) in the X-ray band of the multiwavelength quasar survey. As a by-product, the low dispersion spectra of 22 M dwarfs are obtained in which the spectra of 16 sources are taken for the first time. The spectral types and distances of the sample are given based on spectral indices CaH2, CaH3, and TiO5. The parameter ζTiO/CaH is calculated to separate the different metallicity classes among dwarfs, subdwarfs and extreme subdwarfs. We also discuss the distributions in the diagrams of log(Lx/Lbol), the ratio between X-ray and bolometric luminosity versus spectral type and infrared colors.

Spectroscopic and photometric observations of unidentified ultraviolet variable objects in the GUVV-2 Catalog

An NUV-optical diagram composed from sources from the second Galaxy Evolution Explorer (GALEX) Ultraviolet Variability (GUVV-2) Catalog provides us with a method to tentatively classify the unknown GUVV-2 sources by their NUV-optical magnitudes. For the purpose of testing the correctness and generality of the method, we carry out a program on the spectroscopic observations of the unidentified GUVV-2 sources. The spectroscopic identification of these 37 sources are 19 type -A to -F stars, 10 type -G to -K stars and 7 M dwarf stars together with an AGN. We also present the light curves in the R-band for two RR Lyrae star candidates selected from the NUV-optical diagram, both of which undergo cyclic variations. Combining their light curves and colors, we classify them as RR Lyrae stars. To confirm the results, we show a color-color diagram for the 37 newly identified spectroscopic objects compared with previously identified ones, which are consistent with our previous results, indicating that the ultraviolet variable sources can initially be classified by their NUV/optical color-color diagram.

Re-Identification of the `Enigmatic' X-ray Source 1RXS J114003.0+124112

The ROSAT X-ray source 1RXS J114003.0+124112 was identified as a starburst galaxy at redshift 0.177 by He et al. The authors also noted that the source is almost two orders of magnitude brighter in X-ray than the X-ray-brightest starburst galaxy and it seems to be in a merging system, making this source an enigmatic system demanding further observations. Here we report a re-identification of 1RXS J114003.0+124112 using observations on the 2.6 m telescope at Byurakan Astrophysical Observatory, Armenia and the SDSS data. The results indicate that the starburst activity is associated with the brighter object of the system, while the fainter object is a typical Seyfert 1 galaxy at a different redshift (0.282). Therefore, the two objects are not in a merging system, and the Seyfert 1 galaxy naturally accounts for the high X-ray flux. Three more objects reside in the vicinity, but they are all too faint to be responsible for the high X-ray flux.